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David Sang, Graham Jones,Richard Woodside and Gurinder Chadha

Cambridge International AS and A Level

PhysicsCoursebook

Sang, Jones, Woodside and C

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S and A Level Physics

Coursebook

Cambridge International AS and A Level Physics matches the requirements of the Cambridge International AS and A Level Physics syllabus (9702). It is endorsed by Cambridge International Examinations for use with their examination.

The ! rst 17 chapters cover the material required for AS Level, while the remaining 16 chapters cover A2 Level.

ends with a summary.

for students to consolidate their learning as they progress, with answers at the end of the book.

question.

to revise the content, and a series of exam style questions to give practice in answering longer, structured questions. Answers to these questions are available on the accompanying Teacher’s Resource CD-ROM.

Communications systems cover the Applications of Physics section of the syllabus.

tested in examinations, as well as providing other useful reference material and a glossary.

The accompanying CD-ROM includes animations designed to develop a deeper understanding of various topics. It also contains revision questions with answers for each chapter.

Also available:Teacher’s Resource CD-ROM ISBN 978-0-521-17915-7

Completely Cambridge – Cambridge resources for Cambridge qualifi cations Cambridge University Press works closely with Cambridge International Examinations as parts of the University of Cambridge. We enable thousands of students to pass their Cambridge exams by providing comprehensive, high-quality, endorsed resources.

To ! nd out more about Cambridge International Examinations visit www.cie.org.uk

Visit education.cambridge.org/cie for more information on our full range of Cambridge International A Level titles including e-book versions and mobile apps.

Cambridge International AS and A Level PhysicsCoursebookDavid Sang, Graham Jones, Richard Woodside and Gurinder Chadha

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David Sang, Graham Jones, Richard Woodside and Gurinder Chadha

Cambridge International AS and A Level

Physics Coursebook

Completely Cambridge -Cambridge resources for Cambridge qualifi cations

Cambridge University Press works closely with University of Cambridge International Examinations (CIE) as parts of the University of Cambridge. We enable thousands of students to pass their CIE exams by providing comprehensive, high-quality, endorsed resources.

To fi nd out more about University of Cambridge International Examinations visit www.cie.org.uk

To fi nd out more about Cambridge University Press visit www.cambridge.org/cie

!"#$%&'() *+&,)%-&./ 0%)--Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo, Delhi, Mexico City

Cambridge University Press1 e Edinburgh Building, Cambridge CB2 8RU, UK

www.cambridge.orgInformation on this title: www.cambridge.org/9780521183086

© Cambridge University Press 2010

1 is publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press.

First published 2010

Printed in

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ISBN 978-0-521-18308-6 Paperback with CD-ROM for Windows and Mac

Cambridge University Press has no responsibility for the persistence oraccuracy of URLs for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.

+2.&!) .2 .)"!3)%-

1 e photocopy masters in this publication may be photocopied or distributed electronically free of charge for classroom use within the school or institute which purchases the publication. Worksheets and copies of them remain in the copyright of Cambridge University Press and such copies may not be distributed or used in any way outside the purchasing institution.

6th printing 2012

India by Replika Press Pvt. Ltd.

Contents

iiiContents

1 Kinematics – describing motion 1 Speed 1 Distance and displacement, scalar and vector 4 Speed and velocity 5 Displacement–time graphs 7 Combining displacements 9 Combining velocities 10

2 Accelerated motion 15 The meaning of acceleration 15 Calculating acceleration 15 Units of acceleration 16 Deducing acceleration 17 Deducing displacement 18 Measuring velocity and acceleration 19 Determining velocity and

acceleration in the laboratory 19 The equations of motion 21 Deriving the equations of motion 24 Uniform and non-uniform acceleration 25 Acceleration caused by gravity 26 Determining g 27 Motion in two dimensions – projectiles 30 Understanding projectiles 32

3 Dynamics – explaining motion 40 Calculating the acceleration 40 Understanding SI units 42 The pull of gravity 44 Mass and inertia 46 Top speed 47 Moving through fl uids 49 Identifying forces 50 Newton’s third law of motion 53

Centre of gravity 62 The turning effect of a force 63 The torque of a couple 67

5 Work, energy and power 73 Doing work, transferring energy 74 Gravitational potential energy 78 Kinetic energy 79 g.p.e.–k.e. transformations 80 Down, up, down – energy changes 81 Energy transfers 82 Power 85

6 Momentum 92 The idea of momentum 92 Modelling collisions 93 Understanding collisions 95 Explosions and crash-landings 98 Momentum and Newton’s laws 100 Understanding motion 100

Introduction vii

Acknowledgements viii

7 Matter 107 Macroscopic properties of matter 107 The kinetic model 110 Explaining pressure 112 Changes of state 113

8 Deforming solids 119 Compressive and tensile forces 119 Stretching materials 121 Describing deformation 124 Strength of a material 125 Elastic potential energy 126

9 Electric fields 132 Attraction and repulsion 132 Investigating electric fi elds 134 Electric fi eld strength 136 Force on a charge 138

4 Forces – vectors and moments 57 Combining forces 57 Components of vectors 59

iv Contents

10 Electric current, potential difference and resistance 144Circuit symbols and diagrams 144Electric current 145The meaning of voltage 148Electrical resistance 149Electrical power 151

11 Kirchhoff ’s laws 157Kirchhoff ’s fi rst law 157Kirchhoff ’s second law 159Applying Kirchhoff ’s laws 160Resistor combinations 162Ammeters and voltmeters 165

15 Superposition of waves 212 The principle of superposition of waves 212Diffraction of waves 213Interference 216The Young double-slit experiment 220Diffraction gratings 223

18 Circular motion 258Describing circular motion 258Angles in radians 259Steady speed, changing velocity 260Angular velocity 261Centripetal forces 262Calculating acceleration and force 264The origins of centripetal forces 266

