Posted by **Kristen** on Sunday, September 9, 2007 at 2:40pm.

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?

- AP Physics -
**bobpursley**, Sunday, September 9, 2007 at 5:42pm
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) ....

- AP Physics -
**Kristen**, Sunday, September 9, 2007 at 7:31pm
I still dont 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 dont know how to get the time

- AP Physics -
**Kristen**, Sunday, September 9, 2007 at 7:59pm
I still dont 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 dont know how to get the time it takes to decelerate

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