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Isaac Physics - Gravitational Fields

Hi there, stuck on this question big time (part b) https://isaacphysics.org/questions/gravity_collision?board=4a0f25a1-1822-4b0e-8e6c-a6de4de41d7c
So far my working is:
Momentum is conserved, no external forces.
Loss of GPE = Kinetic energy

So in terms of equations I have gone for:

2GMm/(2r) - 2GMm/(2r+R) = KE of mass 1 + KE of mass 2
Is this correct, and if not why? Many thanks, I can post more detailed working if required :smile:
Original post by tande33
Hi there, stuck on this question big time (part b) https://isaacphysics.org/questions/gravity_collision?board=4a0f25a1-1822-4b0e-8e6c-a6de4de41d7c
So far my working is:
Momentum is conserved, no external forces.
Loss of GPE = Kinetic energy

So in terms of equations I have gone for:

2GMm/(2r) - 2GMm/(2r+R) = KE of mass 1 + KE of mass 2
Is this correct, and if not why? Many thanks, I can post more detailed working if required :smile:


The conservation laws that you stated are good.
But your equation for the conservation law of energy is incorrect. You are double-counting the GPE.
2GMm/(2r) - 2GMm/(2r+R) = KE of mass 1 + KE of mass 2
Reply 2
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tande33
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Ah ok that makes sense, thanks! So then we are talking about two objects GPE, we are actually referring to the GPE of the whole system, is that correct?

Original post by Eimmanuel
The conservation laws that you stated are good.
But your equation for the conservation law of energy is incorrect. You are double-counting the GPE.
2GMm/(2r) - 2GMm/(2r+R) = KE of mass 1 + KE of mass 2
Original post by tande33
Ah ok that makes sense, thanks! So then we are talking about two objects GPE, we are actually referring to the GPE of the whole system, is that correct?



Not really sure I understand you correctly.
There is “really” no such thing as “gravitational potential energy of an object”, although we tend to find such phrases or terms in some physics texts. “gravitational potential energy of an object” has the implication that gravitational potential energy belongs to the object or is a property of the object.

However, gravitational potential energy does not belong to an object or is NOT a property of the object.

In physics, gravitational potential energy is always associated with a system of two or more objects interacting via gravitational force, so it is the property/characteristic of the system and not of any of the individual objects within the system.
Reply 4
Avatar for tande33
tande33
OP
15
Original post by Eimmanuel
Not really sure I understand you correctly.
There is “really” no such thing as “gravitational potential energy of an object”, although we tend to find such phrases or terms in some physics texts. “gravitational potential energy of an object” has the implication that gravitational potential energy belongs to the object or is a property of the object.

However, gravitational potential energy does not belong to an object or is NOT a property of the object.

In physics, gravitational potential energy is always associated with a system of two or more objects interacting via gravitational force, so it is the property/characteristic of the system and not of any of the individual objects within the system.

Well thank you for clearing that up, an *awful* misunderstanding on my front. I have no idea where that came from but I am very grateful for you correcting my fundamental error in my thinking!! Thank you :smile:

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