Thread: 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.

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

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:

But no matter. You know and v. You do not know t. . We need another equation with a and t in it. You also have . <-- 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