1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
    Gravitational
    3. The attempt at a solution
    By using the principle of conservational of energy.
    1/2mv^2=-GMm/R-(-GMm/4R)
    which is the final point-initial point

    but the solution is
    1/2mv^2=-GMm/4R-(-GMm/R)
    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

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Hello,

    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?
    https://scontent-kul1-1.xx.fbcdn.net/hphotos-xtp1/v/t34.0-12/11922950_10204785470108174_1313158553_n.jpg?oh=f63e57fca18f483818a21a849bed979c&oe=55E67634

    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

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    No need for correction. You are doing fine !
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Gravitational problem
  1. Gravitation Problem (Replies: 3)

  2. Gravitational Problem (Replies: 1)

  3. Gravitation problem! (Replies: 4)

  4. Gravitational Problem (Replies: 1)

  5. Gravitation problem (Replies: 6)

Loading...