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## Worked example 2.2: Speed trap

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