Centripetal force= work?

[ClandestineStar]

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Hi there!,

I jave a question on TBR Physics section 2 question 13 out of 52. The question is " As the earth orbits from point B to point D, (it orbits closer to the sun), the gravitaional force from the sun...?"

I assumed that centripetal force does no work on planets since the forces are perpendicular to each other, but the answer says gravitational force does work on the planet. Can someone explain why?

 

[indianjatt]

In this case, it isn't the true centripetal motion we are used to. Usually when we deal with centripetal motion, the orbital is circular in nature and the speed is constant throughout. Thus the KE doesn't change and no work is done. In this case, however, we are moving in an ellipse so the speed increases as the planet gets closer to the sun because it must pass the same amount of area/time. As a result of a change in KE, work must be done. The only force likely acting on the earth in this case is the sun.

 

[circulus vitios]

I don't think you need to know anything about centripetal acceleration or force to solve this problem? The gravitational potential energy (quantitatively: U=-Gm1m2/r) changes so the gravitational force of the sun must do work on the planet.

 

[milski]

I don't think you need to know anything about centripetal acceleration or force to solve this problem? The gravitational potential energy (quantitatively: U=-Gm1m2/r) changes so the gravitational force of the sun must do work on the planet.

Both approaches rely on knowing the same fact - that the orbit is not really a circle.