# Deceleration (Driving a Car-Help)

1. Aug 30, 2006

### Kalie

Deceleration (Driving a Car--Help)

You're driving down the highway late one night at 20m/s when a deer steps onto the road 35m in front of you. Your reaction time before stepping on the brakes is .5s, and the maximum deceleration of your car is 10m/s.

A. How much distance is between you and the deer when you come to a stop?
B. What is the maximum speed you could have and still not hit the deer?

So I don't think I started the problem correctly....
I used the equation d=v0*t+1/2a*t^2
so then d=20*.5-1/2*10*.5^2
d=8.75

so then I subtracted 8.75m from 35m and got 26.25m

It was wrong, then I thought 8.75 was it but that was wrong so I don't know what I did wrong. Could someone tell what I did wrong. I think it might of been the equation so I look for a problem like that in my book but couldn't find anything. Please someone help. Just the equation don't ruin the fun

2. Aug 30, 2006

### durt

It does not take .5 seconds for the car to stop. You drive for another .5 seconds (or 10m) until you begin to stop. Since you know the speed and deceleration of the car, now use another equation: $$v_f^2 - v_i^2 =2 a d$$.

3. Aug 30, 2006

### Kalie

d=distance right? but then
0-20=2*-10*d
-20/-20=1=d
so d doesn't have any sign so it doesn't make since.....

4. Aug 30, 2006

### Kalie

sorry being an idiot thank you it work just fine