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Homework Help: AP physics question about velocity/acceleration

  1. Sep 13, 2009 #1
    Hi everybody,

    I had 2 HW questions that I didn't get for my physics class, and I would really appreciated if you could explain them to me. They're pretty simple, I think, but the way they're worded makes it difficult to picture.

    1) Two trains, each having a speed of 30 km/hr, are headed at each other on the same straight track. A bird that can fly 60 k /hr flies off the front of one train when they are 60 km apart and heads directly for the other train. On reaching the other train it flies directly back to the first train, and so forth. ( We have no idea why a bird would behave this way). What is the total distance the bird travels?

    2) If a particle's position is given by x= 4mp - (12m/s)t+(3 m/s^2)(t^2), what is its velocity at t1 = 1 s?

    For this problem, the correct answer is -6m/s (answers on the back of the book) but I don't know how to get that.

    Thank you so much for your time!
  2. jcsd
  3. Sep 13, 2009 #2
    Imagine it like this, there is a gap between the trains that gets smaller and smaller untill they collide. The bird can keep flying in this gap untill the trains collide, when they collide there is no gap and so the bird can't fly anymore. We know how fast the bird can fly, so if we can work out how long it is able to fly in the gap we can work out how far it has travelled. So we need to calculate how long the gap exists for, or in other words at what point in time the trains will collide. Once you work out the timeyou can work out how far the bird was able to fly, because we know how fast the bird was flying :)

    For the second question you need to use calculus. Remember that dx/dt, the change in distance with respect to time, gives us velocity? You should be able to work it out from that.
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