A bullet travelling with velocity of 100 ms-1 pierces a block of wood and moves with a velocity of 10 ms-1 . If the thickness of the block reduces to one half of the previous value, what wiil be the emerging velocity of the bullet?

Answer 1

Is this block of wood stationary ?
If so
loss of energy in block = F of friction time thickness

energy loss = (1/2)m (100)^2 - (1/2)m(10^2)

= (m/2)(9900)

If we only lost half as much then
(m/2)(100)^2 - (m/2)v^2 = (m/2)(4950)
v^2 = 5050
v = 71.1 m/s

Answer 2

the question was, what if the block of wood were only half as thick?

Anyway (assuming a constant deceleration), half the thickness means half as long spent decelerating, so the change in velocity will be only half as much, ending up with a speed of 55 m/s.

Answer 3

maybe, I think half as thick means losing half as much energy, not losing half as much speed