# Friction

## Key Points

• occur in many different situations.

• Frictional forces stop us from slipping when we walk and slow down a cyclist when they stop pedalling.

• Friction is a contact force and acts against the movement of an object.

## What are frictional forces?

When an object is moving there are almost always forces which act against it, unless it is in a vacuum as in space. These are frictional forces and act in the opposite direction to the movement. Frictional forces make it more difficult for objects to move.

## Friction

Friction is a contact force. It acts against the movement of an object.

There are many examples of where friction is a useful force. For example, friction is why we do not slip when we walk along the pavement. It is also useful in bikes and cars. Without friction, they would not be able to accelerate, turn or brake.

Friction can also be unhelpful. For example when the mechanical parts of a bike, like the chain and axles, rub together the friction can cause the metal to wear away.

Friction also causes objects to heat up. For example, rubbing hands together on a cold day to keep warm. The friction between the palms of your hands causes them to heat up. Friction between moving objects causes thermal energy to be out to the surroundings.

We can limit the amount of friction using . Substances like motor oil can be used to stop metal parts from rubbing and wearing away.

Modern taps use moving parts made from a low friction plastic called PTFE, rather than rubber and brass.

## Investigating friction

Investigate by comparing the amount of force needed to move an object on different types of surfaces by following these steps.

Step 1 - what are the three experimental variables?

• - the type of surface
• - the amount of force needed to move at a constant velocity
• - the speed of the object and the mass of the object

Step 2 - carry out the experiment

Method

1. Set up the equipment by attaching a newton metre to a mass.
1. Place your first surface type on the table or bench.

2. Pull the mass for a 30 cm distance across the surface at a constant speed. Measure the reading on the newton meter.

3. Return the mass back to the starting position and repeat a further 2 times.

4. Calculate a mean force.

5. Swap the surface type and repeat steps 3, 4 and 5.

Step 3 - how to make your experiment valid

• You should always repeat your measurements three times.

• You can then identify any and calculate a mean.

Step 4 - presenting and interpreting the data?

1. Calculate the mean force for each surface type.

2. Plot a bar chart of mean force against surface type. For example:

You should find that more force is needed to move an object over a surface where there is lots of friction.

## Air resistance and drag

is the force which acts against the movement on an object when it moves through a fluid (a liquid or gas). The faster the object moves the more drag it experiences. When the fluid is air, drag is usually described as .

The effects of air resistance can be reduced by the object, which will allow the moving object to go faster for the same force. Cycling teams and Formula 1 motor racing teams work to reduce the effects of air resistance. They design cars, helmets and bikes which are streamlined so that the cyclist or car can travel as quickly as possible.