Question

Carter pushes a bag full of basketball jerseys across the gym floor. Then he pushes with a constant force of 21 newtons. If he pushes the bag 9 meters in 3 seconds, how much power does he use? (Hint: 1 watt = 1 .)
A.
3 watts
B.
7 watts
C.
27 watts
D.
63 watts
E.
189 watts

Answer 1

Careful. You don't *use* power. Power is the instantaneous rate at which energy +/or work is supplied/ applied. If i am doing a bench-press, I am doing work and using energy, but power is just a metric that measures all of this against time.

Work is like Energy, and is measured in Joules (J):

- W = F * s

Power [units: W, Watts = J per sec] is about the delivery of energy (J), or work (J), per unit time (sec):

- P = W/t (J/sec)

Here, you have: 1) F = 21N, 2) s = 9m 3) t = 3 sec

He does work W = 21 [N] * 9 [m] = 189[ J]

That's the work done.

He did it in t = 3 sec

So the average Power over that t = 3 sec period of time is \(\dfrac{ 189}{3} = 63 W\)