@Swan There is some interesting physics here but i's a bad example. The man in the lift sees a current as well. He sees the electrons in the wire stationary but now the positive nuclei are moving down relative to him.

So the man on the ground sees a current due to moving electrons and the man in the lift sees the same current due to moving nuclei.

@Swan this is a better experiment:

This time we have a stream of electrons moving - just electrons.

Both observers see a negative charge generating an electric field, but the observer on the ground also sees a magnetic field because he sees a current moving past him.

The observer moving with the electrons sees only the electric field and no magnetic field.

@HarjotDhillon Hi :-)

I think I am getting wrong because there could be some dissipation of energy as bullet pass through block

But question seems to assume that there is not wastage

@HarjotDhillon Yes. You cannot use conservation of energy because this is an inelastic collision and an unknown amount of energy gets converted to heat.

The way to do this is to find the speed of the block after the bullet has passed through it, then use conservation of momentum to find the final speed of the bullet.

Ok I would try that

I got 120. Thanks for giving hint

You're welcome :-)

You'll see many questions like this where there is some inelastic process so energy is not conserved. You need to be careful about this as some of the questions seem like they are designed to mislead you!