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Homework Help: Biomechanics - Center of Gravity and Projectile Motion - PLEASE help

  1. Dec 7, 2011 #1
    1. My professor explained this concept before, but I didn't quite understand. I need help with understanding how if two players jump for the ball with the exact same mass and body segment dimensions, taking off with the same vertical and horizontal velocity of the CoG, one can have an advantage over the other by reaching with one arm up (and the other arm by her side) while the other one reaches with both arms. I don't even understand the concept, or even know where to start. :( We have to write 2 paragraphs on it using "knowledge of projectile motion and the factors that affect the location of the CoG to explain why the player that has one up arm up has an advantage. Please help me?

    2. I don't know any relevant equations

    3. All I remember him saying is that CoG goes up when arms are raised. Also, there is more force being used because there is less mass? I don't even know. Honestly, that is where I am starting. :(
  2. jcsd
  3. Dec 7, 2011 #2


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    The forces are the same, and the height H the centre of gravity will reach is the same for both players: where the potential energy mgH equals to the initial kinetic energy, 1/2 mv^2 (v is the initial velocity of the player jumping up, and m is his mass.) The position of the CoG depends on the distribution of mass. If both arms are down, it is somewhere in the height of the belly, and rises if one or both arms are up.

    Imagine that the CoG is at the stomach when one arm is raised and at the chest when both arms are up. When both players jump up, the stomach of the first player reaching out by one arm is at the same height as the chest of the other one. Whose fingertips reach higher?

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