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Momentum of bouncing ball

  1. Mar 14, 2013 #1
    1. The problem statement, all variables and given/known data
    I throw a ball horizontally

    During the first collsion between the ball and the floor
    Select one:
    a. Both horizontal and vertical momentum of the ball are conserved
    b. The horizontal momentum of the ball decreases and the vertical momentum decreases
    c. The horizontal momentum of the ball decreases and the vertical momentum remains the same
    d. The horizontal momentum of the ball remains the same and the vertical momentum increases
    e. The horizontal momentum of the ball remains the same and the vertical momentum decreases


    2. Relevant equations
    p=mv


    3. The attempt at a solution
    I know that vertical momentum of a ball decreases but I'm not sure if the horizontal momentum decreases or remains constant.
     
  2. jcsd
  3. Mar 14, 2013 #2

    Simon Bridge

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    Please show reasoning.
    What makes you think that vertical momentum decreases?
    Why doesn't the same argument apply to the horizontal?

    Presumably the ball does not gain or lose mass - so something else must change if the momentum is to change. What is it?
    Dividing the motion of the ball into horizontal and vertical components - which direction has the change and when?
     
  4. Mar 14, 2013 #3
    ok well think about it like this. if you drop a ball and it bounces back up its velocity decrease as it goes back up because its distance decreases as it bounces back up. and since momentum=velocity multiply by mass i think momentum decreases.

    Now for the case of the horizontal momentum i just don't know. I think the horizontal distance should decrease thus decreasing horizontal velocity thus decreasing horizontal momentum but that part I am not to sure of because something in my head tells me the distance is content but something else in my head tells me the distances decreases like how a series would by a certain amount each time until it reaches a certain distance travelled. thats why i need help to understand the horizontal part.
     
  5. Mar 14, 2013 #4

    Simon Bridge

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    Not bad ... but I don't know what you mean by "distance decreases". Vertical displacement decreases after the bounce sure, then it increases again as it changes direction and horizontal displacement surely increases (you've seen a ball bouncing along right?)

    Recall that momentum is a vector.
    Looking only at the vertical motion - assume no losses in the collision to make it simple - what is different about the vertical momentum before and after the collision?

    Recall Newton's laws - under what circumstances does momentum change?
     
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