Could you please help with a physics question? Thanks.

Question

A bullet with a mass of 20 g is traveling with a speed of 600 m/s. What is its kinetic energy?

This is a separate one that was listed below it, but It appears separate. Thanks again.

A 60 kg skydiver is falling at a constant terminal speed of 50 m/s. At what rate is the potential energy of the skydiver - earth system reducing? What happens to this energy?

Answer 1

kinetic energy = 0.5 m * v (squared)

                        = 0.5 * 0.02(kg) * 600(squared)

                        = 3600 joules

As for the second question I'm less certain:

potential energy = mgh, the height (h) is decreasing by 50 meters per second.. I donno how to go on from here tho sorry...

The potential energy is converted to kinetic energy.  

Answer 2

1st part:

Kinetic energy = 0.5 * m * v^2

= 0.5 * 0.02 * 600^2

= 3600 Joule

2nd part

Suppose the skydiver comes down by height h in time t.

Then potential energy PE reduces by mgh

h = vt because v is constant.

Therefore, PE reduces by mgvt

Differentiate the above with respect to t to get the rate at which PE is reducing.

Rate = d/dt of mgvt

= mgv

m = 60 kg

g = 9.8 m/s2

v = 50 m/s

Therefore, rate = 60 * 9.8 * 50

= 29400 joule/second

Another person has written that potential energy is converted into kinetic energy. But that is wrong because speed is constant. This means kinetic energy also is constant.

When something falls only under gravity, then its speed increases because of gravity. But, in the problem, speed is constant. This means that there is some force to balance gravity.

The remaining energy is used up in doing work against that force.

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