Posted by **Bob** on Sunday, October 3, 2010 at 3:57pm.

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 80.0 km/h, and 24.4 m when the initial speed is 47.8 km/h.

(a) What is your reaction time?

(b) What is the magnitude of the deceleration?

I tried eliminating the values in order to find accel and time, but the values canceled out and when I tried separating them, I got the answer wrong. What am I doing wrong?

- Physics -
**bobpursley**, Sunday, October 3, 2010 at 4:07pm
I don't see how it is solvable, unless you assume the acceleration is different on each, or the time during deacceleartion is the same.

You have as variables: a, time to slow, tr.

- Physics -
**Bob**, Sunday, October 3, 2010 at 4:22pm
Let me try to clarify

v1=22.222 m/s

v2=13.278 m/s

x=vi(t)+1/2a(t^2)

56.7=22.222t+1/2a(t^2)

24.4=13.278t+1/2a(t^2).

Through elimination

32.3=8.94t

t=3.611s

From there I separated to -vi=at as vf is 0

-22.222=a(3.611)

-13.278=a(3.611)

-35.5=7.222a

a=-4.92 m/s^2

- Physics -
**Bob**, Sunday, October 3, 2010 at 4:30pm
Forgot to put my question at the end of it XD

This is from me trying to interpret what you've done in the previous times this question has been asked, but obviously, I'm wrong somewhere. At what point in the work am I messing up?

- Physics -
**John**, Monday, August 15, 2016 at 4:36pm
2.53 seconds

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