What is the work done on a comet orbiting a star?

[ccarit3007]

If a comet is orbiting a star is there any work done? I understand that work is force * displacement, but the force must be in the direction of motion. In the case of a comet and star the star exerts a gravitational force on the comet, but this force is not in the direction of the motion of the star (as the motion is circular). Is this the correct logic? For the question in the image shown I understand that potential energy would be greatest at A and kinetic would be greatest at D, but I still am stumped on the work side of things.

 

[phyzguy]

You are correct that if the orbit is circular then the force and the displacement are perpendicular, so no work is done. However, a comet's orbit is not circular...

 

[ccarit3007]

You are correct that if the orbit is circular then the force and the displacement are perpendicular, so no work is done. However, a comet's orbit is not circular...

Yes, I understand that. So when you are not at point A or point D the force will not be perpendicular to the velocity (and momentum). I'm not sure exactly what this means/where I go from there? I'm guessing that means the work at A & D is 0?

 

[ccarit3007]

Yes, I understand that. So when you are not at point A or point D the force will not be perpendicular to the velocity (and momentum). I'm not sure exactly what this means/where I go from there? I'm guessing that means the work at A & D is 0?

Ah thinking about this again, the net force which would be directed towards the star would have a component that is in the direction of the momentum. Is this correct?

 

[Drakkith]

Ah thinking about this again, the net force which would be directed towards the star would have a component that is in the direction of the momentum. Is this correct?

It would have a component of the force directed in the direction of motion, yes.