# AP Physics

To stop a car, you require first a certain reaction time to begin braking. Then the car slows under the constant braking deceleration. Suppose that the total distance moved by your car during these two phases is 56.7 m when its initial speed is 82.5 km/h, and 24.4 when the initial speed is 50.3 km/h. What is your reaction time? What is the magnitude of the deceleration?

1. 2
1. Let tr be reaction time.

The distance traveled during the reaction time is vi*tr, so the distance traveled during stopping is (distancetotal-vi*tr). Now you have an equation...
Vf^2=Vi^2 + 2ad Use that. I will set up one equation for you, you do the second, and solve for tr, and a.

1) 0=24.4^2 + 2a (56.7-24.4tr)
2) ....

posted by bobpursley
2. I still don't get how to get the time it takes to get from 82.5 to 50.3 km/h, i know its a constant deceleration, but i don't know how to get the time

posted by Kristen
3. I still don't get how to get the time it takes to get from 82.5 to 50.3 km/h, i know its a constant deceleration, but i don't know how to get the time it takes to decelerate

posted by Kristen
4. I still don't get how to get the time it takes to decelerate from 82.5 to 50.3 km/h, i know its a constant deceleration, but i know how to find either the deceleration or the time

posted by Kristen

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