Intro to astrophysics:
(a) somewhere in our galaxy resides a cloud of neutral hydrogen gas with a radius r. the gas density is n. (a) show that the number of 21-cm photons the cloud emits every second is given by:
n_{ph}=\frac{\pi r^{3}n}{t}
(b) if the cloud is a distance d from the sun, show that the energy flux of this radiation at the sun is given by:
f=\frac{hc r^{3}n}{4d^{2}\lambda t}

W= fd
where f is an applied force and d is the distance over which it is applied. when a parachuting skydiver is at constant velocity, he is experiencing "terminal velocity", the point at which his acceleration down is canceled out by air resistance. that means that the resisting force of the air is equal to his weight, but in the opposite direction. with this, we can solve for the work done by the force.
w = fd
w = (92.0)(9.81 m/s^2) (325)
w= 2.93 x 10^5 j
the value for work in this case is positive because it is pushing up on the skydiver, who has a downward force of gravity.
therefore the answer is 2.93 x 10^5 j