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Posers and Puzzles

Posers and Puzzles

  1. Subscriber joe shmo On Vacation
    Strange Egg
    26 Mar '09 02:52 / 1 edit
    So my instructor holds a bowling ball up to his face , the ball attached to the cieling by a chain, lets it go ...then points out that if momentum is a conserved quantity "where does it go if at the extreems of its motion its momentum is 0"

    I would say it goes into changing the PEg ( gravitational potential energy)

    i would like to look at this mathematically

    So i state that

    Change in KE= Change in PEg

    1/2*m*V^2 = m*g*h

    Now in order to relate momentum to this I'll take the derivative of both sides with respect to velocity

    m*V = d ( m*g*h)/dV

    so in words, momentum (in this instance) is the rate of change of potential energy with respect to velocity...

    now its the " with respect to velocity" that Im having trouble grasping as a physical concept

    Im thinking that this means that the only way its potential energy can change is if it experiences a change in velocity ( or vice versa) This seems logical to me, but I'm not quite sure if that is in fact how that quantity could be described and or will my instructor buy it..

    perhaps it doesnt show anythig at all, I dont know

    any thoughts appriciated

    Eric
  2. 26 Mar '09 11:28
    I would say that when the total momentum of the system ball+earth is conserved. When the ball swings to the left, the earth moves (very slowly) to the right, reaching maximum speed when the ball stops at the leftmost point. When the ball starts moving to the right again, the earth's speed changes accordingly. In short, the momentum is conserved IN A CLOSED SYSTEM. But the ball isn't a closed system - it is attached to the earth.
  3. 26 Mar '09 13:26
    Originally posted by David113
    I would say that when the total momentum of the system ball+earth is conserved. When the ball swings to the left, the earth moves (very slowly) to the right, reaching maximum speed when the ball stops at the leftmost point. When the ball starts moving to the right again, the earth's speed changes accordingly. In short, the momentum is conserved IN A CLOSED SYSTEM. But the ball isn't a closed system - it is attached to the earth.
    ...is the right answer.
  4. Subscriber joe shmo On Vacation
    Strange Egg
    27 Mar '09 10:55 / 2 edits
    Originally posted by David113
    I would say that when the total momentum of the system ball+earth is conserved. When the ball swings to the left, the earth moves (very slowly) to the right, reaching maximum speed when the ball stops at the leftmost point. When the ball starts moving to the right again, the earth's speed changes accordingly. In short, the momentum is conserved IN A CLOSED SYSTEM. But the ball isn't a closed system - it is attached to the earth.
    I have trouble coming up with ideas like " the earth moves, because of the ball" The concept is simple, yet it is so displaced from day to day experience that it totally escapes my thought process! i guess the key to physics is to live within your mind, not within your world?

    How long do you suppose it took Newton to conceptualize the earths momentum changing as a result of pendulum?

    And, If the earths momentum changes as a result of the ball swinging does the momentum of the galaxy, universe...ect. change?

    well thanks for the help
    Eric