# [further mechanics] Which particle gets pushed over?

So let's say you have object A and object B projected towards each other and they collide. Given that one of them is at rest after the collision, and the other one moves forward, how do you know which one (A or B) is the one put in motion - i.e. the one "pushed over"?

Is it the object with a lower velocity, lower mass or lower momentum??
That's a good question, resolved by the fact that there is no state of absolute motion. In other words, by adding an arbitrary velocity to all objects, you can make either object A or object B "at rest" or "in motion" as you like, without changing the physics. This can be used to help simplify mechanics problems using something called the centre of momentum frame, in which the total momentum is zero.
Original post by olivier_
So let's say you have object A and object B projected towards each other and they collide. Given that one of them is at rest after the collision, and the other one moves forward, how do you know which one (A or B) is the one put in motion - i.e. the one "pushed over"?

Is it the object with a lower velocity, lower mass or lower momentum??

What do you mean by pushed over? One particle goes through the other?
Original post by lordaxil
That's a good question, resolved by the fact that there is no state of absolute motion. In other words, by adding an arbitrary velocity to all objects, you can make either object A or object B "at rest" or "in motion" as you like, without changing the physics. This can be used to help simplify mechanics problems using something called the centre of momentum frame, in which the total momentum is zero.

omg you're right, technically you can take any pair of particles in motion and say one of them is at rest and the other one has velocity equal to the difference in velocities of the two! I didn't realise that when I was writing the question. The PPQ I was working on specified that I had to work out which sphere (modelled as a particle) physically moved tho
Original post by RDKGames
What do you mean by pushed over? One particle goes through the other?

one particle (A) is set in motion due to collision with another particle (B)
given that B was initially moving and was put to rest following the collision