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Crime Scene Problem

  1. Nov 16, 2004 #1
    I was thinking up a physics problem and could someone help me find the answer to it.

    Someone has committed suicided at a crime scene and this is what they know.

    Mass of Bullet: 0.05 kg
    Velocity of Bullet: 1500 m/s
    Acceleration of Bullet: -3.08 m/s2
    Acceleration due to gravity: 9.80 m/s2

    What is the velocity of the bullet the instant it enters the victims head if the gun is held 0.2 meters away?

    /i\Fn=4.9 x 10-1
    <-------------l----->Fp= ? N
    \l/ Fg= 4.9 x 10-1
  2. jcsd
  3. Nov 17, 2004 #2


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    Science Advisor

    I take it your "Acceleration of Bullet: -3.08 m/s2" is horizontal decelleration due to air resistance?

    Horizontally, v= 1500- 3.08 t m/s
    x= 1500t- 1.54t2 m (distance bullet has gone)

    Just as the bullet is "entering the victim's head", x= 1500- 15.4t2= 0.2
    Solve for x to find t.

    Vertically, v= -9.8 t

    Knowing t, you can solve from the horizontal and vertical components of the bullet's velocity and then use Pythagoras to find the speed of the bullet.
  4. Nov 17, 2004 #3
    I'd concider the velocity of the bullet to be 1500 m/s. 0,2 m is way to short a distance for any signifficant change in velocity given a retardation of only 3,08 m/s^2.

    Just don't you go do anything stupid..
  5. Nov 17, 2004 #4
    I'd concider the retardation to be along the direction of the bullets' motion. It's an iteresting problem, still I think there's to little information to say anything more precise than ~1500 m/s
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