Stopping Distances

What is the stopping distance of a car defined as?

The total distance travelled during the time it takes for a car to stop in response to some emergency

What is the equation for it?

Stopping distance = thinking distance + braking distance

For a given braking force, the greater the speed of the vehicle, the … the stopping distance

greater

For a given breaking force, what determines the stopping distance?

The speed of the vehicle

What does the area under a velocity time graph showing thinking and breaking represent?

The stopping distance

What is the reaction time defined as?

A measure of how much time passes between seeing something and reacting to it

What is the human reaction time for someone who is alert?

0.2 - 0.9 seconds

Describe a simple reaction time experiment with two people

• Person A holds a 30 cm ruler vertically, such that the bottom end of the ruler hovers over the top of the hand of Person B

• Person A should release the ruler unexpectedly

• As soon as Person B sees the ruler move, they should close their hand, catching it

• The ruler is marked at the point at which it was caught by Person B - i.e. in line with the top of their hand

• This gives a measurement of the distance the ruler fell

  • The greater the distance, the longer the reaction time

What is thinking distance defined as?

The distance travelled by a car from when a driver realises they need to brake to when they apply the brakes

What is the equation for reaction distance?

Reaction distance = speed of car x Driver’s reaction time

What is the main factor affecting thinking distance?

The car’s speed

What are additional factors that can increase the distance? (name 3)

• tiredness

• distractions

• intoxications

What is the braking distance defined as?

The distance travelled by a car under the braking force - i.e. whilst it is slowing down

What is the main factor affecting braking distance?

speed

What are some others? (name 3)

• vehicle condition

• road condition

  • vehicle mass

What happens when a driver applies the brakes?

There is a frictional force between the brakes and the wheels of the car

What does the frictional force do?

What does this then mean for the kinetic energy of the car and the thermal energy of the brakes?

What does this then mean for the car?

• Does work on the brakes - i.e. it transfers energy from the car to the brakes

• kinetic energy of the car decreases and the thermal energy of the brakes increases - i.e. the brakes heat up

• the car decelerates (slows down)

The greater the speed of a vehicle, the … the braking force required to bring the vehicle to a halt for a given distance

Since the breaking force would need to be …, what would the deceleration of the vehicle be and why?

• greater

• large as well because of newton’s second law linking resultant force and acceleration

what could large decelerations lead to?

brakes overheating and / or loss of control of the vehicle

If you are asked to explain why the temperature of the brakes increases when a vehicle stops, remember, work is done by the frictional force between the … and the wheel.

brakes

What is the equation for the work done by brakes when a vehicle slows to a halt?

braking force x braking distance = ½ x mass x velocity²

what is the ½ x mass x velocity² part of the equation?

what is the braking force x braking distance part of the equation?

• the kinetic energy of the car

• work done by the brakes

what does this equation show?

what can we use it for?

• the work done is the transfer of kinetic energy

• the braking distance is proportional to the speed squared (if the speed is doubled, the distance increases 4 times)

• we can use it to estimate decelerating forces required for a typical vehicle moving at everyday speeds

What is important to note about this equation at high speeds?

It doesn’t apply because the brakes get hot and become less effective, so the braking distance will increase even further while it reduces the braking force