# Homework Help: Force and Equations of Motion

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1. Oct 22, 2014

1. The problem statement, all variables and given/known data
If the engine of a car provides an acceleration of 2ms-2 to start it from rest, assuming the mass to be roughly 1000kg, calculate the time after which the car comes to rest if the engine is turned off after 15 seconds and the frictional force is 15N)

2. Relevant equations
v=u + at
F= m*a

3. The attempt at a solution
v= 0 + (2)(15) = 30ms-1
F= m*a = -15N= 1000 * a
Therefore, acc. caused by friction= (-15)/1000= -0.015
v=u+at, but car comes to rest, so v = 0
0= 30 +(-0.015)(t) = 2000sec

I'm not really sure of the answer as it seems to be quite impractical (:P).
Thanks.

2. Oct 22, 2014

### spl-083902

Looks like you did it right.

Other than writing
0= 30 +(-0.015)(t) = 2000sec
which is technically wrong since 0 != 2000sec. You should put t=2000s on a separate line

3. Oct 22, 2014

Oops....I didn't quite notice that....Thanks a lot for your help!

4. Oct 22, 2014

### PeroK

You missed the obvious solution. If you apply an accelerating force of 2000N for 15secs, how long do you have to apply a decelerating force of 15N to return to rest?