CIE IGCSE PHYSICS
Forces
Physics for IGCSE Physics pages 36-41
July 13th 2011
CIE IGCSE SpecificationSection 1: Effects of Force •State that a force may produce a change in size and shape of a body.
•Describe the ways in which a force may change the motion of a body.
•Find the resultant of two or more forces acting along the same line.
•Demonstrate an understanding that mass is a property which ‘resists’ change in motion. Inertia
•Recall and use the relation between force, mass and acceleration (including the direction).
•Describe, and use the concept of, weight as the effect of a gravitational field on a mass.
Vectors and Scalers
•Demonstrate an understanding of the difference between scalars and vectors and give common examples
•Add vectors by graphical representation to determine a resultant
•Determine graphically the resultant of two vectors
Homework due Wednesday 19th October
• To write in your books 100 words each on the lives of Galileo and Newton 200 word total
Include the following
• When he was born• When he died • Where he lived
• What three things was he known for
• One interesting fact
Homework Markscheme• To write in your books 100 words each on the lives
of Galileo and Newton 200 word total • When he was born – 1 mark B• When he died – 1 mark D• Where he lived - 1 mark L
• What three things was he known for – I mark for each mentioned K1, K2, K3
• One interesting fact - 1 mark I• 100 handwritten words – 1 mark 100• Quality 1 or 2 marks ( Plagiarism 1 mark max)
ForceA force is a push or a pull, it is a vector it has magnitude and direction.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
19/04/23
What is a force?What is a force?A force is a “push” or a “pull” or a “twist”.
Q. How many different forces do you know?
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Learning today Learning today Newton 1
Balanced Forces
What inertia is
Newton 2
Unbalanced Forces cause a body to speed up slow down or change direction.
Also given as
F=ma Unit N
Gravity Mass and Weight
W=mg Unit Newton – N g= 10 m/s2
Newton 3 For every action there is an opposite and equal reaction
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.
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What is a force?What is a force?A force is a “push” or a “pull”. Some common examples:
________ – pulls things downwards
___ ________ (drag) – acts against anything moving through air
______ – keeps things afloat_____ – acts against anything moving
Words – upthrust, air resistance, friction, weight
19/04/23Balanced and unbalanced Balanced and unbalanced forcesforces
What would happen if we took the road away?
Weight
Reaction
19/04/23Balanced and unbalanced Balanced and unbalanced forcesforces
What would happen if we took the road away?
The camel’s weight is no longer balanced by anything, so the camel falls downwards…
Weight
19/04/23
What would happen if we took the road away?
The camel’s weight is no longer balanced by anything, so the camel falls downwards…
Balanced and unbalanced Balanced and unbalanced forcesforces
Balanced forces- Newton 1Balanced forces- Newton 1stst 19/04/23
When forces are balanced, it means that:If an object is stationary, it will continue to be stationary.
If you are sitting on a chair, your weight is balanced by an upwards force from the chair.
If an object is moving, it will move at constant velocity.
If you are pedalling a bicycle at a constant speed, the force you put into the pedals is balanced by the air resistance and friction.
InertiaInertia19/04/23
When is at rest it takes a force to make it move.
All objects resist a change in velocity, this is
called inertia.
Mass and paper demonstration
19/04/23
Learning today Learning today Newton 2
Unbalanced Forces cause a body to speed up slow down or change direction.
Also given as
F=ma
What a resultant force is
Gravity Mass and Weight
W=mg Unit Newton – N g= 10 m/s2
Newton 3 For every action there is an opposite and equal reaction
19/04/23Balanced and unbalanced Balanced and unbalanced forcesforces
1) This animal is either ________ or moving with _____ _____…
4) This animal is…
2) This animal is getting _________…
3) This animal is getting _______….
19/04/23Balanced and unbalanced Balanced and unbalanced forcesforces
Newton 2Newton 219/04/23
Unbalanced Forces cause a body to speed up slow down in the direction of the force or they change direction.