16 Stationary waves 231From moving to stationary 231Nodes and antinodes 232Formation of stationary waves 232Observing stationary waves 233Determining the wavelength and speed of sound 237Eliminating errors 238

17 Radioactivity 242Looking inside the atom 242Alpha-particle scattering and the nucleus 242A simple model of the atom 245Nucleons and electrons 246Discovering radioactivity 249Radiation from radioactive substances 249Properties of ionising radiation 251Randomness and decay 254

12 Resistance and resistivity 171The I–V characteristic for a metallic conductor 172Ohm’s law 173Resistance and temperature 174Resistivity 177

13 Practical circuits 183Internal resistance 184Potential dividers 186Potentiometer circuits 189

14 Waves 195Describing waves 195Longitudinal and transverse waves 198Wave energy 199Intensity 199Wave speed 200Electromagnetic waves 202Electromagnetic radiation 203Orders of magnitude 203The nature of electromagnetic waves 204Polarisation 204

vContents

19 Gravitational fields 272Representing a gravitational fi eld 273Gravitational fi eld strength g 275Energy in a gravitational fi eld 277Gravitational potential 277Orbiting under gravity 278The orbital period 279Orbiting the Earth 280

20 Oscillations 287Free and forced oscillations 287Observing oscillations 288Describing oscillations 289Simple harmonic motion 291Graphical representations of s.h.m. 292Frequency and angular frequency 294Equations of s.h.m. 295Energy changes in s.h.m. 299Damped oscillations 300Resonance 302

21 Thermal physics 312Changes of state 312Energy changes 313Internal energy 315The meaning of temperature 317Thermometers 318Calculating energy changes 320

22 Ideal gases 330Molecules in a gas 330Measuring gases 331Boyle’s law 332Changing temperature 333Ideal gas equation 334Modelling gases – the kinetic model 336Temperature and molecular kinetic energy 338

23 Coulomb’s law 343Electric fi elds 343Coulomb’s law 343Electric fi eld strength for a radial fi eld 345Electric potential 346Comparing gravitational and electric fi elds 350

24 Capacitance 356Capacitors in use 356Energy stored in a capacitor 358Capacitors in parallel 361Capacitors in series 362Comparing capacitors and resistors 363Capacitor networks 364

Producing and representing magnetic fi elds 371Magnetic force 374Magnetic fl ux density 375Measuring magnetic fl ux density 376Currents crossing fi elds 378Forces between currents 380Relating SI units 381

25 Magnetic fields and electromagnetism 371

26 Charged particles 387Observing the force 387Orbiting charges 389Electric and magnetic fi elds 391Discovering the electron 393

Observing induction 399Explaining electromagnetic induction 400Faraday’s law of electromagnetic induction 405

27 Electromagnetic induction 399

vi Contents

33 Communications systems 512Radio waves 512Analogue and digital signals 517Channels of communication 520Comparison of different channels 523

29 Quantum physics 431Modelling with particles and waves 431Particulate nature of light 433The photoelectric effect 437Line spectra 440Explaining the origin of line spectra 441Photon energies 443Isolated atoms 444The nature of light – waves or particles? 444Electron waves 444The nature of the electron – wave or particle? 448

30 Nuclear physics 454Mass and energy 454Energy released in radioactive decay 457Binding energy and stability 458The mathematics of radioactive decay 460Decay graphs and equations 462Decay constant and half-life 464

The inverting amplifi er 478The non-inverting amplifi er 479Output devices 480

32 Medical imaging 487The nature and production of X-rays 487X-ray attenuation 490Improving X-ray images 491Computerised axial tomography 494Using ultrasound in medicine 498Echo sounding 500Ultrasound scanning 502Magnetic resonance imaging 504

28 Alternating currents 416Sinusoidal current 416Alternating voltages 417Measurements using a cathode-ray oscilloscope 418Power and a.c. 419Why use a.c. for electricity supply? 421Transformers 422Rectifi cation 424

Lenz’s law 407Using induction: eddy currents, generators and transformers 410

Appendices 534

A1 Practical skills at AS level 534

A2 Further practical skills 546

B Physical quantities and units 554 C Data, formulae and relationships 555 D The Periodic Table 557

Answers to Test yourself questions 558

Glossary 585

Index 595

31 Direct sensing 469Sensor components 469The operational amplifi er (op-amp) 473Negative feedback 477

viiIntroduction

Introduction

! is book covers the entire syllabus of CIE Physics for A and AS level. It is in three parts:

Chapters 1–17: the AS level content, covered in the • fi rst year of the course;Chapters 18–33: the remaining A level content, • including Applications of Physics, covered in the second year of the course;Appendices: practical skills, and useful data and • formulae.

! e main task of a textbook like this is to explain the various concepts of physics which you need to understand and to provide you with questions which will help you to test your understanding and prepare for the examinations. You will fi nd that each chapter has the same structure.

At the start there are objectives, mapping out the • content of the chapter.! e main text explains ideas and outlines the evidence • which supports them.Worked examples show how to tackle particular types • of problems.‘Test yourself ’ questions allow you to check your • understanding as you go along.At the end, there is a detailed summary which links • back to the objectives at the start.Finally, at the end of each chapter there are questions of • two sorts: general questions which relate to the material covered in the whole chapter, and exam-style questions which will give you practice at tackling the sort of structured questions which appear in exams.

When tackling questions, it is a good idea to make a fi rst attempt without referring to the textbook or to your notes. ! is will help to reveal any gaps in your understanding. By sorting out any problems at an early stage you will progress faster as the course continues.

! e CD-ROM which accompanies this book includes a worksheet for each chapter with more questions. ! e later questions on each worksheet are designed to provide more of a challenge for able students.

In this book, the maths has been kept to the minimum required by the CIE Physics AS and A level syllabus. If you are also studying mathematics, you may fi nd that more advanced techniques such as calculus will help you with many aspects of physics.

