The discussion centers on the concept of reference frames in collision problems, specifically regarding two balls (Ball 1 and Ball 2) colliding elastically. It is established that while the rest frame of Ball 2 can be considered, it is not an inertial frame due to Ball 2 being accelerated during the collision. Consequently, there is no inertial reference frame in which Ball 2 remains fixed throughout the collision process. The relative velocity of the balls changes sign but remains consistent in magnitude, indicating a lack of significant change during the collision.
PREREQUISITESPhysics students, educators, and professionals interested in mechanics, particularly those focusing on collision dynamics and reference frame analysis.
Yes, the rest frame of Ball 2 is such a frame of reference (surprise!), but it is not inertial and therefore not necessarily a good choice to describe the process.feynman1 said:Is there any sort of reference frame in which Ball 2 is always fixed (at rest) so that one can look at their relative velocity always in that reference frame?
Do you mean it's not inertial at the instant of the collision but inertial before and after? If so, Ball 2 won't be fixed in that frame, which isn't what I look for.DrStupid said:Yes, the rest frame of Ball 2 is such a frame of reference (surprise!), but it is not inertial and therefore not necessarily a good choice to describe the process.
Relative velocity is per definition the velocity of ball A in the rest frame of ball B (or vice versa). But if the relative velocity doesn't change, then there is not much of a collision.feynman1 said:2 balls (Ball 1 and Ball 2) collide fully elastically and their relative velocity stays the same as that before the collision. Is there any sort of reference frame in which Ball 2 is always fixed (at rest) so that one can look at their relative velocity always in that reference frame? Here 'always' includes before and after the collision.
It is not inertial at the instant of the collision because Ball 2 (which is accelerated during the collision) is fixed in that frame.feynman1 said:Do you mean it's not inertial at the instant of the collision but inertial before and after? If so, Ball 2 won't be fixed in that frame, which isn't what I look for.
Just edited the original question, relative v changes sign.A.T. said:Relative velocity is per definition the velocity of ball A in the rest frame of ball B (or vice versa). But if the relative velocity doesn't change, then there is not much of a collision.
Right, then have you an answer to the original question?DrStupid said:It is not inertial at the instant of the collision because Ball 2 (which is accelerated during the collision) is fixed in that frame.
He already answered it in post 2!feynman1 said:Right, then have you an answer to the original question?
I knew all along that such a frame written in post 2 doesn't work well. Then can we conclude that there's no inertial reference frame in which Ball 2 is always fixed?Dale said:He already answered it in post 2!
Yes.feynman1 said:Then can we conclude that there's no inertial reference frame in which Ball 2 is always fixed?