How do i conceptually understand this physics question?

[phattestlewt]

This question is from a BR passage.

If a moon orbited a planet in uniform circular motion, the force of gravity from the planet acting on the moon.

A. would do work on the moon, otherwise it would cease to orbit the planet
B. Would do no work on the moon, otherwise the moon would change orbital speed
C. Would do work on the moon keeping speed constant
D. Would do no work on the moon, which changes the orbital speed of the moon.




I put down C. The force of earth's gravity is acting on the moon to keep it in uniform circular motion and hence is doing work as the moon is going around. It is accelerating because it's change directions (the magnitude of acceleration does not change and hence the speed stays constant.)

The answer is actually B. Because (acc to the soln manual) the orbit of the moon will always be perpendicular the force of earth acting on the moon. Work = force parallel to the direction of movement x distance; and because its perpendicular, no work is done. I understand that, so what I'm trying to ask is, what should i have immediately seen when i saw this question? what concept was it testing. thanks!

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[EndlessEntropy]

you could have drawn a diagram of the question, the moon orbiting in circular motion and drawn the the acceleration and velocity vectors, acceleration to the center and velocity straight, perpendicular. So you can see movement is perpendicular to the force so there is 0 work done.

 

[snowboarder37]

I know you said you understand why there is no work being done but I'll go through how I answer questions like this.

The first question I ask myself is, is work=0. I usually do this by using the equation w=f*d*cos(theta). Since it is perpendicular, the angle would by 90 and cos(90)=0, thus the work is 0.

The answer would be narrowed to B and D. Next, it seems like a logic question.

D says it does no work and that changes the orbital speed. But how can something that does no work change speed? To my knowledge, it can't and doesn't make sense to me. B says it does no work and if it did, it would change the moon's speed. Since this is uniform circular motion the orbital speed is constant. So B sounds right.