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Homework Help: Linear/angular Momentum / tripping / conservation

  1. Mar 30, 2014 #1
    1. The problem statement, all variables and given/known data

    An rectangular object(mass m) sliding on a horizontal plane (surface is frictionless), with a speed V, object height H.
    then hits a rectangular obstacle with height h. V is large enough to cause tripping

    2. Relevant equations

    what is the relation between linear momentum / angular momentum / conservation of momentum

    3. The attempt at a solution

    is this what's happening; the object has a momentum of P=mV.
    then an impulse I=t*F is applied to the object by the obstacle.
    This impulse reduces the linear momentum to P'=m*V-F*x
    ---This m*V-Fx gives the angular momentum(with respect to the point of contact) causing the rotation?---

    I am not sure if my thinking is correct, especially my last statement.

    An alternative is; taking initial mV as an angular momentum with changing radius wrt the contact point??? ie not mentioning linear momentum at all?

    Thanks in advance.
  2. jcsd
  3. Mar 30, 2014 #2


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    It won't be useful to introduce F or t. Just leave it as I.
    I provides the angular momentum.
    To solve the problem, you need to use a fact concerning subsequent motion of the block.. like, some point that is stationary immediately after impact.
  4. Mar 31, 2014 #3
    Thanks a lot haruspex.

    x was wrong, well spotted. It should be 't'.

    what I am trying to do is derive a formula to show the affect of speed on the tripping.

    Assuming the object is a car and the obstacle is a concrete block; I would like to find out what's the speed above which the car will trip. So the top of concrete can be the reference point.

    I know the mass, I know the object height and center of gravity and obstacle height. I am still struggling to figure out writing the equation. Not sure if I need to use momentum equations or energy equations.
  5. Mar 31, 2014 #4


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    You need to pick the one that's conserved. There is an unknown impulse at the point of impact, and it clearly does not conserve work. The best, then, is to take angular momentum, picking a reference point that makes the unknown impulse have zero moment.
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