Studying physics can be a stimulating and worthwhile experience. It is an international subject; no single country has a monopoly on the development of the ideas. Discovering how men and women from many countries have contributed to our knowledge and well-being can be a rewarding exercise. We hope that this book will help you to succeed in examinations but also that it will stimulate your curiosity and fi re your imagination. Today’s students become the next generation of physicists and engineers, and we hope that you will learn from the past to take physics to ever greater heights.

Acknowledgements

We would like to thank the following for permission to reproduce images:

Cover, Omikron / SPL

Figure 1.1, Edward Kinsman / Photo Researchers, Inc.; 1.2, Yves Herman / Reuters / Corbis; 1.3, © Dominic Burke / Alamy; 1.13a, Robert Harding Picture Library / Alamy; 2.1, Time & Life Pictures / Getty Images; 2.7, David Scharf; 2.12, Lockheed Martin Astronautics; 2.22, Michel Pissotte / Action Plus; 2.27, Professor Harold Egerton / SPL; 3.1, Getty Images; 3.6, Blickwinkel / Schmidbauer / Alamy; 3.7, Keith Kent / SPL; 3.9, Richard Francis / Action Plus; 3.11, Jamie Squire / Sta! / Getty; 3.12, Mira / Alamy; 4.1, Phil Cole / Sta! / Getty; 4.9, Andrew Lambert / SPL; 4.14, AFP / Getty; 4.16, Corbis Premium RS / Alamy; 5.1, George Steinmetz; 5.2, Popperfoto; 5.3, Mike Clarke / Getty; 5.12, Images Colour Library; 5.15, Bob Martin / Allsport; 5.16, SPL; 5.19, AustraliaLK; 6.1, TRL Ltd. / SPL; 6.2, arabianEye; 6.3, Andrew Lambert; 6.4, Andrew Lambert; 6.7, Motoring Picture Library / Alamy; 6.12, SPL; 6.14, SPL; 7.1, Andrew Dunn; 8.1, Stacy Walsh Rosenstock / Alamy; 8.17, Alex Wong / Getty; 9.1, Kent Wood / SPL; 9.4, Sheila Terry / SPL; 9.6, Andrew Lambert / SPL; 9.14, Andrew Lambert / SPL; 10.1, Adam Hart-Davis / SPL; 10.2, Adam Hart-Davis / SPL; 10.3, Adam Hart-Davis / SPL; 10.4, Adam Hart-Davis / SPL; 10.11, Andrew Lambert; 10.13, Leslie Garland Picture Library ; 10.14, Maxamillian Stock LTD / SPL; 11.1, TEK Image / SPL; 11.2, CC Studio / SPL; 11.19, Andrew Lambert; 11.23, Andrew Lambert / SPL; 12.1, Takeshi Takahara / SPL ; 12.4, imagebroker / Alamy; 12.5, Richard Megna / Fundamental Photos / SPL; 13.1a, SPL; 13.1b, Sheila Terry; 13.10, Andrew Lambert; 14.1, Jim Reed Photography / SPL; 14.2, SPL; 14.4, Hermann Eisenbeiss / SPL; 14.6, sciencephotos / Alamy; 14.10, Tom Pietrasik / Corbis; 14.11, © Popperfoto / Reuters; 14.12, Chris Pearsall / Alamy; 14.13, Dr Morley Read / SPL; 14.14, Physics Today Collection / American Institute of Physics / SPL; 14.20, Michael Brooke; 14.21, Peter Aprahamian / Sharples Stress Engineers Ltd / SPL; 15.1, Pascal Goetgheluck / SPL; 15.2, ImageState / Alamy; 15.7, ImageState / Alamy; 15.9, © superclic / Alamy; 15.14, © Bruce Coleman Inc. / Alamy; 15.21, © Phototake Inc. / Alamy; 15.24, Photography by Ward; 16.1a, Alex Bartel / SPL; 16.1b, Bettmann / CORBIS; 16.2, Tim Ridley; 16.7, Andrew Lambert; 16.13, hipp stenoglepsis / Ian Sanders /

Alamy; 17.1, Karen Kasmauski; 17.2, University Of Cambridge, Cavendish Laboratory; 17.8, Stocktrek Images, Inc. / Alamy; 17.9, Jean-Loup Charmet / SPL; 17.14, N Feather / SPL; 17.17, Leslie Garland Picture Library / Alamy; 17.18, Andrew Lambert / SPL; 18.1, Dinodia Images / Alamy; 18.12, Marvin Dembinsky Photo Associates; 18.16, Action plus sports images; 19.1, Ken Fisher; 19.10, © NASA Images / Alamy; 20.1, iStock; 20.2, Andrew Lambert; 20.3, Andrew Lambert / SPL; 20.14, Gurinder Chadha; 20.29, Robin MacDougall; 20.31, Andrew Lambert / SPL; 20.33, David Leah / SPL; 20.35, Simon Fraser; 21.1, Tony Camacho / SPL; 21.6, Martyn F. Chillmaid / SPL; 21.12, Andrew Lambert / SPL; 22.1, Patrick Dumas / Eurelios / SPL; 22.2, David Mack / SPL; 23.1, Health Protection Agency / SPL; 24.1, David Hay Jones / SPL; 24.2, Andrew Lambert / SPL; 25.1, Alex Bartel / SPL; 25.12, Martyn F. Chillmaid / SPL; 26.1, Omikron / SPL; 26.2, Andrew Lambert / SPL; 26.8, Andrew Lambert / SPL; 26.12, SPL; 27.1, Sean Gallup / Getty Images; 27.15, Bill Longchore / SPL; 28.1, Tom Bonaventure; 28.3, Adam Gault / SPL; 28.7, Andrew Lambert Photography / SPL; 28.10, Chris Knapton / SPL; 28.13, Greenshoots Communications / Alamy; 20.28, Andrew Lambert / SPL; 20.30, Andrew Lambert / SPL; 29.1, © PHOTOTAKE Inc. / Alamy; 29.2, © STphotography / Alamy; 29.3, Paul Broadbent / Alamy; 29.5, Volker Steger / SPL; 29.12, Department of Physics Imperial College / SPL; 29.13, Department of Physics Imperial College / SPL; 29.14, Department of Physics Imperial College / SPL; 29.19, SPL; 29.20, SPL; 29.21, Professor Dr Hannes Lichte, Technische Universitat, Dresden; 29.23, Dr David Wexler, coloured by Dr Jeremy Burgess / SPL; 29.24, Dr Tim Evans / SPL; 30.1, US Navy / SPL; 31.1, Ria Novosti; 31.9, Cristian Nitu; 31.10, sciencephotos / Alamy; 31.21, Simon Belcher; 32.1, Mauro Fermariello / SPL; 32.3, AJ Hop Photo / Hop Americain / SPL; 32.8, Edward Kinsman / SPL; 32.14, Zephyr / SPL; 32.15, GustoImages / SPL; 32.17, Michelle Del Guercio / SPL; 32.20, Scott Camazine / SPL; 32.21, medicimage; 32.25, GustoImages / SPL; 32.28, Professor AT Elliott, Department of Clinical Physics and Bioengineering, Glasgow University; 32.29, Geo! Tomkinson / SPL; 32.35, Alfred Pasieka / SPL; 32.36, Sovereign, ISM / SPL; 33.1, SPL; 33.2, S. Hammid; 33.13, iStock; 33.14, iStock; 33.15, iStock; 33.17, Peter Ryan / SPL; 33.19, Eyebyte / Alamy.

