# Homework Help: Average speed and deceleration

1. Sep 2, 2007

### misshobo

hi. I'm fairly new at physics but i got put in AP Physics and i think i could make it with some help. Could anybody please explain how to solve this problem?

A speed trap is set with 2 pressureactivated strips placed across a highway, 110 m apart. A car is speeding along at 33 m/s, while the speed limit is 21 m/s. At the instant the car activates the first strip, the driver begins slowing down. What minimum deceleration is needed in order that the average speed is within the limit by the time the car crosses the second marker?

2. Sep 2, 2007

3. Sep 2, 2007

### EricVT

Hello!

Try thinking about how long it would take someone to get from one strip to the next if they were driving precisely the speed limit, and then use the kinematic equations that I hope you've learned to work out the needed acceleration of the speeding car in order to cover 110m in that same time. You want the average velocity to be within the speed limit, and the MINIMUM deceleration will put that average right at 21 m/s (the speed limit).

So you have several known pieces of the puzzle, and all you need is to put them into the equations to find your answer. I think the rest can be safely left to you. Hope that advice wasn't too telling.

Cheers.

4. Sep 2, 2007

### misshobo

thanks a lot. :D