How Is Power Calculated in a 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.

 

[wais]

You are not screwing up, your calculations are correct. The only issue is that the answer choices provided are not all correct. The correct answer should be 12.5W, which is not listed as an option. This could be a mistake or error in the question or answer choices. It is always important to double check your calculations and compare them to the given answer choices to ensure accuracy. In this case, your calculations are correct and the given answer choices are incorrect.