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Need help w/ Angular Momentum please

  1. Dec 6, 2005 #1
    Need help w/ Momentum & equation of motion please

    I was completely dumbfounded when i first read the question... How can a momentum problem like this ask for such a specific thing as the angle with which the object departed the ground?

    "At time t = 0, a ball is struck at ground level and sent over level ground. Figure 9-7 gives the magnitude p of the ball's momentum versus time t during the flight. (p1 = 7.0 kgm/s and the vertical axis is marked in increments of 2.0 kgm/s.) At what initial angle above the horizontal is the ball launched?"


    So I've tried finding the area under the graph (integral) to find my KG * m... but that doesn't seem to get me anywhere.

    I've also tried to find the slope to find acceleration (KG * m/s^2), but I don't see how that does me any good either...

    Any help, even how to approach the problem, would be much appreciated. Thank you.
    Last edited: Dec 6, 2005
  2. jcsd
  3. Dec 6, 2005 #2


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    The graph will tell you everything you need to know.

    For example, you can see that the ball reaches maximum height at t=2.25 s, from which you can find the vertical component of the initial velocity.
    Now from the magnitude of the initial momentum you can find the horizontal component as well from which you can find the angle.

    BTW: This problem has nothing to do with angular momentum.
    Last edited: Dec 6, 2005
  4. Dec 6, 2005 #3

    now to find the vertical component of the velocity am i using conservation of energy? .5mv^2 = mgh?

    If so, how do i determine the mass of the object?

    and woops oin the title... is it possible to change it?
    Last edited: Dec 6, 2005
  5. Dec 6, 2005 #4


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    mgh will not help you here, since you do not know how high the ball goes and the kinetic energy at the top is not 0. You need the time variable, so you the equation of motion.

    I think you can change the title. Just click the 'EDIT' button in the first post.
  6. Dec 6, 2005 #5
    oh woops, misread the question slightly: at the lowest point the momentum is 7kg m/s... and at it's highest points it's 15.

    So the equation of motion i have in mind then is:

    vf = vi + a*t

    since that's the only one that disregards distance...

    Now my next question then is how do we figure out what the final velocity is when we dont know what the mass is? or is it possible to determine the mass?
  7. Dec 6, 2005 #6
    Try assuming the mass of the object is 1kg... Since momemtum is p=mv, you now have the initial velocity, 15m/s, and the velocity at the top, 7m/s... Think carefully about the direction of the velocity at the top of projectile's flight launched initially at some angle. A sketch might help. Compare the sketch of the velocity at the top and the initial velocity... also think about the x and y components of the velocities at both locations.
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