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Homework Help: Kinetic energy and normal force

  1. Dec 12, 2009 #1
    1. The problem statement, all variables and given/known data
    You and your sled have a mass of 100 kg and starting from rest, slide down a very icy and frictionless road which is 20 degrees steep and 200 m long.
    a. What is your kinetic energy at the bottom of the hill?
    b. What is your net acceleration?
    c. How long does it take you to slide to the bottom?
    d. What is the normal force exerted on you and your sled?
    At the bottom of the hill you change direction but not your speed and go up a hill which is 30 degrees steep and with friction coefficient .1.
    e. What distance along the hill do you go before stopping?

    I am not sure how to start this one. I think you have to find the speed first since KE=(1/2)mv^2. I think normal force is mgcos(angle)yhat but I dont know how to find yhat. Also is net acceleration the sum of all forces divided by the mass? I'm stuck!
  2. jcsd
  3. Dec 12, 2009 #2


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    Speed is not needed, this is a conservation-of-energy question. You can check your textbook for the relevant equation if you do not know it.
    It's universally agreed that the normal force's direction is perpendicular to the sloping surface (by definition), so it should suffice to give it's magnitude.
    Yes, that is what they are asking for. Personally I dislike the term "net acceleration", an object only has one acceleration even if there are several forces acting on it. Just my opinion.
  4. Dec 13, 2009 #3
    Ohh okay thank you!! I'll work on it!
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