Forces On Vehicles

Understand the terms motive force and braking force.

Be able to explain how driving wheels can generate a motive force.

Explain the importance of friction in acceleration and braking.

Know that stopping distances of vehicles relate to frictional forces and

speed.Know how to interpret distance/time

graphs.

Basics – A Reminder

• Do you remember Newton’s Third law?

• It states “For every action there is an equal and opposite reaction”

An Example

• This coach is stationary on the ground.

• There are two forces acting on it….what are they?

1. Its weight acting down2. The floor pushing up

As the coach is not moving, these two must be equal and opposite.

What if they were not equal?

• If the floor did not push up on the coach, or it did not push up enough, then the coach would sink into the ground.

• If it pushed up too much, the coach would fly up into the air.

• This doesn’t only happen in a vertical arrangement – it happens if you lean on something.

• In fact it happens everywhere, all of the time!

Motive Force

• “Motive” comes from the word “motion”.

• So this is a force which causes motion.

• Lets think of a car:

•Where does the motive force come from?

Motive Force

• The engine provides a torque to the driving wheels.• However, this is not really the motive force.• The motive force occurs at the wheels.• The wheels turning push backwards on the road.• By Newton’s third law, the road must be pushing

forwards on the wheel, like this:

Wheel pushing backRoad pushing forwards

Motive Force

• This means that the purpose of the engine is to provide a torque to the wheels, so they push backwards on the road.

• It is the road which then pushes the car forwards.

Braking Force

• The car is moving – so the wheels are turning – usually at high speed.

• The braking force is provided by the brake pads gripping the wheel.

• The brakes provide a torque opposite to the torque driving the wheels round.

• This reduces the force of the wheels on the road, and hence the push of the road on the car.

Brakes

Another Reminder - Friction

• Friction occurs when things rub together.• It is caused by tiny imperfections in the

surfaces locking and unlocking.• The result is heat is generated, and

things wear down.• Friction can be reduced by lubricating

surfaces: coating them with liquid, which allows surfaces to glide over each other.

Why is Friction so Important?

• Lets look at the motive force first:

If friction is absent, the road will not be able to push back on the wheel.

The result will be that the wheel will spin – there will be no grip.

This sometimes happens when people accelerate too harshly-the motive force exceeds the force of friction, and we get a wheelspin.

Movement and Friction

• If the car is moving and the friction force reduces, you may not notice…

• Unless you are going round a bend!• If friction disappears while you are turning a

corner, your car could go into a spin.• This is more likely to happen at high

speeds – when greater forces are acting.

What About Braking?

• If friction is absent during braking, the wheels will lock – they will stop turning.

• This is very dangerous – mainly because the car will slide, but also because the car cannot be steered.

Summary

• The Motive force is the force which makes something accelerate.

• The braking force causes bodies to decelerate.

• Friction is very important for both forces.

Road Safety

Know that stopping distances of vehicles relate to frictional

forces and speed.

Know how to interpret distance/time graphs.

Stopping Distance• This is the distance moved by the vehicle

between the driver seeing the obstacle and the vehicle stopping.

• The vehicle keeps moving at a steady speed whilst the driver reacts(thinking distance)

• The vehicle slows down whilst the brakes are applied(braking distance)

• Stopping = Thinking + Braking

Speed(m/s)

Time (s)

Thinking distance

Braking distance

Reaction time

Microsoft Excel Worksheet

Other factors affecting stopping distance • Apart from speed,• What will increase the thinking distance (make

reaction time greater)?• What will increase the braking distance (reduce

the frictional force)?