SPL = Science Photo Library

viii Acknowledgements

" e author and publishers are grateful for the permissions granted to reproduce materials in either the original or adapted form. While every e! ort has been made, it has not always been possible to identify the sources of all the materials used, or to trace all copyright holders. If any omissions are brought to our notice, we will be happy to include the appropriate acknowledgements on reprinting.

1 Kinematics – describing motion


Describing movementDescribing movement

Our eyes are good at detecting movement. We notice even quite small movements out of the corners of our eyes. It’s important for us to be able to judge movement – think about crossing the road, cycling or driving, or catching a ball.

Figure 1.1 shows a way in which movement can be recorded on a photograph. ! is is a stroboscopic photograph of a boy juggling three balls. As he juggles, a bright lamp fl ashes several times a second so that the camera records the positions of the balls at equal intervals of time.

If we knew the time between fl ashes, we could measure the photograph and calculate the speed of a ball as it moves through the air.

1ObjectivesObjectivesAfter studying this chapter, you should be able to:

defi ne displacement, speed and velocity draw and interpret displacement–time graphs

describe laboratory methods for determining speed use vector addition to add two or more vectors

SpeedSpeedWe can calculate the average speed of something moving if we know the distance it moves and the time it takes:

distance=average speedtime

In symbols, this is written as: = xv t

where v is the average speed and x is the distance travelled in time t. ! e photograph (Figure 1.2) shows Ethiopia’s Kenenisa Bekele posing next to the score board after breaking the world record in a men’s 10 000 metres race. ! e time on the clock in the photograph enables us to work out his average speed.

If the object is moving at a constant speed, this equation will give us its speed during the time taken. If its speed is changing, then the equation gives us its average speed. Average speed is calculated over a period of time.

Figure 1.2 Ethiopia’s Kenenisa Bekele set a new world record for the 10 000 metres race in 2005.

If you look at the speedometer in a car, it doesn’t tell you the car’s average speed; rather, it tells you its speed at the instant when you look at it. ! is is the car’s instantaneous speed.

Figure 1.1 This boy is juggling three balls. A stroboscopic lamp fl ashes at regular intervals; the camera is moved to one side at a steady rate to show separate images of the boy.

2 1 Kinematics – describing motion

1 Look at Figure 1.2. ! e runner ran 10 000 m, and the clock shows the total time taken. Calculate his average speed during the race.

Test yourself

UnitsUnitsIn the Système Internationale d’Unités (the SI system), distance is measured in metres (m) and time in seconds (s). ! erefore, speed is in metres per second. ! is is written as m s"1 (or as m/s). Here, s"1 is the same as 1/s, or ‘per second’.

! ere are many other units used for speed. ! e choice of unit depends on the situation. You would

probably give the speed of a snail in di# erent units from the speed of a racing car. Table 1.1 includes some alternative units of speed.

Note that in many calculations it is necessary to work in SI units (m s"1).

m s"1 metres per secondcm s"1 centimetres per secondkm s"1 kilometres per secondkm h"1 or km/h kilometres per hourmph miles per hour

Table 1.1 Units of speed.


Modern cars are designed to travel at high speeds – they can easily exceed national speed limits. However, road safety experts are sure that driving at lower speeds increases safety and saves the lives of drivers, passengers and pedestrians in the event of a collision. ! e police must identify speeding motorists. ! ey have several ways of doing this.

On some roads, white squares are painted at intervals on the road surface. By timing a car between two of these markers, the police can determine whether the driver is speeding. Speed cameras can measure the speed of a passing car. ! e camera shown in Figure 1.3 is of the type known as a ‘Gatso’. ! e camera is usually mounted in a yellow box, and the road has characteristic markings painted on it.

! e camera sends out a radar beam (radio waves) and detects the radio waves refl ected by a car. ! e frequency of the waves is changed according to the instantaneous speed of the car. If the car is travelling above the speed limit, two photographs are taken of the car. ! ese reveal how far the car has moved in the time interval between the photographs, and these can provide

the necessary evidence for a prosecution. Note that the radar ‘gun’ data is not itself su$ cient; the device may be confused by multiple refl ections of the radio waves, or when two vehicles are passing at the same time.

Note also that the radar gun provides a value of the vehicle’s instantaneous speed, but the photographs give the average speed.

Figure 1.3 A typical ‘Gatso’ speed camera (named after its inventor, Maurice Gatsonides). The box contains a radar speed gun which triggers a camera when it detects a speeding vehicle.

