EDEXCEL IGCSE PHYSICS 1-2

Forces & ShapeEdexcel IGCSE Physics pages 12 to 22

July 13th 2011

Content applying to Triple Science only is shown in red type on the next slide and is indicated on

subsequent slides by ‘TRIPLE ONLY’

Edexcel IGCSE SpecificationSection 1: Forces and motionc) Forces, movement, shape and momentumexpress a force as a push or pull of one body on anotheridentify various types of force (for example gravitational, electrostatic etc)distinguish between vector and scalar quantitiesappreciate the vector nature of a forceadd forces that act along a lineunderstand that friction is a force that opposes motiondescribe how extension varies with applied force for helical springs, metal wires and rubber bandsrecall that the initial linear region of a force-extension graph is associated with Hooke’s lawassociate elastic behaviour with the ability of a material to recover its original shape after the forces causing deformation have been removed.

Red type: Triple Science Only

ForceA force is a push or a pull.A force can cause an object to:

– speed up– slow down– change direction– change shape

Force is measured in newtons (N).Force is measured with a newtonmeter.

newtonmeters

Some types of force1. GravitationalThis is the attractive force exerted between bodies because of their masses.

This force increases if either or both of the masses is increased and decreases if they are moved further apart.

Weight is the gravitational force of the Earth on an object.

Bathroom scales measure weight.

A mass of 1kg weighs about 10N

1 stone is about 63N.

2. Normal reaction or contactThis is the repulsive force that stops two touching bodies moving into each other.

The word ’normal’ means that this force acts at 90° to the surfaces of the bodies.

It is caused by repulsive molecular forces.

normal reaction forces

weight

The two upward reaction forces on the tyres balance the

downward weight of the car

3. FrictionThis is the force that opposes motion.The kinetic energy of the moving object is converted to heat energy by the force of friction.

Friction is needed for racing cars to grip the road

Friction is needed for walking!

4. Air resistance or dragThis is the force that opposes the movement of objects through air.Drag is a more general term used for the opposition force in any gas or liquid.Objects are often streamlined to reduce this force.

streamlined cara parachute

maximises drag force

5. UpthrustThis is the force experienced by objects when they are placed into a fluid (liquid or gas).

An object will float on a liquid if the upthrust force equals its weight.

A hot air balloon rises when the upthrust from the surrounding air is greater than the balloon’s

weight.

6. MagneticBetween magnets but also the force that allows electric motors to work.

7. ElectrostaticAttractive and repulsive forces due to bodies being charged.

Electrostatic force causes the girls’ hair to rise when they

touch the Van der Graaff generator.

Choose appropriate words to fill in the gaps below:

A _____ is a push or a pull. A force can cause an object to ___________ or change shape.

Force is measured in _______ (N) with a newtonmeter.

There are many types of force. ________ force occurs when two bodies touch each other.

Friction is a force that _______ the _______ of one body relative to another. It is caused by the _________ forces between ___________.

attractive

motioncontact

newtons

opposes

forceaccelerate

WORD SELECTION:

molecules

attractive

motion

contact

newtons

opposes

force

accelerate

molecules

Vectors and Scalars

All physical quantities (e.g. speed and force) are described by a magnitude and a unit.

VECTORS – also need to have their direction specified

examples: displacement, velocity, acceleration, force.

SCALARS – do not have a direction

examples: distance, speed, mass, work, energy.

TRIPLE ONLY

Representing Vectors

An arrowed straight line is used.

The arrow indicates the direction and the length of the line is proportional to the magnitude.

Displacement 50m EAST

Displacement 25m at 45o North of East

TRIPLE ONLY

Addition of vectors 1

The original vectors are called COMPONENT vectors.

The final overall vector is called the RESULTANT vector.

4N

6Nobject

4N6N

object

resultant = 10N

object

4N 6N

object

4N6N

object

resultant = 2N

object

TRIPLE ONLY

Resultant forceA number of forces acting on a body may be replaced by a single force which has the same effect on the body as the original forces all acting together.

This overall force is called resultant force.

In the example opposite, 5N is the resultant force of the 3N and 2N forces.

3N2N

5N

TRIPLE ONLY

Determine the resultant force in the cases below:

4N

6N1.

4N3N2.

7N

3N3.

2N

6N4.

4N

4N5.

10N 1N

4N

4N

There is no resultant force in this case

TRIPLE ONLY

Resultant force and motion

Resultant force Effect on the motion of an object

Zero Object’s velocity stays the same including staying stationary

In the direction the object is moving

Object accelerates

In the opposite direction in which the object is

moving

Object decelerates

TRIPLE ONLY

Examples 1 & 2

The box will move when the man’s push force is greater than the friction force.

The plane will accelerate provided that the engine force is greater than the drag force.

TRIPLE ONLY

Examples 3 & 4

The brakes exert a resultant force in the opposite direction to the car’s motion causing the car to decelerate.

Once released, the glider moves at a near constant velocity as it experiences a nearly zero horizontal resultant force.

TRIPLE ONLY

Choose appropriate words to fill in the gaps below:

A single force, called _________ force, can be used to replace a _______ of forces that act on a body.

If the resultant force is _____ then the body will either remain at _____ or continue to move at a constant ________.

