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Homework Help: Which equation do I use to find time to a certain speed?

  1. Nov 1, 2005 #1
    The question
    A train is at rest and it accelerates to a velocity of 100 meters per second. How long does it take to reach that speed?

    I can do the work, I just don't know what equation to use.

    I know the constant force being created by the train's engine (10,000 Newtons)
    I know the mass of the train (5,000 Kg)
    I can find out the drag force working against the train at 100 meters per second by using the Drag equation at Wikipedia D=.5pV^2ACd = 150 Newtons


    Force ----> (object @ 100 M/S velocity) <----Drag+Mass

    The thing that kind of confuses me is that drag isn't constant from 0 to 100 M/S. It's steadily increasing. I know this isn't a hard one but it is for me.


  2. jcsd
  3. Nov 1, 2005 #2


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    Homework Helper

    The drag would depend on the shape of the train. A race car would have less drag than a truck, even if they had the same mass. In questions like this you normally don't need to take it into account.

    Calculate the acceleration of the train, then calculate how long it will take to reach 100m/s.
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