Drive slower, live longer


2 Here are some units of speed: m s!1 mm s!1 km s!1 km h!1 mph Which of these units would be appropriate when

stating the speed of each of the following?a a tortoiseb a car on a long journeyc lightd a sprintere an aircraft

3 A snail crawls 12 cm in one minute. What is its average speed in mm s!1?

Test yourself

Determining speedDetermining speedYou can fi nd the speed of something moving by measuring the time it takes to travel between two fi xed points. For example, some motorways have emergency telephones every 2000 m. Using a stopwatch you can time a car over this distance. Note that this can only tell you the car’s average speed between the two points. You cannot tell whether it was increasing its speed, slowing down, or moving at a constant speed.

Laboratory measurements of speedLaboratory measurements of speedHere are some di" erent ways to measure the speed of a trolley in the laboratory as it travels along a straight line. # ey can be adapted to measure the speed of other moving objects, such as a glider on an air track, or a falling mass.

Using two light gates# e leading edge of the card in Figure 1.4 breaks the light beam as it passes the fi rst light gate. # is

starts the timer. # e timer stops when the front of the card breaks the second beam. # e trolley’s speed is calculated from the time interval and the distance between the light gates.

Using one light gate# e timer in Figure 1.5 starts when the leading edge of the card breaks the light beam. It stops when the trailing edge passes through. In this case, the time shown is the time taken for the trolley to travel a distance equal to the length of the card. # e computer software can calculate the speed directly by dividing the distance by the time taken.


Figure 1.4 Using two light gates to fi nd the average speed of a trolley.

stop

timer

lightgates

start

Figure 1.5 Using a single light gate to fi nd the average speed of a trolley.

stopstart

lightgate

timer

Using a ticker-timer# e ticker-timer (Figure 1.6) marks dots on the tape at regular intervals, usually 1

50 s (i.e. 0.02 s). (# is is because it works with alternating current, and in most countries the frequency of the alternating mains

Figure 1.6 Using a ticker-timer to investigate the motion of a trolley.

ticker-timer

power supply

0 1 2 3 4 5

starttrolley


Distance and displacement, Distance and displacement, scalar and vectorscalar and vectorIn physics, we are often concerned with the distance moved by an object in a particular direction. # is is called its displacement. Figure 1.9 illustrates the di" erence between distance and displacement. It shows the route followed by walkers as they went from town A to town C. # eir winding route took them through town B, so that they covered a total distance of 15 km. However, their displacement was much less than this. # eir fi nishing position was just 10 km from where they started. To give a complete statement of their displacement, we need to give both distance and direction:

displacement = 10 km 30° E of N

Displacement is an example of a vector quantity. A vector quantity has both magnitude (size) and direction. Distance, on the other hand, is a scalar quantity. Scalar quantities have magnitude only.

is 50 Hz.) # e pattern of dots acts as a record of the trolley’s movement.

Start by inspecting the tape. # is will give you a description of the trolley’s movement. Identify the start of the tape. # en look at the spacing of the dots:

even spacing – constant speed • increasing spacing – increasing speed.•

Now you can make some measurements. Measure the distance of every fi fth dot from the start of the tape. # is will give you the trolley’s distance at intervals of 0.1 s. Put the measurements in a table. Now you can draw a distance–time graph.

Using a motion sensor# e motion sensor (Figure 1.7) transmits regular pulses of ultrasound at the trolley. It detects the refl ected waves and determines the time they took for the trip to the trolley and back. From this, the computer can deduce the distance to the trolley from the motion sensor. It can generate a distance–time graph. You can determine the speed of the trolley from this graph.

4 A trolley with a 5.0 cm long card passed through a single light gate. # e time recorded by a digital timer was 0.40 s. What was the average speed of the trolley in m s!1?

5 Figure 1.8 shows two ticker-tapes. Describe the motion of the trolleys which produced them.

start

a

b

Figure 1.8 Ticker-tapes; for Test yourself Q 5.

6 Four methods for determining the speed of a moving trolley have been described. Each could be adapted to investigate the motion of a falling mass. Choose two methods which you think would be suitable, and write a paragraph for each to say how you would adapt it for this purpose.

Test yourself

Choosing the best methodEach of these methods for fi nding the speed of a trolley has its merits. In choosing a method, you might think about the following points.

Does the method give an average value of speed or can • it be used to give the speed of the trolley at di" erent points along its journey?How precisely does the method measure time – to the • nearest millisecond? How simple and convenient is the method to set up in • the laboratory?

Figure 1.7 Using a motion sensor to investigate the motion of a trolley.

motionsensor

trolley

computer


Speed and velocitySpeed and velocityIt is often important to know both the speed of an object and the direction in which it is moving. Speed and direction are combined in another quantity, called velocity. ! e velocity of an object can be thought of as its speed in a particular direction. So, like displacement, velocity is a vector quantity. Speed is the corresponding scalar quantity, because it does not have a direction. So, to give the velocity of something, we have to state the direction in which it is moving. For example, an aircraft fl ies with a velocity of 300 m s"1 due north. Since velocity is a vector quantity, it is defi ned in terms of displacement:

change in displacementvelocity = time taken

Alternatively, we can say that velocity is the rate of change of an object’s displacement. From now on, you need to be clear about the distinction between velocity and speed, and between displacement and distance. Table 1.2 shows the standard symbols and units for these quantities.

Speed and velocity calculationsSpeed and velocity calculationsWe can write the equation for velocity in symbols:

= sv t

= sv t!!

! e word equation for velocity is:

change in displacement=velocitytime taken

Note that we are using !s to mean ‘change in displacement s’. ! e symbol !, Greek letter delta, means ‘change in’. It does not represent a quantity (in the way that s does); it is simply a convenient way of representing a change in a quantity. Another way to write !s would be s2 " s1, but this is more time-consuming and less clear.

