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Homework Help: Help momentum, recoil speed [simple]

  1. Nov 3, 2009 #1
    1. The problem statement, all variables and given/known data
    A bullet m= 9.0 g leaves the barrel of a rifle with a speed = 8.0 x 10 ^2 ms-1 (by the may im sure - is this 80 ms-1 ?)

    The mass of the rifle = 1.8 kg
    The rifle is free to move back
    Calculate the speed of recoil

    2. Relevant equations
    I figured I would need these :

    I'm not sure wether that's all I need.

    3. The attempt at a solution

    Well, first of all I think that the momentum 1 way = the momentum the other way.


    m1 x v1 = m2 x v2

    9 [g] x 80 [ms-1] = 1800 g (1.8 kg right ?) x V2

    720 = 1800 V2

    V2= 720/1800

    V2= 0.4

    Is this correct guys ?
  2. jcsd
  3. Nov 3, 2009 #2
    the way you solve the problem is correct however i think you have a mistake about the speed of the bullet. 8.0x10^2 means the bullet moves with a velocity of 800 m/s.

    according to my calculations the answer is 40 m/s....
  4. Nov 3, 2009 #3
    hmm not sure about that .. maybe I have another mistake

    m1 x v1 = m2 x v2

    9 x 800 = 1800 x v2

    7200 = 1800 v2

    v2 = 7200 / 1800
    v2 = 4

    isnt it 4 ms -1 ?
  5. Nov 3, 2009 #4
    yes the method you have used is correct however as cryphonus said, the velocity is 800 m/s. also grams is not the si unit for mass, kilograms is. so you want to use 0.009kg and 1.8kg as the masses.

    i got an answer of 4 m/s
  6. Nov 3, 2009 #5
    thanks a lot guys
  7. Nov 3, 2009 #6
    Ok; here is how i solved it; initial momentum is
    800 m/s.0.09 kg (mass of the bullet in kilogram);

    which is 72 m.kg/s

    so if the initial momentum is 72 m.kg/s the final value should be equal to 72 also;

    P(final)=1,8kg (mass of the rifle).V(recoilspeed)


    V=40 m/s ,
    it is in the opposite direction of the bullet so if you express the speed of the bullet as a positive value, you have to express the recoil speed as negative

    any corrections are welcome....
  8. Nov 4, 2009 #7
    9 g =0.009kg not 0.09kg
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