IP2.7.6 Stopping cars © Oxford University Press 2011 Stopping cars

IP2.7.6 Stopping cars © Oxford University Press 2011  We normally think of friction as being unhelpful because it is a resistive force which opposes the desired motion. In cars we think of wasteful friction within the engine and air resistance. All these make cars less efficient.  However, friction is also very positive in some key processes in a car.  If it wasn’t for the friction between the tyres and the road the car would not move anywhere at all. If there were no friction the wheels would just spin on the ground. The grip given by static friction enables the car to push the road away and so move forward.

IP2.7.6 Stopping cars © Oxford University Press 2011  Friction is also vitally important when the driver wants to slow the car down.  If there was no friction between the tyres and the road the car would just slide without being able to slow down.  It is actually friction within the braking system of the car which gives the force to slow the car down.

IP2.7.6 Stopping cars © Oxford University Press 2011  The brake discs that are fixed to the wheels turn at the same speed as that of the car. The brake pads are fixed in position.  When the brakes are applied the break pads are closed around the disc under a high pressure.  Friction between the disc and the pads produce a very high amount of kinetic friction and this force slows down the car.  The kinetic energy in the wheels is transformed to heat energy in the break pads and discs.

IP2.7.6 Stopping cars © Oxford University Press 2011  If the car has to stop in a certain distance, the force needs to be greater if the car is travelling faster.  If the braking force is always the same (remains constant), then the distance needed to stop the vehicle increases as the speed of the vehicle increases.