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Pe and Ke problem

  1. Nov 5, 2012 #1
    A block of mass m and height h slides without friction up a hill rise of height (3/2)h as shown. In order to make it to the top of the hill, the block must have a minimum initial speed of:



    Pe+Ke = Pe(f) + Ke(f)



    I tried figuring out what the velocity of the block would be if it had slid down the hill. I was assuming it would require that amount of velocity to go back up the hill (maybe I assumed wrong). The answer listed is (3gh)^1/2 However my answer was (2gh)^1/2. The block has a height h but I don't understand why the size of the block should matter. Nor do I have any idea of how to incorporate that fact into my equation, please help.
     
  2. jcsd
  3. Nov 5, 2012 #2

    SammyS

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    Please don't use a bold font when posting a thread.

    You're right that the height of the block doesn't matter.

    Check you algebra.

    Show your work.
     
  4. Nov 5, 2012 #3
    Um, when you start a thread it says (without the forward / in the beginning):

    [/b]1. The problem statement, all variables and given/known data[/b]



    [/b]2. Relevant equations[/b]



    [/b]3. The attempt at a solution[/b]

    So, the site is asking me to put it in bold, that's why I did it.

    In any case here is my algebra in steps:

    mgh = 1/2mV^2 (mass'es cancel)
    gh=1/2V^2 (multiply both sides by 2)
    2gh=V^2 (square both sides)
    V=2gh^1/2
     
  5. Nov 5, 2012 #4
    The answer suggest that it was raised to a height of 3/2 h. So the bottom of the block was raised up to the top of the hill then.
     
  6. Nov 5, 2012 #5

    SammyS

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    No. The site assumes that you leave its titles alone and in bold, then after the title you put in your text -- not in bold.

    As follows:

    1. The problem statement, all variables and given/known data
    A block of mass m and height h slides without friction up a hill rise of height (3/2)h as shown. In order to make it to the top of the hill, the block must have a minimum initial speed of:

    2. Relevant equations
    Pe+Ke = Pe(f) + Ke(f)

    3. The attempt at a solution
    I tried figuring out what the velocity of the block would be if it had slid down the hill. I was assuming it would require that amount of velocity to go back up the hill (maybe I assumed wrong). The answer listed is (3gh)^1/2 However my answer was (2gh)^1/2. The block has a height h but I don't understand why the size of the block should matter. Nor do I have any idea of how to incorporate that fact into my equation, please help.
    I fully understand why you did post in bold.
    The hill has a rise of (3/2)h so that should be

    mg((3/2)h) = 1/2mV^2 ,

    so all will work out fine.
     
  7. Nov 5, 2012 #6
    Right I noticed what I did wrong when I was going to sleep, I must have been tired. Thanks for the help.
     
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