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    Suppose an object A with momentum p collides with an object B at rest (in our frame of reference anyway).
    The objects collide not necessarily head-on and move off with momenta p_1 and p_2 (this is irrelevant but anyway).

    What are the internal forces in this system?
    How would you answer this?

    I believe this to be an ambiguous question. There seems to be in some sense a "delta" of force, and furthermore all forces at all times sum to zero.
    Is the correct answer there are no forces? F=dp/dt and F is not differentiable at the point of impact, assuming elasticity.

    ...what does the marker want to hear? I assume something a bit more simplistic but...sigh.

    Thanks.

    [edit] Or would it be better to mention the Newtonian pair of the objects on each other for an instant (literally) of time?
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    I think the Newton III pair is what they want. Internally, equal and opposite forces acting for the same dt give equal and opposite changes of momentum.
    From the external point of view there is, of course, no resultant force and no change in momentum.
    Other internal forces you could mention would be the elastic forces within the two objects. This would depend on the nature of the objects and the collision. (elastic/inelastic deformation etc).
    The answer also depends on the level you are working at here. GCSE. A Level. Undergrad?
    I assumed GCSE / A Level.
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    I think it's assumed to be elastic.
    The time interval of the collision is hence zero, right? Or is it, I can't remember...

    And the level is undergrad, but the person who asked me this isn't doing a physics course exactly, so it's more like A-level. (it's not my a question I'm asking as such...I do maths at university so the problem just doesn't make any sense to me without a nice set of definitions haha)
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    The time interval is not zero. It it were then F times dt would be zero. (No impulse)
    As I said before, there are a number of ways of answering.
    Internally, object A applies a force F to object B, and object B applies a force F to object A while they are in contact for a time t.
    Beyond that, without knowing the scope of this question, the context, and what its aim is in eliciting some aspect of physics theory, it's difficult to be more specific.
    If it's an open ended question to probe your friends general physics knowledge, I would suggest talking about the electrostatic repulsion between the atoms on the surface of the two objects, and the forces involved when the material deforms elastically or inelastically.
 
 
 
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