assuming collisions are elastic...

right 5 equal masses, if you displace the first ball then you observe balls 2-4 stationary and 5 moves off with equal speed to the right, as all the momentum and energy is passed to ball 2, ball 1 stops, ball 2 then passes all its momentum and energy to ball 3 and then stops etc...

This conserves momentum and k.e.

However, if balls 2 and 5 are glued together, and you displace ball 1 with velocity 'u', then the other 4 must move of withspped 4/u to conseve momentum as mu = 4m*u/4, but then kinetic energy is not conserved...?

thankssss !

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Doc Al
Mentor
However, if balls 2 and 5 are glued together,
I assume you mean that balls 2 through 5 are glued together?
and you displace ball 1 with velocity 'u', then the other 4 must move of withspped 4/u to conseve momentum as mu = 4m*u/4, but then kinetic energy is not conserved...?
Don't assume that ball 1 stops dead.

Doc Al is right
in case I, ball 1 stops as both ball 1.2 has same mass
but now its different case, ball 1 collides with mass 4m, so it will now recoil!!!with somewhat lesser velocity than u.