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*Question:* In a speed trap, two pressure-activated strips are placed
apart on a highway on which the speed limit is
. A driver going
notices a police car just as he/she activates the first strip, and immediately slows down. What
deceleration is needed so that the car's average speed is within the speed limit
when the car crosses the second strip?

*Answer:* Let
be the speed of the car at the first strip.
Let
be the
distance between the two strips, and let
be the time taken by the
car to travel from one strip to the other. The average velocity of the car is

We need this velocity to be
. Hence, we require

Here, we have changed units from to . Now,
assuming that the acceleration of the car is uniform, we have

which can be rearranged to give

Hence, the required deceleration is
.

** Next:** Worked example 2.3: The
** Up:** Motion in 1 dimension
** Previous:** Worked example 2.1: Velocity-time
Richard Fitzpatrick
2006-02-02