Friction

Things that are moving continue to do so unless a force makes them stop. The force that makes most moving objects stop is friction. Friction exists whenever two surfaces are in contact. Friction is the force that provides the grip needed by cars, bikes, trucks and your shoes to get moving, change direction and slow down. Friction can be a problem. It causes machines with moving parts to heat up, which wastes energy.

• Answers
  1. When you rub your hands together quickly you feel your hands getting hotter. When you rub the piece of wood with sandpaper the surface of the wood gets smoother.
  2. Both observations were the result of friction. Rubbing your hands together causes friction. The friction between sandpaper and wood rubs off the bumps on the wood making it soft.

What is friction?

If you roll a soccer ball along a patch of grass, then it will slow down and eventually stop. The force that slows it down is friction. Friction occurs whenever one object tries to move over another. Because it occurs between surfaces in contact, friction is called a contact force.

Friction acts in an opposite direction to motion, as the image below.

Two sheets of sandpaper will appear to ‘stick’ when you try to slide them over each other. This is because the grit on one sheet grabs and catches bumps on the other. This causes friction, and collisions between these bumps create heat.

All surfaces have bumps on them, even ‘smooth’ materials like glass or steel.

Some surfaces are so smooth that you will slip if you try and walk on them. An example is ice, people will slip because the friction is so low.

Factors affecting friction

Friction depends on:

• how rough the surfaces in contact are
• how hard the surfaces are pushed together

The greater the weight of a sliding object, the greater the force of friction.

Useful friction

When you walk, you push your foot backwards against the ground. The force of friction acts in the opposite direction to push you forwards. For this to happen, there must be enough traction, or grip, between your feet and the ground, otherwise you will slip. This is shown underneath Without friction you could not walk, run, ride a bike or travel in a car, and you could not pick things up.

Unwanted friction

You rely on friction in your daily life, but it also has some unwanted effects. Moving parts, such as those in a machine, are gradually worn away and made thinner by friction. Friction also produces heat. Much of the energy put into a machine, such as a car, an aircraft, a mixer or an electric knife, is converted into heat caused by friction. This makes the machine Jess efficient and wastes energy, because it is not being converted into the useful forms of energy required. The friction of moving parts in a car engine would quickly cause it to overheat if the car did not have a radiator. A car radiator releases excess heat to the air that passes through it as the car drives along.

Friction between an object and the air around it slows its motion. This type friction is called air resistance, or drag.

Let’s look at objects in a wind tunnel- a wind tunnel can test how well an object cuts through the air.

Reducing friction

Reducing friction makes machines more efficient, cheaper to run and longer lasting. Friction can be reduced by a number of different methods. Here are some examples:

Your answers

1. 1 – Sit on a chair

2 – Picking up anything

3 – Walking

2. a) F

b) T

c) F

3. a) Your foot and the ball

b) Your arms and the water

c) Your foot and the ground

d) Your hands and the car

4. Friction exists because all surfaces have bumps on them, and when those bumps rub together they cause friction.

5. The reason why you cant pick up an apple with friction is because, when you pick up and apple the apples bumps and your hand bumps stick so it doesn’t fall.

6. the reason why a car needs a radiator to work is because without a radiator the car would heat up and damage the car.

7. The reason why its harder to push a toy chest on the ground when its full is because when the chest is full there’s more friction in the box which makes it harder to push.

8. The reason why ball bearings are used for skateboard wheels is because its used to reduce friction.

9. 1 – You can take all the stuff out of the fridge

2 – You can put the fridge on a trolley

3 – You can tilt the fridge and put a cloth underneath it

10. The mountain bike is heaver because if its more heaver than there’s more friction

11. a) The ice would have less friction because its more smooth than the foot paths, and the footpath would have more bumps which means more friction.

b) The ice makes the ice-skater move differently because its has less friction.

c) The narrow blade is smaller so its has less friction.

14. a) 1 – The car and air

2 – The car’s wheels and the road

b) The direction its going in is left

c) The driver can maintain the speed by keeping the driving forced and the friction balanced

d) i) Its would decrease because there’s less air resistance.

ii) when you take off the surfboard the air resistance would decrease, so the friction would as well.

e) When the tyres are inflated there would be more friction.

15. The reason why the electric mixer is hot after beating the cream because the blades and the cream are rubbing against each other which cause friction and friction cause heat.

16. a) The reason why they use chalk on their hands is because chalk adds more grip.

b) To much chalk causes more friction which makes your skin tear.