A ball of mass m moving with a velocity $\mathrm{v}$ hits a massive wall moving towards the ball with a velocity$\mathrm{u}.$An elastic impact lasts for a time $∆\mathrm{t}.$ 

Two blocks A and B each of mass $‘\mathrm{m}’$ are connected by a massless spring of natural length $\mathrm{L}$ and spring constant $\mathrm{k}.$The blocks are initially resting on a smooth horizontal plane. A third block C also of mass m moves on the plane with a speed $‘u’$ along the line joining A and B and collides elastically with A then which of the following is/are correct :

The figure shows a string of equally spaced beads of mass $\mathrm{m},$ separated by distance $\mathrm{d}.$The beads are free to slide without friction on a thin wire. A constant force$\mathrm{F}$ acts on the first bead initially at rest till it makes collision with the second bead. The second bead then collides with the third and so on. Suppose all collisions are elastic, then :

A bullet of mass $\mathrm{m}=1\mathrm{kg}$ strikes a block of mass$\mathrm{M}=2\mathrm{kg}$ connected to a light spring of stiffness $\mathrm{k}=3\mathrm{N}/\mathrm{m}$ with a speed ${\mathrm{V}}_0=3\mathrm{m}/\mathrm{s}.$ If the bullet gets embedded in the block then.