A motorist drives at 30m/S along a straight road.The speed limit on the road 72km/h.The motorist applies brakes 25m before crossing a speed trap.What must the cars acceleration be to reduce its speed to 72km/h as it's reaches the speed trap?

Answer 1

72000 meters / 3600 seconds = 20 meters/second
so reduce speed from 30 m/s to 20 m/s
average speed during slowdown = 25 m/s
goes 25 meters during that slowdown
so went 25 meters / 25 meters/second = 1 second with the brakes on
change in velocity = -10 m/s
in time 1 second
a = change in velocity / change in time = -10 m/s^2

Answer 2

A motorist drives at 30m. Second on a straight level road.The speed limit is 70 km.h. 50 m before he drives over a speed trap, he slams on brakes.