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Homework Help: How much force does the train exert on Superman?

  1. Jan 8, 2010 #1
    1. The problem statement, all variables and given/known data

    Superman must stop a 120-kmh train 150m to keep it from hitting a stalled car on the tracks. If the train's mass is 3.6 x 10^5 kg, how much force must he exert? Compare to the weight of the train (give as %). How much force does the train exert on Superman?"

    2. Relevant equations

    We just started this unit but I am not entirely sure which equation to use. I know I must find the amount of time it takes to stop, the deceleration of the train, and the forces from the problem.

    3. The attempt at a solution

    So far I've found the time, 9 seconds, and the deceleration, -3.703 (would it be negative?) I need help from this point on and I know it's relevant to Newton's third law of motion. Any help is greatly appreciated.
  2. jcsd
  3. Jan 8, 2010 #2


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    Homework Helper

    Since you have the deceleration, use Newton's 2nd law and find the force. F=ma.
  4. Jan 8, 2010 #3
    First of all, deceleration includes a negative sign. So it would be negative acceleration or positive deceleration.

    The first step in your answer is to find the deceleration which you have done and it looks correct to me.

    Newton's second law and Newton's third law overlap here. You know that force equals mass multiplied by acceleration. You have an acceleration and a mass so the force should be easy to find. As for Newton's third law. If Superman is applying 'x' amount of force, the train applies 'x' amount of force back.
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