If the resultant force is in the same _________ as an object’s motion, the object will __________. A car is decelerated when the braking force acts in the _________ direction to the car’s motion.

acceleratevelocity

rest direction

resultant

number zero

WORD SELECTION:

opposite

accelerate

velocityrest

direction

resultant

number

zero

opposite

TRIPLE ONLY

Changing shapeForce can also change the shape of an object.

A stretching force puts an object such as a wire or spring under tension.

A squashing force puts an object under compression.

Brittle materials such as glass do not change shape easily and break before noticeably stretching.

Resilient materials do not break easily.

Elastic materials return to their original shape when the forces on them are removed.

Plastic materials retain their new shape.

Stretching SpringsExperimental procedure:1. Place the weight holder only on the spring and note the position of the pin against the metre rule.2. Add 1N (100g) to the holder and note the new position of the pin.3. Calculate the extension of the spring.4. Repeat stages 1 to 3 for 2N, 3N, 4N, 5N and 6N. DO NOT EXCEED 6N. metre rule

pin

spring

weights

Typical resultsPin position with holder only (mm)

Added weight or Force (N)

Pin position with weight

(mm)

Extension (mm)

450 1 480 30

450 2 509 59

450 3 541 91

450 4 570 120

450 5 601 151

450 6 629 179

Force against extension graph

0

Fo

rce

(N

)

0Extension (mm)

Hooke’s lawHooke’s law states that the extension of a spring force is proportional to the force used to stretch the spring.

‘Proportional’ means that if the force is doubled then the extension also doubles.

The line on a graph of force against extension will be a straight AND go through the origin.

QuestionA spring of original length 150mm is extended by 30mm by a force of 4N. Calculate the length of the spring if a force of 12N is applied.

12N is three times 4N

Therefore the new extension should be 3 x 30mm

= 90mm

New spring length = 150mm + 90mm

= 240mm

Elastic limitUp to a certain extension if the force is removed the spring will return to its original length. The spring is behaving elastically.

If this critical extension is exceeded, known as the elastic limit, the spring will be permanently stretched.

Hooke’s law is no longer obeyed by the spring if its elastic limit is exceeded.

The right hand spring has been

stretched beyond its elastic limit

Force against extension graph if the elastic limit is exceeded

0

Fo

rce

(N

)

0Extension (mm)

elastic limit

Stretching an elastic band

0

Fo

rce

0Extension

An elastic band does not obey Hooke’s law.

Choose appropriate words to fill in the gaps below:

Hooke’s law states that when a wire or spring is _________ the increase in length or _________ is proportional to the load ______ applied.

This law is not obeyed if the spring is taken beyond its ______ limit after which it will become _____________ stretched.

A ________ band does not obey Hooke’s law.

A graph illustrating Hooke’s law will have a line that is ___________ and passes through the _______.

straight

elastic

origin

stretched

rubberforce

extension

WORD SELECTION:

permanently

straight

elastic

origin

stretched

rubber

force

extension

permanently

Online SimulationsEffect of forces on motion using a space module - Freezeray.com Force combination balloon game - eChalk Electric & Magnetic Forces - 'Whys Guy' Video Clip (3:30mins) - Shows Charged Balloon & Effect of a magnet on a TV screen. Resultant of two forces - Fendt Forces on objects immersed in liquids - NTNU BBC KS3 Bitesize Revision: What is a force Balanced forces Unbalanced forces BBC AQA GCSE Bitesize Revision: Resultant force Types of forces

Vector Addition - PhET - Learn how to add vectors. Drag vectors onto a graph, change their length and angle, and sum them together. The magnitude, angle, and components of each vector can be displayed in several formats.

Representing vectors - eChalk

Vectors & Scalars - eChalk

Vector addition - eChalk

Vector Chains - eChalk

Fifty-Fifty Game on Vectors & Scalars - by KT - Microsoft WORD

Vector addition - Explore Science

Stretching Springs - PhET - A realistic mass and spring laboratory. Hang masses from springs and adjust the spring stiffness and damping. You can even slow time. Transport the lab to different planets. A chart shows the kinetic, potential, and thermal energy for each spring.

Forces & ShapeNotes questions from pages 4 and 12 to 22

1. (a) What is force? (b) Explain the meaning of the following types of force: gravitational, normal reaction, drag, electrostatic and friction. (see pages 12 to 17)

2. Explain the difference between vectors and scalars quantities and give two examples of each. (see pages 4 and 13)

3. State what is meant by Hooke’s law and explain how a graph can be drawn to verify that a spring obeys this law.

4. What is meant by ‘elastic limit’?5. Sketch a graph showing how the loading force varies with

extension when extending an elastic band.

6. Answer the questions on pages 21 & 22.7. Verify that you can do all of the items listed in the end of chapter

checklist on page 21

TRIPLE ONLY

Forces & ShapeNotes questions from pages 12 to 22

1. (a) What is force? (b) Explain the meaning of the following types of force: gravitational, normal reaction, drag, electrostatic and friction. (see pages 12 to 17)

2. State what is meant by Hooke’s law and explain how a graph can be drawn to verify that a spring obeys this law.

3. What is meant by ‘elastic limit’?4. Sketch a graph showing how the loading force varies

with extension when extending an elastic band.

5. Answer questions 1, 2, 6 and 9 on pages 21 & 22.

DOUBLE ONLY