Question

You pull a wagon carrying your younger brother 1000 meters (1 km) to the store using 20 newtons of force the whole way. It takes you 15 minutes to get there. How much power did you generate, in joules per minute? In watts?

Answer 1

22.2 W

Explanation:

First of all, we calculate the work done by moving the wagon, using the formula:

where

F = 20 N is the magnitude of the force

d = 1000 m is the displacement of the wagon

is the angle between the direction of the force and of the displacement (assuming the force is applied in the direction of motion)

Substituting, we find

Now we can find the power generated, which is equal to the ratio between the work done and the time taken:

where

W = 20,000 J

t = 15 min = 900 s

Substituting,

And the same value in Joules/second (remember that 1 Watt = 1 Joule/second)

Answer 2

Lifting Weights: On earth it takes about 10 Newton-meters (N-m) of energy to raise a 1 kilogram mass to a height of 1 meter. Since 1 N-m equals 1 Joule, that's 10 Joules. If it takes 1 second to lift the weight 1 meter, than you have converted 10 Joules of energy to potential energy in one second.