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Force and Energy Question

  1. Jul 7, 2008 #1
    Force and Energy Question!!!

    1. The problem statement, all variables and given/known data
    the force that must be applied to bring it to a stop at point E
    (this is only one part of a multi-part question - the rest of the information: Consider the frictionless rollar coaster. The height at A (the start of the rollar coaster) is 95m, it is downward and then upward to C, at 65m high, then down again and then up again to 25m at D until a downward slope to a stop at ground level, point E.
    [sorry for the written explanation, but a picture needs approval - takes too long lol]

    2. Relevant equations
    Et = Ek + Ep
    W = Fd
    W = Ek

    3. The attempt at a solution
    Et = Ek + Ep
    Ek = Et - Ep
    Ek = Et - 0
    Ek = 11172000 J {calculated in previous question}

    W = Fd
    F = W/d
    F = Ek/d
    F = 11172000 J/ ???

    ** so my dilemma is that it does not give me the distance in the question. Is there any other equations or methods that I could use instead to solve for the force??
    My thanks for your help in advance!!
     
  2. jcsd
  3. Jul 7, 2008 #2
    Re: Force and Energy Question!!!

    Try using the work kinetic energy thm which states that
    [tex]\Delta[/tex]W =[tex]\Delta[/tex]K.E
     
  4. Jul 7, 2008 #3
    Re: Force and Energy Question!!!

    but then how will i find F (force)?? because Ek = 1/2mv^2
     
  5. Jul 8, 2008 #4
    Re: Force and Energy Question!!!

    How did you figure out Ek if you are not given the mass of the object?
     
  6. Jul 8, 2008 #5
    Re: Force and Energy Question!!!

    i am given mass, sorry i forgot to include it, its 12000 kg
    so:

    Ek = Et - Ep
    Ek = 11172000 - mgh
    Ek = 11172000 - (12000)(9.8)(65)
    Ek = 3528000 J
     
  7. Jul 8, 2008 #6
    Re: Force and Energy Question!!!

    ok I see, so you found the potential energy on top of the 95 m hill and then found the kinetic energy on the 65 m hill. You need to do this again for the 25 m hill, and finally find the final speed of the cart.

    hmm, you're not given the stopping distance? Nor the time it takes for the cart to stop? Is there any other information at all?
     
  8. Jul 8, 2008 #7
    Re: Force and Energy Question!!!

    Maybe the stopping distance can be determined from your diagram. Try to upload it or take a closer look yourself.
     
  9. Jul 8, 2008 #8
    Re: Force and Energy Question!!!

    no distance is given and no stopping time but ill upload a picture of the track
     
  10. Jul 8, 2008 #9
  11. Jul 10, 2008 #10
    Re: Force and Energy Question!!!

    What if i were to just use F=mg, because it is a downward slope plus there is no friction acting on it to consider???
     
  12. Jul 10, 2008 #11
    Re: Force and Energy Question!!!

    hmm afraid not because F = mg is the equation for free-fall but the object here is certainly not in free fall and is therefore not accelerating at -9.8 m/s^2
     
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