# Work, Power, Energy, Efficiency

danielsmith123123
Homework Statement:
Calculate the work done by planet X on its moon.
Planet X applies a force of 5.6 x 10^10 N on its moon and the moon is 4.0 x 10^8 km away from the planet and it continues to circle the planet in a circular orbit of circumference of 2.5 x 10^9 m.
Relevant Equations:
Gravity
The answer in the textbook says 0J but I am not sure what the concept behind it is. Does it have something to do with gravity?

Mentor
"Gravity" is not an equation. What is the mathematical definition (equation) of work?

danielsmith123123
"Gravity" is not an equation. What is the mathematical definition (equation) of work?
W = fd

Mentor
W = fd
Great, so can you apply it to this problem?

danielsmith123123
Great, so can you apply it to this problem?
W = Fd -------------- W = (5.6 x 10^10 N) (d) Would "d" be 0 because Planet X keeps orbiting the moon and essentially coming back to where it initially started therefore making it's displacement 0?

Mentor
W = Fd -------------- W = (5.6 x 10^10 N) (d) Would "d" be 0 because Planet X keeps orbiting the moon and essentially coming back to where it initially started therefore making it's displacement 0?
Yep, that's one way to look at it - the other is along the direction of motion there is no force. Essentially, the force and displacement have to be in the same direction. Since you don't have that, they'll multiply to zero.

danielsmith123123
Yep, that's one way to look at it - the other is along the direction of motion there is no force. Essentially, the force and displacement have to be in the same direction. Since you don't have that, they'll multiply to zero.
Ok, thank you so much

• russ_watters