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Homework Help: Gravitational problem

  1. Aug 31, 2015 #1
    1. The problem statement, all variables and given/known data
    A stationary object is released from a point P at distance 4R from the centre of the earth which has radius R and mas M.What is the speed of the object when it hits the Earth's surfacehttps://scontent-kul1-1.xx.fbcdn.net/hphotos-xpt1/v/t34.0-12/11921921_10204785254702789_1979507807_n.jpg?oh=4a62967277ee876cb2de85befad6b89a&oe=55E68D61

    2. Relevant equations
    3. The attempt at a solution
    By using the principle of conservational of energy.
    which is the final point-initial point

    but the solution is
    which is the initial point-final point

    So which 1 is correct i got little bit confused here.Arent we should use the final-initial to find the changes of gravitatinal potential energy?
  2. jcsd
  3. Aug 31, 2015 #2


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    The kinetic energy is positive. Potential energy is negative, so the book solution is correct. The energy balance you want to solve for is:

    ##E_{\rm kin, final } + E_{\rm potential, final} = E_{\rm kin, initial } + E_{\rm potential, initial}## with ## E_{\rm kin, initial } = 0 ##. If you write that out, which one do you get ?
  4. Aug 31, 2015 #3
    Hi, thank you for your explaination :smile:
    another thing that confused me is this question

    The figure below shows a point P which at a distance R from the centre of the Earth, has gravitional potential of -6kJ kg^-1 . What is the change in gravitational potential energy when a 1kg mass is moved from point P to point Q, 2R away from the centre of the earth?

    So Gravitational potential energy at point Q is -3kJkg^-1
    The change in gravitational potential energy is P-Q or Q-P. I get really confused here. since the question above use Inital - final, so i should use gravitational in point P- point Q also?
  5. Aug 31, 2015 #4


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    Better to write down an expression for the ratio of these energies...

    Sorry, too fast. Does the potential energy become less negative ? That is the same as saying it increases, so the difference is positive.

    And in the question above you did not use initial - final but initial = final. See the expression ##E_{...}##
  6. Aug 31, 2015 #5
    Owh so the second question is using final - initial to find up the change?
    -3-(-6)=+3 ?

    correct me if i did mistake :D
    by the way thanks for answer i truthly appreciate:woot:
  7. Aug 31, 2015 #6


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    No need for correction. You are doing fine !
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