Physics question?

Question

I'm taking AS physics, and this was on a GCSE recap test, and was the only one I got wrong, so it's been annoying me.

A mass of 5 kg is raised 20m into the air, and this takes 4s. What was the power used?

I have 2 different equations for power -

Power = mass/velocity

and

Power = work done/time

I used the first one, worked out the velocity as 5ms-1, and got the power as 1 watt. But this was wrong. Since I have used the second one and got 125 watts (im not sure if this is right), and I really dont think it is relevant, but in the previous question I found that the potential energy of the mass from 20m in the air is 1000J.

Can someone please go through this step by step, and explain where Im going wrong with having two different power equations. Thankyou.

Answer 1

Your first equation (Power = mass/velocity) is wrong.

It should be:
Power = force / velocity

Since the force in this case is equal to the weight, this is:
Power = weight / velocity

Remember that "mass" and "weight" aren't the same. You have to multiply mass times "g" to get the weight.

BTW, the answer is not 1 watt nor 125 watts. You got the 1000J about right; this is the PE, and it also equals the work done. Check this against your 2nd equation one more time (might have been just a division error).

Answer 2

Im not sure if this is right or that i've missed something out but i hope it is helpful to you anyway.

1. Force(N) = mass x gravity (9.8ms/s)
2. Work(J) = force x distance
3. Power(W) = work / time taken

1. 49N = 5kg x 9.8ms/s
2. 980J = 49N x 20m
3. 245W = 980J / 4s

Answer 3

power=forcexvelocity