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a particle, moving at speed u (u is fast enough for relativistic effects) with rest mass m0 collides with a stationary particle with rest mass m0. They coalesce to form a new particle of mass M (observer fame, not rest mass M) and move at speed v. find v in terms of y (y is gamma factor).

Conservation of momentum:

m0y(u)u=M0y(v)v

Is M(the rest mass of the 2 particles): 2m0 (just adding the rest masses and multiplying by the new gamma factor)

or

m0y(u) +m0 (adding up the masses as they were before the collision)

Thank you