Physics Question?

Question

A 60-kg sky diver falls from rest from a height of 2400m above ground. By the time she has fallen the first 1000m, she has reached a constant speed of 60 m/s. (a) How much work has been done by the force of air resistance during the first 1000m? (b) How much work does this force do during the next 800m of fall?

Answer 1

You must use the energy in the first moment, and the energy in the second moment :

At the first moment : There is no kinetic energy, but there is potential energy :

E = 60*g*2400

at the second moment there is kinetic energy and potential energy :

E' = 60*60^2 / 2 + 60*g*(2400-1000), so then :

E = 144000g

E' = 108000 + 84000g

if you rest : E - E', you will have the positive value of the work been done bye the air resistance :

g = 9.8 m/s^2

W = 1411200 - 108000 - 823200 = 480000 J >>> first answer

Now, the W = F*distance

the distance is 1000 m

So the force of resistance will be = 480 Newtons

b ) The work that the force will do durin the next 800 m, will be :

480 * 800 = 384000 Joules

Answer 2

a) work done =loss in its energy
= -1/2mv^2+mgh
= - 1/2*60*60^2 + 60*g*1000

b) after attaining constant velocity , force f =wt of person
=60g
work done by this force =f*displacement = - 60g*1000