1. EDEXCEL IGCSE PHYSICS 1-2 Forces & Shape
Edexcel IGCSE Physics pages 12 to 22
Content applying to Triple Science only is shown in red type on the next slide and is indicated on subsequent slides by ‘TRIPLE ONLY’
July 13th 2011

2. Edexcel IGCSE Specification
Section 1: Forces and motion
c) Forces, movement, shape and momentum
express a force as a push or pull of one body on another
identify various types of force (for example gravitational, electrostatic etc)
distinguish between vector and scalar quantities
appreciate the vector nature of a force
add forces that act along a line
understand that friction is a force that opposes motion
describe how extension varies with applied force for helical springs, metal wires and rubber bands
recall that the initial linear region of a force-extension graph is associated with Hooke’s law
associate 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

3. Force newtonmeters A 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. 3

4. Some types of force 1. Gravitational
This 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. 4

5. 2. Normal reaction or contact
This 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 5

6. 3. Friction This 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 walking!
Friction is needed for racing cars to grip the road 6

7. a parachute maximises drag force
4. Air resistance or drag
This 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.
a parachute maximises drag force
streamlined car 7

8. 5. Upthrust
This 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. 8

9. 6. Magnetic 7. Electrostatic
Electrostatic force causes the girls’ hair to rise when they touch the Van der Graaff generator.
6. Magnetic
Between magnets but also the force that allows electric motors to work.
7. Electrostatic
Attractive and repulsive forces due to bodies being charged. 9

10. 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 ___________.
force
accelerate
newtons
contact
opposes
motion
attractive
molecules
WORD SELECTION:
newtons
accelerate
force
attractive
opposes
molecules
contact
motion 10

11. TRIPLE ONLY
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. 11

12. Representing Vectors An arrowed straight line is used.
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 12

13. Addition of vectors 1 TRIPLE ONLY 4N 6N object 4N 6N object
resultant = 10N
object
The original vectors are called COMPONENT vectors.
The final overall vector is called the RESULTANT vector. 4N 6N
object 4N 6N
object
resultant = 2N
object 13

14. TRIPLE ONLY
Resultant force
A 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. 3N 2N 5N 14

15. Determine the resultant force in the cases below:
TRIPLE ONLY
Determine the resultant force in the cases below: 4N 6N 1. 1N
10N 4N 3N 2. 4N 2N 6N 3. 4. 5. 4N 3N
There is no resultant force in this case 4N 7N 15

16. Resultant force and motion
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 16

17. 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. 17

18. 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. 18

19. Choose appropriate words to fill in the gaps below:
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.
resultant
number
zero
rest
velocity
direction
accelerate
opposite
WORD SELECTION:
number
rest
direction
zero
opposite
velocity
accelerate
resultant 19

20. Changing shape Force 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.

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

22. Elastic materials return to their original shape when the forces on them are removed.
Plastic materials retain their new shape.

23. Stretching Springs Experimental 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 23

24. Typical results
Pin position with holder only (mm)
Added weight or Force (N)
Pin position with weight (mm)
Extension (mm)
450 1
480 30 2
509 59 3
541 91 4
570
120 5
601
151 6
629
179 24

25. Force against extension graph
Force (N)
Extension (mm) 25

26. Hooke’s law
Hooke’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.

27. Question
A 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

28. The right hand spring has been stretched beyond its elastic limit
Up 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

29. Force against extension graph if the elastic limit is exceeded
Force (N)
Extension (mm)
elastic limit 29

30. Stretching an elastic band
Force
Extension
An elastic band does not obey Hooke’s law. 30

31. 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 _______.
stretched
extension
force
elastic
permanently
rubber
straight
origin
WORD SELECTION:
stretched
elastic
permanently
extension
origin
force
rubber
straight 31

32. Online Simulations
Effect 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.

33. Forces & Shape Notes questions from pages 4 and 12 to 22
TRIPLE ONLY
Forces & Shape Notes questions from pages 4 and 12 to 22
(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)
Explain the difference between vectors and scalars quantities and give two examples of each. (see pages 4 and 13)
State what is meant by Hooke’s law and explain how a graph can be drawn to verify that a spring obeys this law.
What is meant by ‘elastic limit’?
Sketch a graph showing how the loading force varies with extension when extending an elastic band.
Answer the questions on pages 21 & 22.
Verify that you can do all of the items listed in the end of chapter checklist on page 21

34. Forces & Shape Notes questions from pages 12 to 22
DOUBLE ONLY
Forces & Shape Notes questions from pages 12 to 22
(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)
State what is meant by Hooke’s law and explain how a graph can be drawn to verify that a spring obeys this law.
What is meant by ‘elastic limit’?
Sketch a graph showing how the loading force varies with extension when extending an elastic band.
Answer questions 1, 2, 6 and 9 on pages 21 & 22.