# Uniform Accelerated Motion

• Feb 16th 2011, 04:21 PM
Kels14
Uniform Accelerated Motion
Spotting a police car, you hit the breaks on your new porsche to reduce your speed from 90 mi/hr to 60 mi/hr at a constant rate over a distance of 200ft.
a)Find the accerleration in ft/s^2
b)How long does it take to reduce your speed to 55 mi/hr?
c)At the acceleration obtained in part (a), how long would it take for you to bring your porsche to a complete stop from 90 mi/hr?

I know this has something to do with these formulas
s(t)=So+Vot+1/2gt^2
v(t)=Vo+at
but I am not sure how to apply them and where to start.
• Feb 16th 2011, 05:01 PM
Kels14
I know that the answer is
a) -121/5 ft/s^2
b) 70/33 ft/s^2
c)60/11s
I just don't know how to get to this
• Feb 16th 2011, 05:11 PM
topsquark
Quote:

Originally Posted by Kels14
Spotting a police car, you hit the breaks on your new porsche to reduce your speed from 90 mi/hr to 60 mi/hr at a constant rate over a distance of 200ft.
a)Find the accerleration in ft/s^2

You missed a rather useful equation for when a is constant: $\displaystyle \displaystyle v^2 = v_0^2 + 2a(s - s_0)$

But no matter. You know $\displaystyle v_0$ and v. You do not know t. $\displaystyle v = v_0 + at$. We need another equation with a and t in it. You also have $\displaystyle \displaystyle s = s0 + v_0t + \frac{1}{2}at^2$. <-- $\displaystyle s_0$ is 0 ft. Why?

You now have two equations in two unknowns, a and t.

Try that and we can get to the next two after you've got this.

-Dan