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Homework Help: 2d Momentum Problem Physics 30

  1. Sep 18, 2016 #1
    1. The problem statement, all variables and given/known data
    Question is on the attachment photo.

    2. Relevant equations

    I tried to create an equation but I didn't know the momentum for each of the people in the scenario because none of them have a given speed.



    3. The attempt at a solution

    I just need the velocity of Superman to solve the problem. How do we find the speed?
     

    Attached Files:

  2. jcsd
  3. Sep 18, 2016 #2

    haruspex

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    How far will Lois have fallen when she is caught?
     
  4. Sep 18, 2016 #3
    It has to be the difference between the 2 heights. 13.2 - 6.8.
     
  5. Sep 18, 2016 #4

    haruspex

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    So what is Lois' velocity when they meet?
     
  6. Sep 18, 2016 #5
    I don't know, I need time to figure that out
     
  7. Sep 18, 2016 #6

    haruspex

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    When you say you need time, do you mean you think you need to know how much time elapsed, or merely that you cannot work it out immediately?
     
  8. Sep 18, 2016 #7
    Like I need the time to figure out the velocity from the formula d/t
     
  9. Sep 18, 2016 #8
    Yes how much time was elapsed because there's no mention of time in the problem
     
  10. Sep 18, 2016 #9

    haruspex

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    For motion under constant acceleration, there are five standard variables (usually known as SUVAT, s for displacement, u for initial speed, v for final speed, a for acceleration, t for time). There are correspondingly five equations. Each equation omits one of the five variables. Are you familiar with these?
     
  11. Sep 18, 2016 #10
    Actually no, sorry I haven't learned these
     
  12. Sep 18, 2016 #11
    Oh wait, Lois is falling with gravity, so we know acceleration. Now I can figure out time
     
  13. Sep 18, 2016 #12
    I'll try to figure it out and get back to you thanks
     
  14. Sep 18, 2016 #13

    haruspex

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    Right. I suggest you Google SUVAT. Plenty of references. Or, in this case, you can use work conservation.

    Oh, and you do not need to find the time. You can get the speed directly.
     
  15. Sep 18, 2016 #14
    Oh alright. Thanks
     
  16. Sep 18, 2016 #15
    Hi, I got 16.1 m/s for Lois velocity. How do I find superman's initial velocity?
     
  17. Sep 18, 2016 #16

    haruspex

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    Stick with vertical motion for now. You know the vertical speed of each just before they meet. Can you find their vertical speed just afterwards?
     
  18. Sep 18, 2016 #17
    Yes I will try
     
  19. Sep 18, 2016 #18
    Hi again. I tried my best to solve it but I'm not getting the right answer. Can you possibly give it a shot yourself? Or just tell me another hint on how to solve it. Thanks
     
  20. Sep 18, 2016 #19

    haruspex

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    Please post your working.
     
  21. Sep 18, 2016 #20
  22. Sep 18, 2016 #21
  23. Sep 18, 2016 #22
    Then I found superman's velocity using the time for Louiss to reach down to Superman and the length of the pit 1474246781661.jpg
     
  24. Sep 18, 2016 #23
    Then I put it into a momentum equation initial and final, solved for vx and vy and total velocity 1474247044125.jpg
     
  25. Sep 18, 2016 #24
    Answer I got was 7.37m/s
     
  26. Sep 18, 2016 #25

    haruspex

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    I cannot follow what you did there, not only because it was upside down and hard to read. (The posting of images is intended for diagrams and textbook extracts. Please take the trouble to type in your working. This makes it both easier to read and easier to make specific comments on.)
    What is going on in that first line? Why are you multiplying Superman's mass by 9.8 in a line titled X:?
    In which line are you applying conservation of vertical momentum?

    By the way, in calculating Lois' vertical speed you had a negative value inside the square root, so you just ignored the minus sign. To be consistent with 'positive up' you should have set Lois' displacement as -6.4m. Then you would have had a positive value to square root.
     
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