Stopping Safely

Question 1

what should drivers to on the road to avoid and accident

Answer 1

• leave enough space in front so they could stop safely

- at least equal to the stopping distance for their speed

Question 2

what happens as the speed of the car increases and why

Answer 2

• the thinking distance increases at the same rate
• this is because the driver’s reaction time stays fairly constant
• but the higher the speed, the further you go in that time

Question 3

what is the relationship between speed and braking distance

Answer 3

• if the speed doubles, the braking distance increases by a factor of 4 (2^2)
• and is the speed trebles, the braking distance increases by a factor of 9 (3^2)

Question 4

how do brakes stop a car

Answer 4

• the brakes of the car do work on the car’s wheels
• this transfers energy from the car’s kinetic energy store to the thermal energy store of the brakes
• so to stop a car, the brakes must transfer all its energy

Question 5

what is the equation which shows the energy transfer from the kinetic energy stores of the car to the thermal energy stores of the brakes

Answer 5

• energy in the car’s kinetic stores = work done by the brakes
• (1/2 x mass x velocity^2 = force x (braking) distance)

Question 6

what are the estimated masses of cars, single decker buses and a loaded lorry

Answer 6

• car’s mass is 1000kg
• single decker bus is 10,000kg
• loaded lorry is 30,000kg

Question 7

what is the estimated reaction time of a driver

Answer 7

1 second

Question 8

what does the first part of a thinking and braking distance velocity time graph look like

Answer 8

• there is a solid block at the beginning
• its area gives the thinking distance
• the length of it on the time axis gives the reaction time

Question 9

what does the second part of a thinking and braking distance velocity time graph look like

Answer 9

• a triangle, with the line at the top of the solid block going down into the time axis
• the area of the triangle gives the braking distance
• the gradient (deceleration) shows the maximum force applied by the brakes

Question 10

if there were two velocity time graphs, one showing the thinking and braking distance of a man before and after being tired, would the gradient change and why

Answer 10

• it wouldnt
• as the deceleration is the same on both graphs
• because the maximum force applied on the brakes hasnt changed