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Question on conservation of momentum

  1. Dec 30, 2011 #1
    When you jump on a weighing scale the instant that you hit you're weight is higher than it would be if you were to gently step on on.Can someone give me a proper answer? I think its somehow related to conservation of momentum but i cant think of a proper answer.

    Also,suppose you two balls and you know the mass and the velocity of both and then they both collide then is there any way of knowing there velocities after the collision?
    Last edited: Dec 30, 2011
  2. jcsd
  3. Dec 30, 2011 #2
    The force on the weight from your body is higher initially, because your body decelerates to zero speed. The weight measures the force of deceleration + gravity, and shows higher value till center of mass of the system has stopped, and deceleration has ceased.

    To second: Energy is preserved, kinetic energy is constant, Forces are equal in contact
  4. Dec 30, 2011 #3
    1> Is this right-the scale shows more because the scale must provide the force to decelerate your body and also to negate the effect of gravity.
    2>I didn't understand your answer.
  5. Dec 30, 2011 #4


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    The scale measures the force of gravity pulling your body down towards the Earth. If you are falling onto it then there is more force against the scale than if you were just standing on it. This is called "apparent weight" and is the weight measured if you were accelerating in something such as an elevator. This contrasts with a purely "gravitational weight" which is only measured as the force of gravity on your mass.

    To answer your 2nd question you would need to know what the balls were made of. Two beach balls would lose much more energy from the collision than two billiard balls would since the beach balls deform much more and have lots of air.

    Assuming a "perfect" elastic collision yes, you could calculate the velocities and other properties of the balls after the collision.
  6. Dec 30, 2011 #5


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