! e equation for velocity, = sv t!! , can be rearranged

as follows, depending on which quantity we want to determine:

change in displacement !s = v % !t

Figure 1.9 If you go on a long walk, the distance you travel will be greater than your displacement. In this example, the walkers travel a distance of 15 km, but their displacement is only 10 km, because this is the distance from the start to the fi nish of their walk.

CB

A

7 km

8 km

10 km

Quantity Symbol for quantity Symbol for unit

distance d mdisplacement s, x mtime t sspeed, velocity v m s"1

Table 1.2 Standard symbols and units. (Take care not to confuse italic s for displacement with s for seconds. Notice also that v is used for both speed and velocity.)

7 Which of these gives speed, velocity, distance or displacement? (Look back at the defi nitions of these quantities.)a ! e ship sailed south-west for 200 miles.b I averaged 7 mph during the marathon.c ! e snail crawled at 2 mm s"1 along the

straight edge of a bench.d ! e sales representative’s round trip was 420 km.

Test yourself


change in time = st v!!

Note that each of these equations is balanced in terms of units. For example, consider the equation for displacement. ! e units on the right-hand side are m s"1 % s, which simplifi es to m, the correct unit for displacement.

Note also that we can, of course, use the same equations to fi nd speed and distance, that is:

=speed dv t

distance d = v % t

=time dt v

Step 1 Start by writing what you know. Take care with units; it is best to work in m and s. You need to be able to express numbers in scientifi c notation (using powers of 10) and to work with these on your calculator.

v = 3.0 % 108 m s"1

x = 150 000 000 km = 150 000 000 000 m = 1.5 % 1011 m

Step 2 Substitute the values in the equation for time:

11

8

%1.5 10= = = 500s

%3.0 10xt v

Light takes 500 s (about 8.3 minutes) to travel from the Sun to the Earth.

In examinations, many candidates lose marks because of problems with calculators. Always use your own calculator. To calculate the time t, you press the buttons in the following sequence:

[1.5] [EXP] [11] [!] [3] [EXP] [8]

or

[1.5] [%10 n] [11] [!] [3] [%10 n] [8]

1 A car is travelling at 15 m s"1. How far will it travel in 1 hour?

Step 1 It is helpful to start by writing down what you know and what you want to know:

v = 15 m s"1

t = 1 h = 3600 s d = ?

Step 2 Choose the appropriate version of the equation and substitute in the values. Remember to include the units:

d = v % t = 15 % 3600 = 5.4 % 104 m = 54 km

! e car will travel 54 km in 1 hour.

2 ! e Earth orbits the Sun at a distance of 150 000 000 km. How long does it take light from the Sun to reach the Earth? (Speed of light in space = 3.0 % 108 m s"1.)

Worked examples

continued

Making the most of unitsMaking the most of unitsIn Worked example 1 and Worked example 2, units have been omitted in intermediate steps in the calculations. However, at times it can be helpful to include units as this can be a way of checking that you have used the correct equation; for example, that you have not divided one quantity by another when you should have multiplied them. ! e units of an equation must be balanced, just as the numerical values on each side of the equation must be equal.

If you take care with units, you should be able to carry out calculations in non-SI units, such as kilometres per hour, without having to convert to metres and seconds.


8 A submarine uses sonar to measure the depth of water below it. Refl ected sound waves are detected 0.40 s after they are transmitted. How deep is the water? (Speed of sound in water = 1500 m s!1.)

9 " e Earth takes one year to orbit the Sun at a distance of 1.5 # 1011 m. Calculate its speed. Explain why this is its average speed and not its velocity.

Test yourself

For example, how far does a spacecraft travelling at 40 000 km h!1 travel in one day? Since there are 24 hours in one day, we have:

distance travelled = 40 000 km h!1 # 24 h = 960 000 km

Displacement–time graphsDisplacement–time graphsWe can represent the changing position of a moving object by drawing a displacement–time graph. " e gradient (slope) of the graph is equal to its velocity (Figure 1.10). " e steeper the slope, the greater the velocity. A graph like this can also tell us if an object is moving forwards or backwards. If the gradient is negative, the object’s velocity is negative – it is moving backwards.

Figure 1.10 The slope of a displacement–time (s–t) graph tells us how fast an object is moving.

s

t0

0

s

t

low v

high v

00

s

t0

0

s

t0

0 T

s

t0

0

The straight line shows that theobject’s velocity is constant.

The slope of this graph is 0.The displacement s is not changing.Hence the velocity v = 0.The object is stationary.

The slope of this graph suddenlybecomes negative. The object ismoving back the way it came.Its velocity v is negative after time T.

This displacement – time graph iscurved. The slope is changing.This means that the object’s velocity is changing – this is considered in Chapter 2.

The slope shows which object ismoving faster. The steeperthe slope, the greater the velocity.

11 Sketch a displacement–time graph to show your motion for the following event. You are walking at a constant speed across a fi eld after jumping o$ a gate. Suddenly you see a bull and stop. Your friend says there’s no danger, so you walk on at a reduced constant speed. " e bull bellows, and you run back to the gate. Explain how each section of the walk relates to a section of your graph.

10 " e displacement–time sketch graph in Figure 1.11 represents the journey of a bus. What does the graph tell you about the journey?

s

t0

0

Figure 1.11 For Test yourself Q 10.

Test yourself

continued


Deducing velocity from a Deducing velocity from a displacement–time graphdisplacement–time graphA toy car moves along a straight track. Its displacement at di# erent times is shown in Table 1.3. ! is data can be used to draw a displacement–time graph from which we can deduce the car’s velocity.

Displacement / m 1.0 3.0 5.0 7.0 7.0 7.0Time / s 0.0 1.0 2.0 3.0 4.0 5.0

Table 1.3 Displacement (s) and time (t) data for a toy car.

It is useful to look at the data fi rst, to see the pattern of the car’s movement. In this case, the displacement increases steadily at fi rst, but after 3.0 s it becomes constant. In other words, initially the car is moving at a steady velocity, but then it stops.

