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When a diver enters a pool after a dive and stops

  1. Oct 20, 2003 #1
    When a diver enters a pool after a dive and stops in the water after 5 meters, how is the work/energy theorm set up?

    I said Wnet = Change in K, so
    Wg + Q = K,

    where Wg is the work done by gravity, mgh, and Q is the work done by the resistance of the water. Are Wg and Q negative (-Wg - Q = K), since they are in the opposite direction of motion, and oppose the divers motion when he hits the water? Or can they be added becuase they are both negative work? Oh, and is positive or negative 5 meters used in the calculations?
     
    Last edited by a moderator: Feb 6, 2013
  2. jcsd
  3. Oct 21, 2003 #2

    russ_watters

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    Staff: Mentor

    As long as you keep them consistent it doesn't matter what is positive and what is negative for the forces. For the sake of logic, I'd use a force working against you as being negative. And distance is an absolute value, its always positive. So your Q will be a negative number, and you add it.

    Also, since the diver stops, he has no kinetic energy and since the surface of the water is our zero altitude, no potential enegy, so k=0.

    Also, what about the work done by the diver's legs? Are you assuming a stiff-legged fall forward.

    Anyway:

    Wg+Q=K=0
    Remember, putting a negative sign in front of a variable doesn't make the variable negative - in fact it doesn't tell you anything at all about the sign of the variable. It just means in your equation you are reversing its sign.
     
  4. Oct 21, 2003 #3

    HallsofIvy

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    Staff Emeritus
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    What is negative and what is positive depends upon how you set up your coordinate system. Assuming you set up so that positive is upward, then the force of gravity is negative because it is downward- not because it is "the opposite direction of motion". You can say that the resistance of the water is negative because it is opposite to the motion but that's a "factor" that has to be multiplied by the speed through the water (no speed, no resistance), not the force itself. Since the dive is downward, the resistance force itself is (negative times negative) positive. The two forces cancel- that's why the diver comes to a stop.
     
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