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Math Help - momentum problem

  1. #1
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    momentum problem

    In a football game, a receiver is standing still, having just caught a pass. Before he can move, a tackler, running at a velocity of +4.0 m/s, grabs him.

    The tackler holds onto the receiver, and the two move off together with a velocity of +3.0 m/s.

    The mass of the tackler is 110 kg. Assuming that momentum is conserved, find the mass of the receiver.
    in kg.

    i tried to solve this like this:

    3.0 m/s = m1 * 4.0 m/s divided by 110 kg

    and i got 82.5 which is not correct.

    please help me with what i've done wrong please.
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  2. #2
    Junior Member Spimon's Avatar
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    Do you have the correct answer? My answer seems too low..unless this particular game of football is between large men and small children... I got 36.6kg

    Momentum = Mass * Velocity

    M1 = 110 * 4 kg/ms
    M1 = 440

    Law of conservation of energy in a closed system means momentum must remain constant.

    M2 = M1
    440 = M2 * V2
    440 = 3(x+110) where x is the unknown players mass)
    x = 36.6kg
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  3. #3
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    hey 36.6 kg was correct.

    it must be small men playing.

    thankyou for the equations and solution.
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  4. #4
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by Spimon View Post

    Momentum = Mass * Velocity

    M1 = 110 * 4 kg/ms
    M1 = 440
    What??

    What is M1? Surely not the mass. The only thing I can think of is that you are referring to the total momentum, but the standard symbol for that is "P."

    Quote Originally Posted by Spimon View Post
    Law of conservation of energy in a closed system means momentum must remain constant.
    Again, what???

    The total momentum of a system is conserved if there is no net external force acting on the system. Done. Finito. It has nothing to do with energy conservation.

    Quote Originally Posted by Spimon View Post
    M2 = M1
    440 = M2 * V2
    440 = 3(x+110) where x is the unknown players mass)
    x = 36.6kg
    Again, again, what???

    In line 1 above you seemed to be using M as a total momentum. But in line 2 you are using it as a mass?

    This whole post is sloppy and barely coherent. PLEASE make sure you know how to say what you mean to say before you post a solution! The fact that you got the correct answer means you know what you are talking about, which is good. But you need to be able to communicate what you are doing clearly.

    -Dan
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  5. #5
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by rcmango View Post
    In a football game, a receiver is standing still, having just caught a pass. Before he can move, a tackler, running at a velocity of +4.0 m/s, grabs him.

    The tackler holds onto the receiver, and the two move off together with a velocity of +3.0 m/s.

    The mass of the tackler is 110 kg. Assuming that momentum is conserved, find the mass of the receiver.
    in kg.
    Let the +x axis be in the direction that the tackler is initially moving in. Initially the receiver of mass m_r is not moving, so his/her momentum before the collision is 0 kg/m s. So the tackler has all of the momentum at the beginning of the problem. After the collision, they move together as one object at the same velocity.

    So. No net external forces acting on the players, so the momentum of the system of two players is conserved:
    P = P_0

    (m_r + m_t)v = m_tv_t

    (m_r + 110)(3) = (110)(4)

    Solve for m_r. I got 36.7 kg.

    -Dan
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  6. #6
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    thanks for the solutions, seeing this multiple ways just makes it easier for me to understand it more thoroughly.

    Thanks again.
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  7. #7
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by rcmango View Post
    thanks for the solutions, seeing this multiple ways just makes it easier for me to understand it more thoroughly.

    Thanks again.
    My method and Spimon's method are the same. It is just that my work was far more organized and lacked the ambiguity of his.

    -Dan
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