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Questions on center of mass

  1. Jun 30, 2015 #1
    Let's say a kid is playing basketball. He has a running start, jumps and while he is the air he shoots. There are no external forces at the time he shoots (disregard the minimal airdrag). If there are no external forces, shouldn't the center of mass after he shoots be the same as it was before he shoots? This wouldnt be true though becuase both the kid and ball move forward (because we say that he has a running start). My guess would be that the position of the center of mass doesn't necessary need to be the same, but the velocity HAS to be the same. Thus, although after the shot, both the boy and ball move forward, the velocity of the center of mass is the same as it was RIGHT before the kid shot the ball (because the kid moves forward slower but the ball forward faster). Is this correct?

    Another question about center of mass:

    Lets continue with the basketball example. This time lets say that the kid has NO running start. He jumps vertically and shoots. In this case the velocity of the mass and boy before he shoots is 0 (because he has no running start). Becuase the center of mass HAS to be the same as where it was before the kid shoots, the kid HAS to move back because the ball moves forward. This however does not happen in real life... because the kid usually comes back vertically down with no backwards displacement. Furthermore, if you watch professional NBA games, you can see that sometimes as basketball players shoot the ball when they jump, they go back very far. What causes this difference? When I shoot the ball with a jump, I come back vertically down, but when an NBA player shoots the ball, they displace backwards by a tremendous amount.
  2. jcsd
  3. Jun 30, 2015 #2


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    Staff Emeritus
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    Seems correct.
    You don't come back perfectly vertical, it just seems so because of the vast mass difference between the ball and yourself.
    Let's say you weigh 160 lbs. This would make your mass 117 times that of the basket ball. We'll also say that you shoot the ball at a speed of 10 ft/sec. You jump to a height of 1 1/2 feet and release the ball at the top of the jump. Your backward motion will be 1/117 that of the basket ball or ~ 1 in/sec. It will take you ~ 1/4 sec to fall back to the floor from 1 1/2 feet, meaning you will land just about 1/4 in. from where you took off. And that is assuming that you jumped perfectly vertically, with no backward or forward motion at all ( not very likely). In order to see a measurable difference in your landing spot caused by throwing the ball, you are going to have to throw something a lot more massive, or throw the ball very very fast.
  4. Jun 30, 2015 #3
    As Janus said. Plus the NBA player jumps backwards to create space between the release point of the ball and a blocker.
  5. Jun 30, 2015 #4
    oh yeah.... good point.
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