# Homework Help: Instantaneous power problem

1. Oct 13, 2005

### lonely86driver

i need a little help on this problem and i'm not sure if i have the equation down right.

a 1990kg car accelerates uniformly fromrest to a speed of 17.1 m/s in 4.21 sec.
now the question states what is the instantaneous power delivered by the net force to the car at 2s. answer in units of w.

so this is what i did.
i know that the equation is P=dw/dt which is F*v
to find F i did F=ma but i need to find out what a is
vf=vi+at

vf=vi+at
17.1m/s = 0 m/s + a(2s)
a = 8.55 m/s

F = 1990kg (8.55m/s) = 17014.5

then i plug all that into P= F*v
P = 17014.5 * 17.1 m/s = 290947.95

but thats wrong (online submit). my instructor told the class that v in that equation is the final velocity. am i doing something wrong here. i already found the average power in for the 4.21s and i already found work done.

2. Oct 13, 2005

### Pyrrhus

You need to find the speed at 2s, then multiply it by the net force.

3. Oct 13, 2005

### lonely86driver

how would i find speed? isnt speed change of x over change of time?

4. Oct 13, 2005

### Pyrrhus

You know the acceleration is uniform and it stars from rest, Kinematics?