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Homework Help: Force and Motion Problem

  1. May 20, 2014 #1
    1. The problem statement, all variables and given/known data

    A rifle bullet leaves the muzzle of a gun at 800. m/sec. The rifle barrel is 80. cm long and the mass of the bullet is 20.g. What force was needed to accelerate the bullet?

    2. Relevant equations


    F(Total)= ma

    F= mg

    F = micrometers (m)(g)

    3. The attempt at a solution

    t= 80 [itex]\div[/itex] 800 = .1s

    a= 800 - 0 [itex]\div[/itex] .1 = 8000 m/s^2

    The answer is provided on my sheet as 8000 N, but I don't have a clue as to how to do that. Its the closest I've gotten to the answer.
  2. jcsd
  3. May 20, 2014 #2

    Simon Bridge

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    Homework Helper

    Welcome to PF;
    The equation you used was for time assumed a constant speed along the barrel - but if the bullet travels at a constant speed, then the force must be zero.

    if you imagine the bullet has constant acceleration from rest to 800m/s along the length of the barrel, then you use a kinematic equation to find the acceleration.

    note: it is best practice to complete the algebra before using the numbers.
  4. May 20, 2014 #3
    Thanks for the help. Did you get the answer "8000 N" just like I did this time around?
  5. May 20, 2014 #4

    Simon Bridge

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    Homework Helper

    I do not, as a rule, actually complete the problems here.
    That's your job ;)

    You'll find that the answer is not as important as your understanding of the physics behind it.
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