Kinematics (2)

The police frequently have to investigate road traffic

accidents. They make use of many aspects of physics,

including the equations of motion. The next two questions

will help you to apply what you have learned to situations

where police investigators have used evidence from skid

marks on the road.

Trials on the surface of a new road show that,

when a car skids to a halt, its acceleration is

−7.0 m s⁻². Estimate the skid-to-stop distance

of a car travelling at a speed limit of 30 m s⁻¹

(approx. 110 km h−1 or 70 mph).

At the scene of an accident on a country road,

police find skid marks stretching for 50 m.

Tests on the road surface show that a skidding

car decelerates at 6.5 m s⁻². Was the car which

skidded exceeding the speed limit of 25 m s⁻¹

(90 km h−1) on this road?