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Weight lifter true/false physics question

  1. Feb 6, 2008 #1
    1. The problem statement, all variables and given/known data

    A weight lifter stands on a bathroom scale. He pumps a barbell up and down. What happens to the reading on the scale as this is done? Suppose he is strong enough to actually throw the barbell upward. How does the reading on the scale vary now? (Select all that apply.)

    True Decreases when barbell is pumped up.
    False Increases when barbell is pumped up.
    True Increases when barbell is pumped down.
    False No change when barbell is thrown.
    True Decreases and then increases when barbell is thrown.
    False Increases and then decreases when barbell is thrown.
    False No change when barbell is pumped up and down.
    False Decreases when barbell is pumped down.

    Could someone please double check my answers to this question and let me know if I have any wrong. Thanks....
     
  2. jcsd
  3. Feb 6, 2008 #2

    Dick

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    Science Advisor
    Homework Helper

    You seem to have some concept backwards. When he pushes the barbell UP, he has to exert a force on the barbell greater than it's weight to accelerate it upward. By Newton's third law, the barbell then exerts the same force on him, hence one greater than it's weight. The scale indicates the total of the weight of the lifter and the downward force that the barbell exerts on him. Conversely, to bring the barbell downwards he has to exert less force on it than it's weight. Try rethinking your answers.
     
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