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Two people in a boat and one person throws a ball

  1. Apr 21, 2015 #1
    1. Two people sit at opposite ends of a boat. Ther person at the front of the boat has a ball and throws it to the person sitting at the back of the boat who catches it.

    What is the motion of the boat at the time immediaately after the ball is thrown and later after the ball is caught ?

    Answer choices.

    After Throw After the Catch
    (A) Boat moves foward. Boat moves foward.
    (B) Boat moves foward Boat moves backward.
    (C) Boat moves foward. Boat does not move.
    (D) Boat moves backwards Boat does not move.
    (E) Boat moves backwards Boat moves foward.

    2. Newton Third Law.

    3. I honestly dont know. What i would say is that that the boat moves foward. After that I am just confused. What actually happens when the person at the back catches the ball? Doesnt he exert a force that us equal and opposite in direction?

    I used my phone and the spacing is messed up. After the throw and Ablfter the Catch should be seperated.
     
  2. jcsd
  3. Apr 21, 2015 #2
    What does Newton's Third Law say?
     
  4. Apr 21, 2015 #3

    haruspex

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    How about a conservation law instead?
     
  5. Apr 21, 2015 #4
    Yes, that is probably a better way to approach the problem.
     
  6. Apr 21, 2015 #5
    If object one exerts a force on object two, then object 2 exerts a force on object one that is equal in magnitude and opposite in direction.
     
  7. Apr 21, 2015 #6
    Conservation Law? What is being conserved here?
     
  8. Apr 21, 2015 #7

    haruspex

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    What external forces act on the boat+occupants+ball system?
     
  9. Apr 21, 2015 #8
    Hmm, would the only external force be the force from throwing the ball?.
     
  10. Apr 21, 2015 #9

    haruspex

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    How is that external to the system?
     
  11. Apr 21, 2015 #10
    Ok. Let me rethink. That would be an internal force then. I think that there is no external force acting on the boat+occupants+ ball system then.
     
  12. Apr 21, 2015 #11

    haruspex

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    Well, no external horizontal force, which is all we need.
    So what conservation laws apply?
     
  13. Apr 21, 2015 #12
    Conservation law. Are you speaking of conservation of kinetic energy?
    Sorry I feel kinda dumb at the moment.......
     
  14. Apr 21, 2015 #13

    haruspex

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    Not that one. Two others to choose from....
     
  15. Apr 21, 2015 #14
    Uhm conservation of momentum maybe?
     
  16. Apr 21, 2015 #15
    Well for every action there is an opposite reaction.....
     
  17. Apr 21, 2015 #16

    haruspex

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    Yes. Apply that to the before throw and after throw conditions.
     
  18. Apr 21, 2015 #17

    haruspex

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    Yes, and conservation of momentum can be derived from that, but for the present problem it's more convenient to invoke the conservation law itself.
     
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