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Homework Help: Newtons Cradle

  1. Mar 20, 2010 #1
    We were told to make our own project, i chose to base mine on the conservation of momentum and used the pendulum.
    Though im kind of stuck, i need more things to write about and more equations i can calculate using the pedulum.

    all i have so far is
    H1=0.145m (90dgrs)

    E=0.063945 J


    so thats the velocity of my first ball on 90 degrees.
    and when that ball hit the ball on the other side went up 0.11m (11cm)

    done same equations as before and got v=1.468ms^-2

    so now i can only prove that the velocity changes to 1.468 from 1.685.
    and that its inelastic because it changes, if you know what i mean.
    i need more things that i can work with, more things i can try and find out with the pendulum. and how can i find out how long it will take before the pendulum will come to rest.
    help anyone?
  2. jcsd
  3. Mar 20, 2010 #2

    Gib Z

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    Newton's Cradle exhibits two of the conservation laws: Conservation of momentum, as we easily see by pulling one ball to the side and releasing, and Conservation of Kinetic Energy in an Inelastic collision, which we can observe by pulling up two adjacent balls to the side at once and releasing. Rather than one ball moving at twice the velocity, which would still satisfy the conservation of momentum, two balls swing. Try use the equations for the conversation of kinetic energy in inelastic collisions to show why this must occur.
  4. Mar 20, 2010 #3
    So for kinetic energy.
    The first ball.

    The second ball.

    but i still don't know how i can find the time to when the cradle comes to rest.
  5. Mar 21, 2010 #4
    help please i need to find out how long it will take before the cradle comes to a rest.
    will this help?
    T= 2pi(sqrtL/g)
  6. Mar 21, 2010 #5
    It never stops in the ideal model. Energy is lost in the real model in the collisions, then it stops.
  7. Mar 21, 2010 #6
    Yes i know because its inelastic.
    Though i need to find out how long it will take for an inelastic model to come to rest.
  8. Mar 21, 2010 #7


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    It is not simple to find out when the cradle will stop. Do some more observations. From the difference of the initial and final height you can follow the lost energy at each collision. It is difficult to predict how the energy will change in time, but you can plot the heights in terms of the number of collisions, and you can extrapolate the curve after a few swings to zero energy.

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