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Homework Help: Physics Collision Problem

  1. Oct 19, 2004 #1
    A 0.165 kg projectile is fired with a velocity of +705 m/s at a 2.00 kg wooden block that rests on a frictionless table. The velocity of the block, immediately after the projectile passes through it, is +55.0 m/s. Find the velocity with which the projectile exits from the block.

    I used the equation vf1=(m1-m2)/(m1+m2) but I got the wrong answer. Can you please help me?
  2. jcsd
  3. Oct 19, 2004 #2
    Conservation of momentum is the answer.

    Before the impact, the total momentum is:

    0.165 * 705 + 0 * 0 kg m/s (bullet + wood)

    After the impact, the total momentum is (vp is the bullet residual velocity):

    0.165 * vp + 2 * 55 kg m/s (bullet + wood)

    Since momentum is conserved, these two momentum should be equal:

    0.165 * 705 + 0 * 0 = 0.165 * vp + 2 * 55

    You can solve this equation for vp and get the result.

    You can check that the bullet final velocity is 38.33 m/s (right ?) since:

    0.165 * 705 + 0 * 0 = 0.165 * 38.33 + 2 * 55

    (hint: never apply an equation, apply the principles)
  4. Jul 27, 2011 #3
    Try using law of conservation of momentum i.e.

    By using it we get velocity of projectile as 38.33
  5. Jul 27, 2011 #4


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    First of all:
    Forum Rules: Do not post your homework/school-type questions here!

    So here you may got a common sense physics answer rather than a solution to most unrealistic school excercise:
    1. there is no such thing as 'frictionless table';
    2. "0.165 kg projectile is fired with a velocity of 705 m/s" means you deal with a bullet of heaviest (20mm) machine gun;
    3. have you ever seen a wooden plank hit by a rifle bullet? So try to imagine what may remain after being hit by 20mm bullet, then try to count the splinters and estimate momentum carried by them; If you haven't seen - watch Discovery Channel military programmes ;)
    4. If you listen to answers like "velocity of projectile as 38.33" try to picture it - the bullet 'passes through it...exits from the block' at the speed lower than block has itself.

    Sorry - I've underestimated caliber. Heaviest 20mm bullets have only 0.11 kg.
    0.165kg may be an armour-piercing bullet for 25mm cannons, used in assault helicopters. Their typical muzzle velocity is 720m/s.
    So now picture yourself a wooden block hit by such bullet...
    Last edited: Jul 27, 2011
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