Speed kills
You’re hired to advise the advertising agency making a Government information film on road safety. Their script for the film currently shows how much further a car travelling at 35 mph travels than one braking from 30 mph.
You don’t think this is punchy enough a message to get over and suggest that it would be more dramatic to show how fast the former is travelling at the point where the latter has come to a stop. They agree to hear your suggestion and the justification for it. What would you say to them?
Hint
Reveal
Solution
Reveal

Solution
- The key assumption here is that the braking acceleration of the car is (a) constant and (b) independent of the speed of travel.
- We can then use the equation v2=u2+2as to analyse both situations.
- In the first, the stopping distances are compared:
For car 1, travelling at 30 mph :
(Note on units: I have not converted to ms-1. All speeds are in miles h-1, all accelerations in miles h-2)
Similarly for car 2, travelling at 35 mph :
Eliminating a,
In the second situation, the speed v2 is calculated after the second car has travelled the same distance that it takes the first car to come to a stop, s1.
Using the expression for s1 calculated above :
- Thinking about that, which would you find more dramatic?