2. What is friction?
If you rub your hands together, they get warm because there is resistance to the rubbing motion.
What is the name of this resistive force?
It is called friction.
What causes this force?
Your hands might look smooth, but on a microscopic level they have rough surfaces, as shown in the photograph.
Photo credit: © Martin Dohrn / Science Photo Library
When you rub your hands together you feel the resistive force of friction.

3. Friction and motion
Friction always tries to slow moving objects down – it opposes motion. Frictional forces occur when two touching surfaces move past each other, in this case the box moving across the ground.
pull of rope on box
friction
Friction also occurs when things move through air. This is called air resistance.

4. What are the sources of friction?
Label all sources of friction acting on this bike.
pedal
bearing
air
resistance
links in
chain
brake pad
Photo credit: © 2008 Jupiterimages Corporation
tyre and
road
wheel
bearing

5. Sources of friction

6. Friction experiment

7. Cars stopping
Friction is a very important force for the movement of cars. It acts in the opposite direction to the movement of the car.
thrust from engine
friction
The time it takes for a car to brake is affected by the frictional forces between the car’s tyres and the road surface.

8. Factors affecting stopping distance
One of the most important sources of friction in cars is that between the tyres and the road.
When the car brakes, it’s important to have as much friction as possible so that the car does not skid.
The friction between the tyres and road is affected by the:
inflation pressure of the tyres
road surface
surface condition caused by the weather (rain, ice, etc).

9. Explaining air resistance
Air resistance is a type of friction that occurs whenever an object moves through the air, and is caused by the frictional forces acting between the air and the object.
400 N
If the area of contact between the air and object is reduced, the object is said to be streamlined.
300 N
Which of these cars is more streamlined?

10. Air resistance and drag
The photo shows a streamlined car which has been shaped so that the flow of air around the body is made as smooth as possible. This minimizes the air resistance.
If you looked at the car from the front, how much surface area would you see?
The air resistance is determined by:
the size of the car
the shape of the car
the speed of the car.
Worksheet 2 accompanies this slide.
In this worksheet, pupils must create the least streamlined car they can design.
Photo credit: © 2008 Jupiterimages Corporation

11. Falling objects on the moon

12. Effects of frictional forces

13. How does a plane take-off?

15. Forces in water What forces are acting on the fish? upthrust
Upthrust is a force that pushes objects upwards in a liquid or a gas. This balances the weight of the fish, and is also known as buoyancy.
thrust
friction
weight
In order to move forward, the fish uses its tail to produce thrust. This force drives it through the water and is always opposed by friction.

16. Weight and upthrust

17. Upthrust quiz

19. Forces – summary 51

20. Glossary
air resistance – A frictional force that acts against an object moving through air.
balanced forces – Opposing forces acting on an object that cancel each other out.
Earth’s gravity – The force of attraction that pulls objects towards the centre of the Earth.
force – A push or a pull, measured in newtons (N).
friction – A force that occurs between two surfaces rubbing against each other.
gravity – The force of attraction between any two objects that have mass.
mass – The amount of matter that an object is made of, measured in kilograms (kg).
newton – The unit used to measure force (N).
resultant force – A single force that has the same effect as all the forces acting on an object added together.
satellite – Any object that orbits another object.
streamlined – A smooth shape designed to reduce drag.
thrust – A force that pushes an object forwards.
unbalanced forces – Forces acting on an object that change its speed or direction.
upthrust – A force that pushes an object upwards.

21. Anagrams

22. Multiple-choice quiz