Now we can plot the displacement–time graph (Figure 1.12).

change in displacement=velocity

change in timev

= s!

!t

(7.0 1.0) 6.0= =(3.0 0) 3.0

–

– = 2.0 m s"1

If you are used to fi nding the gradient of a graph, you may be able to reduce the number of steps in this calculation.

12 Table 1.4 shows the displacement of a racing car at di# erent times as it travels along a straight track during a speed trial.a Determine the car’s velocity.b Draw a displacement–time graph and use it

to fi nd the car’s velocity.

Displacement / m 0 85 170 255 340

Time / s 0 1.0 2.0 3.0 4.0

Table 1.4 Displacement (s) and time (t) data for Test yourself Q 12.

13 A veteran car travels due south. ! e distance it travels at hourly intervals is shown in Table 1.5.a Draw a distance–time graph to represent the

car’s journey.b From the graph, deduce the car’s speed in km h"1

during the fi rst three hours of the journey.c What is the car’s average speed in km h"1

during the whole journey?

Time / h Distance / km0 01 232 463 694 84

Table 1.5 Data for Test yourself Q 13.

Test yourself

Figure 1.12 Displacement–time graph for a toy car; data as shown in Table 1.3.

s /m8

6

4

2

01 2 3 4 50

!s

gradient = velocity

!t

We want to work out the velocity of the car over the fi rst 3.0 seconds. We can do this by working out the gradient of the graph, because:

velocity = gradient of displacement"time graph

We draw a right-angled triangle as shown. To fi nd the car’s velocity, we divide the change in displacement by the change in time. ! ese are given by the two sides of the triangle labelled !s and !t.


Combining displacementsCombining displacements! e walkers shown in Figure 1.13 are crossing di$ cult ground. ! ey navigate from one prominent point to the next, travelling in a series of straight lines. From the map, they can work out the distance that they travel and their displacement from their starting point.

distance travelled = 25 km

(Lay thread along route on map; measure thread against map scale.)

displacement = 15 km north-east

(Join starting and fi nishing points with straight line; measure line against scale.)

3 A spider runs along two sides of a table (Figure 1.14). Calculate its fi nal displacement.

north

east

A B

O

1.2 m

0.8 m

"

"

Figure 1.14 The spider runs a distance of 2.0 m, but what is its displacement?

Step 1 Because the two sections of the spider’s run (OA and AB) are at right angles, we can add the two displacements using Pythagoras’s theorem:

OB2 = OA2 + AB2

= 0.82 + 1.22 = 2.08

OB = 2.08 = 1.44 m # 1.4 m

Step 2 Displacement is a vector. We have found the magnitude of this vector, but now we have to fi nd its direction. ! e angle " is given by:

tan " = opp 0.8=adj 1.2

= 0.667

" = tan"1 (0.667) = 33.7° # 34°

So the spider’s displacement is 1.4 m at an angle of 34° north of east.

4 An aircraft fl ies 30 km due east and then 50 km north-east (Figure 1.15). Calculate the fi nal displacement of the aircraft.

Worked examples

continued

A map is a scale drawing. You can fi nd your displacement by measuring the map. But how can you calculate your displacement? You need to use ideas from geometry and trigonometry. Worked examples 3 and 4 show how.

! is process of adding two displacements together (or two or more of any type of vector) is known as vector addition. When two or more vectors are added together, their combined e# ect is known as the resultant of the vectors.

Figure 1.13 In rough terrain, walkers head straight for a prominent landmark.

START

FINISH

1 2 3 4 5km

river

ridge bridge

valley

cairn


N

E

45°

Figure 1.15 What is the aircraft’s fi nal displacement?

Here, the two displacements are not at 90° to one another, so we can’t use Pythagoras’s theorem. We can solve this problem by making a scale drawing, and measuring the fi nal displacement. (However, you could solve the same problem using trigonometry.)

Step 1 Choose a suitable scale. Your diagram should be reasonably large; in this case, a scale of 1 cm to represent 5 km is reasonable.

Step 2 Draw a line to represent the fi rst vector. North is at the top of the page. ! e line is 6 cm long, towards the east (right).

Step 3 Draw a line to represent the second vector, starting at the end of the fi rst vector. ! e line is 10 cm long, and at an angle of 45° (Figure 1.16).

final di

splace

ment

30 km

45°

50 km1 cm

1 cm

Figure 1.16 Scale drawing for Worked example 4. Using graph paper can help you to show the vectors in the correct directions.

Step 4 To fi nd the fi nal displacement, join the start to the fi nish. You have created a

vector triangle. Measure this displacement vector, and use the scale to convert back to kilometres:

length of vector = 14.8 cm

fi nal displacement = 14.8 % 5 = 74 km

Step 5 Measure the angle of the fi nal displacement vector:

angle = 28° N of E

! erefore the aircraft’s fi nal displacement is 74 km at 28° north of east.

continued

14 You walk 3.0 km due north, and then 4.0 km due east.a Calculate the total distance in km you have

travelled.b Make a scale drawing of your walk, and use it

to fi nd your fi nal displacement. Remember to give both the magnitude and the direction.

c Check your answer to part b by calculating your displacement.

15 A student walks 8.0 km south-east and then 12 km due west.a Draw a vector diagram showing the route. Use

your diagram to fi nd the total displacement. Remember to give the scale on your diagram

and to give the direction as well as the magnitude of your answer.

b Calculate the resultant displacement. Show your working clearly.

Test yourself

Combining velocitiesCombining velocitiesVelocity is a vector quantity and so two velocities can be combined by vector addition in the same way that we have seen for two or more displacements.


Imagine that you are attempting to swim across a river. You want to swim directly across to the opposite bank, but the current moves you sideways at the same time as you are swimming forwards. ! e outcome is that you will end up on the opposite bank, but downstream of your intended landing point. In e# ect, you have two velocities:

the velocity due to your swimming, which is directed • straight across the river;the velocity due to the current, which is directed • downstream, at right angles to your swimming velocity.