Also given as
F=ma
Resultant force is the overall sum of many different forces
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Force and accelerationForce and acceleration
If the forces acting on an object are unbalanced then the object will accelerate or change direction, like these wrestlers:
Force (in N) = Mass (in kg) x Acceleration (in m/s2)
F
AM
19/04/23Force, mass and Force, mass and accelerationacceleration
1) A force of 1000N is applied to push a mass of 500kg. How quickly does it accelerate?
2) A force of 3000N acts on a car to make it accelerate by 1.5m/s2. How heavy is the car?
3) A car accelerates at a rate of 5m/s2. If it weighs 500kg how much driving force is the engine applying?
4) A force of 10N is applied by a boy while lifting a 20kg mass. How much does it accelerate by?
F
AM
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
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
Mass and weightMass is the amount of matter in an object.Mass is measured in kilograms.The mass of an object is the same on the Moon as on the Earth.
Weight is the force of gravity on an object.Weight is measured in newtons.The weight of an object on the Moon is about one sixth that on the Earth.
A newtonmeter is used to determine the weight of the parcel
The acceleration due to gravity (g)The acceleration due to gravity (g) varies with planet, moon and star and depends on the height of an object.
Some examples of the acceleration due to gravity:
Location m/s2 Location m/s2
Earth 10 Jupiter 24
Moon 1.6 Pluto 0.7
Mars 3.7 The Sun 270
Gravitational field strength (g)This is an alternative way of measuring the strength of gravity.
The gravitational field strength is equal to the gravitational force exerted per kilogram.
Near the Earth’s surface, g = 10 N/kg
In most cases gravitational field strength in N/kg is numerically equal to the acceleration due to gravity in m/s2, hence they both use the same symbol ‘g’.
Calculating weight
weight = mass × gravitational acceleration
W = m x g
weight is measured in N
mass is measured in kg
gravitational acceleration is measured in m/s2
On the Earth’s surface a mass of 1kg
has a weight of 10N.
Falling objects
When an object falls through air or some other fluid initially the only significant force acting on it is the downward pull of gravity.
On Earth, it will initially accelerate downwards at 10 m/s2.
19/04/23
Learning today Learning today Terminal Velocity in a fluid
Forces acting – Weight and Drag
Newton 3 For every action there is an opposite and equal reaction
Explain using Newton 3 how a rockets produces a force
The difference between vectors and scalars
Yacapaca results
As the object speeds up frictional forces such as air resistance become greater the faster the object moves.
Eventually the resultant force on the object will be zero when the frictional forces equal the weight of the object.
The object then moves at a constant speed called terminal velocity.
Terminal VelocityVelocity-time graphs for a falling object in fluids
The forces are Weight and Drag
Newton 3Newton 3• For every action there is an equal
and opposite reaction.• Forces act in pairs Two bodies are involved so forces do not cancel
out
A person in a liftEarth and the moonTask 1Explain how a rocket produces a force.Task 2 Question 4
19/04/23
Vectors and Scalars
All physical quantities (e.g. speed and force) are described by a magnitude and a unit.
VECTORS – have magnitude, direction and a unit.
examples: displacement, velocity, acceleration, force.
SCALARS – have magnitude only
examples: distance, speed, mass, work, energy.
Scalar and Vectors Scalar and Vectors
Draw a T-table at the back of your books Scalar Vectors
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Scalar and Vectors Scalar and Vectors
Now draw a T-table in the front of your books. Scalar Vectors
19/04/23
ParachutingA parachutist will have two different terminal velocities.Before opening the parachute it is about 60 m/s (140 m.p.h..).
Afterwards, due the much greater drag force, the terminal velocity is about 5 m/s (12 m.p.h.)
velocity
time
first terminal velocity
initial acceleration
= 10 m/s2
Velocity-time graph of a parachutist
second terminal velocity
parachute opened
ground reached
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 – have magnitude and direction
examples: displacement, velocity, acceleration, force.
SCALARS – have magnitude only
examples: distance, speed, mass, work, energy.
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
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
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
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.
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.
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
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.