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Homework Help: Question on different masses striking balls

  1. Jun 17, 2007 #1
    Everybody knows that if a ball hits an equal mass ball the one that was moving will stop and the other on which was stopped will move, this is the funtioning of newtons craddles.
    My question is rather simple:
    If a 1kg billiard ball at a speed of 1 m/s hits a 2kg still ball, with what speed will rebound the 1 kg ball? what speed will acquire the 2kg ball? what formula have you used to solve this problem?
  2. jcsd
  3. Jun 17, 2007 #2
    No external forces => linear momentum is conserved. If it's elastic, then kinetic energy is also conserved. You can derive the necessary formulae from these facts.
  4. Jun 17, 2007 #3
    Well i suppose its elastic because the balls rebound and there are neither external forces.
    I understand that the moving ball having just half mass than the other it is just able to transfer half of the linear momentum keeping the other half in the rebound.
    then the answer would be the 1 kg ball rebounds with a speed of 0.5 m/s and the 2 kg ball would acquire a speed of 0.25 m/s, is this correct?
    What im unable is to figure it out taking into account conservation of energy, just to make sure i did my homework right.
  5. Jun 17, 2007 #4
    I would really like to have a number as an answer to my question. The problem our teacher asked us to solve have different more dificult numbers so I will still have to do the work, I put easy numbers to understand it and you could easily solve it. Besides my teacher wouldnt be happy if all i showed him of my work was a number without prior steps
    So please is it 0.5 m/s and 0.25 m/s the answer to my question if not what numbers?
  6. Jun 17, 2007 #5


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    Homework Helper

    Do not make guess that half of the momentum will be given to the other ball.

    Take the velocity of the two balls after collision as v1 and v2 and then write the equation according to law of conservation of momentum.

    Other equation between the two unknowns V1 and V2 will come from law of conservation of energy.

    solving the two equations you will get actual values of v1 and v2.

    First go through the text for collision, this will help you and I am sure there will be some example also.
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