! ese combine to give a resultant (or net) velocity, which will be diagonally downstream. In order to swim directly across the river, you would have to aim upstream. ! en your resultant velocity could be directly across the river.

Step 2 Now sketch a vector triangle. Remember that the second vector starts where the fi rst one ends. ! is is shown in Figure 1.17b.

Step 3 Join the start and end points to complete the triangle.

Step 4 Calculate the magnitude of the resultant vector v (the hypotenuse of the right-angled triangle).

v2 = 2002 + 502 = 40 000 + 2500 = 42 500

v = 42500 = 206 m s"1

Step 5 Calculate the angle ":

tan " = 50200

= 0.25

" = tan"1 0.25 = 14°

So the aircraft’s resultant velocity is 206 m s"1 at 14° east of north.

5 An aircraft is fl ying due north with a velocity of 200 m s"1. A side wind of velocity 50 m s"1 is blowing due east. What is the aircraft’s resultant velocity (give the magnitude and direction)?

Here, the two velocities are at 90°. A sketch diagram and Pythagoras’s theorem will su$ ce to solve the problem.

Step 1 Draw a sketch of the situation – this is shown in Figure 1.17a.

v

200ms–1 200ms–1

50ms–1

50ms–1a b

Not toscale

"

Figure 1.17 Finding the resultant of two velocities – for Worked example 5.

Worked example

continued

16 A swimmer can swim at 2.0 m s"1 in still water. She aims to swim directly across a river which is fl owing at 0.80 m s"1. Calculate her resultant velocity. (You must give both the magnitude and the direction.)

17 A stone is thrown from a cli# and strikes the surface of the sea with a vertical velocity of 18 m s"1 and a horizontal velocity v. ! e resultant of these two velocities is 25 m s"1.a Draw a vector diagram showing the two

velocities and the resultant.b Use your diagram to fi nd the value of v.c Use your diagram to fi nd the angle between

the stone and the vertical as it strikes the water.

Test yourself


SummarySummary Displacement is the distance travelled in a particular direction. Velocity is defi ned by the word equation

velocity = displacementtime taken

The gradient of a displacement–time graph is equal to velocity:

v = !!st

Distance and speed are scalar quantities. A scalar quantity has only magnitude. Displacement and velocity are vector quantities. A vector quantity has both magnitude and direction. Vector quantities may be combined by vector addition to fi nd their resultant.

End-of-chapter questionsEnd-of-chapter questions1 A car travels one complete lap around a circular track at a constant speed of 120 km h"1.

a If one lap takes 2.0 minutes, show that the length of the track is 4.0 km.b Explain why values for the average speed and average velocity are di# erent.c Determine the magnitude of the displacement of the car in a time of 1.0 minute. (! e circumference of a circle = 2&R where R is the radius of the circle.)

2 A boat leaves point A and travels in a straight line to point B (Figure 1.18). ! e journey takes 60 s.

B

600 m

800 m

A

Figure 1.18 For End-of-chapter Q 2.

Calculate:a the distance travelled by the boatb the total displacement of the boatc the average velocity of the boat.Remember that each vector quantity must be given a direction as well as a magnitude.


3 A boat travels at 2.0 m s"1 east towards a port, 2.2 km away. When the boat reaches the port, the passengers travel in a car due north for 15 minutes at 60 km h"1.

Calculate:a the total distance travelledb the total displacementc the total time takend the average speed in m s"1

e the magnitude of the average velocity.

4 A river fl ows from west to east with a constant velocity of 1.0 m s"1. A boat leaves the south bank heading due north at 2.40 m s"1. Find the resultant velocity of the boat.

Exam-style questionsExam-style questions1 a Defi ne displacement. [1]

b Use the defi nition of displacement to explain how it is possible for an athlete to run round a track yet have no displacement. [2]

2 A girl is riding a bicycle at a constant velocity of 3.0 m s"1 along a straight road. At time t = 0, she passes a boy sitting on a stationary bicycle. At time t = 0, the boy sets o# to catch up with the girl. His velocity increases from time t = 0 until t = 5.0 s when he has covered a distance of 10 m. He then continues at a constant velocity of 4.0 m s"1.a Draw the displacement–time graph for the girl from t = 0 to 12 s. [1]b On the same graph axes, draw the displacement–time graph for the boy. [2]c Using your graph, determine the value of t when the boy catches up with the girl. [1]

3 A student drops a small black sphere alongside a vertical scale marked in centimetres. A number of fl ash photographs of the sphere are taken at 0.1 s intervals, as shown in the diagram. ! e fi rst photograph is taken with the sphere at the top at time t = 0.

20

30

40

50

60

70

80

90

100

110

120

130

10

cm


a Explain how the diagram shows that the sphere reaches a constant speed. [2]b Determine the constant speed reached by the sphere. [2]c Determine the distance that the sphere has fallen when t = 0.8 s. [2]

4 a State one di# erence between a scalar quantity and a vector quantity and give an example of each. [3]

b A plane has an air speed of 500 km h"1 due north. A wind blows at 100 km h"1 from east to west. Draw a vector diagram to calculate the resultant velocity of the plane. Give the direction of travel of the plane with respect to north. [4]

c ! e plane fl ies for 15 minutes. Calculate the displacement of the plane in this time. [1]

5 A small aircraft for one person is used on a short horizontal fl ight. On its journey from A to B, the resultant velocity of the aircraft is 15 m s"1 in a direction 60° east of north and the wind velocity is 7.5 m s"1 due north.

60°

A E

B

N

a Show that for the aircraft to travel from A to B it should be pointed due east. [2]b After fl ying 5 km from A to B, the aircraft returns along the same path from B to A

with a resultant velocity of 13.5 m s"1. Assuming that the time spent at B is negligible, calculate the average speed for the complete

journey from A to B and back to A. [3]

Documents

Cambridge International AS and A Level Physics: Coursebook with CD-ROM