simple work-energy problem

[StonieJ]

A force of 5N acts on a 2 kg object that starts from rest. When the force has been acting for 2 s, the rate at which it is doing work is:

Here's how I tried to solve it.

Solve for acceleration, which I assumed was constant.


a = F/m = 5N / 2kg = 2.5 m/s/s


Then use the displacement equation to find the distance traveled.


x = xo + (vo)(t) + (1/2)(a)(t^2)
x = 0 + 0 + (1/2)(2.5 m/s/s)(2s)^2
x = 5m

Work = F * x = 5N * 5m = 25 J

Power = Work / t = 25J / 2s = 12.5 W
However, the only possible answer choices are:
a) 5W b) 10W c) 25W d) 50W e) 75W

Where am I screwing up?

[Leong]

P=\frac{dW}{dt}
P=\frac{d(Fs)}{dt}
P=F\frac{ds}{dt}
P=F\frac{d(ut+\frac{1}{2}at^2)}{dt}
P=F*(u+at)
P=5*(2.5*2)
P=25\ W
You formula for power can't be used in this case because P is not constant for P depends on how long the force acts on the object.