12 thoughts on “Homework #9”

  1. rdetroy

    So I am getting stuck on part F of PSS 9.1. I don’t understand why I cannot use the time given for the y-component (0.3 s) and apply it to the x-component of the kinematics equation using the final velocity obtained from part E. Just let me know if you have any hints or if I should be looking at this problem in a different way.

  2. rdetroy

    I have question on written hw #9. When finding the initial velocity of the cart and people using the intial momentum obtained from the two balls, is the mass of the balls included in the total weight of the cart? Also, in order to find the final velocity after the balls have been caught, do we use the total mass provided or do we include the mass of the balls? I

  3. physics131-willocq Post author

    For the written problem, consider the system as consisting of 3 parts:
    1. the ball thrown by James,
    2. the ball thrown by Sarah,
    3. the cart + James + Sarah.

    Require that the total momentum of the system is conserved before the balls are thrown, during the flight of the balls, and after the balls are caught. The total momentum does not change.

  5. epickhol

    I am stuck on problem 9.6. I’m not sure how to attack the problem with the given information. Can anyone give me a little direction?

  6. physics131-willocq Post author

    Do you mean problem 9.60? I explained how to approach it in class on Friday. This is similar to the momentum problem 2 from the lecture notes. You need to work backwards.

    This means that you first solve the projectile motion part of the problem to find the velocity of the rocket+box system going over the cliff. After you have that you use momentum conservation to find the initial rocket velocity. From the vertical drop you should be able to figure out the time it takes to drop, which is the same time it takes to travel 30 m across horizontally, thereby giving you the velocity of the rocket+box system when it is above the edge of the cliff.

  7. cmachaby

    I am also stuck on problem 9.60. I am looking at the momentum problem #2 from class to try to figure it out, but I am getting lost. I can’t figure out why v b+w is 3.96. The notes online skip this step, and I wasn’t able to write down this intermediate step entirely in class. Could you please explain how to find v b=w in this problem? Thank you.

  8. physics131-willocq Post author

    What is similar with the example I showed in class is not the details but the fact that we need to first use kinematics to find the combined velocity of the rocket+box system and then apply momentum conservation to find the initial rocket velocity.

    The v_b+w = 3.96 m/s from the class example came from the solution of the kinematics part of that problem.

  9. jkern

    For part E on question 9.1, I can’t seem to get any answer that fits. Perhaps I am not approaching it correctly. For the magnitude of the horizontal momentum I got 6.45 kg*m/s. I’m not sure what equation might help with this.

  11. jkern

    Yes I understand what it is asking for, what confused me is why when I used the hint it asked for the momentum but then left out how I can use that to find the speed.

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