physics

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?

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asked by Ram
  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

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    posted by Damon
  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.

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    posted by Steve
  3. maybe, I think half as thick means losing half as much energy, not losing half as much speed

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    posted by Damon

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