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!

Homework Help: Finding jumping height on an unknown planet given Mass/Radius

  1. Jul 30, 2012 #1
    1. The problem statement, all variables and given/known data
    Knowing you can jump about 1m high on Earth's surface, how high can you jump on the unknown planet.
    Munknown= 4.19*10^21kg
    Radius Unknown= 1*10^6m

    2. Relevant equations
    Not sure if can be used in this question
    K1+U1 = K2 + U2
    1/2MV2 + mgh = 1/2MV2 + mgh
    U=-GM/r and U = mgh

    3. The attempt at a solution

    I found the force of gravity on the unknown planet using GM/R2
    Giving me 0.28m/s2
    Can I just equate the two equations for Gravitional Potential energy (U) to find the new height? as in -GM/r = mgh?
    Last edited: Jul 30, 2012
  2. jcsd
  3. Jul 30, 2012 #2
    If you were to do that, you are saying that the initial energy on each planet is the same.
    Is it?
  4. Jul 30, 2012 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Hello rkiecaboose. Welcome to PF.

    In the rules of this Forum, there are 4 paragraphs regarding Homework Help. The second paragraph reads as follows:
    You MUST show that you have attempted to answer your question in order to receive help. You MUST make use of the homework template, which automatically appears when a new topic is created in the homework help forums. Once your question or problem has been responded to, do not go back and delete (or edit) your original post.​

    These Guidelines are to help us give you better help that you might get without them.

    How do the mass & radius of Planet X compare to those of Earth?

    Do you know Newton's law of universal gravitation ?

    Added in Edit:

    I see that you did add some information after I quoted the OP.
  5. Jul 30, 2012 #4
    Sorry I posted the question initially while I went and grabbed my work.
    Newtons universal law of gravitation...
    Though I don't know where I would use it.
    Am I on the right track using conservation of energy but instead of using mgh use -GM/r ?
  6. Jul 30, 2012 #5
    Well if we're considering the initial point to be grounded wouldn't the initial just be 0 on both planets? I'm lost as to how to relate Earth to the unknown
  7. Jul 30, 2012 #6
    Were does the energy that makes a person jump come from?

    Does this source maintain its full potential when transferred to a planet with different gravity?
  8. Jul 30, 2012 #7
    I think the energy stored in one's muscle must be the same everywhere.
  9. Jul 30, 2012 #8
    So...can someone tell me if this is correct?
    Since I know the gravitational potential energy (U) is mgh on earth. It gives me 490J assuming a mass of 50kg. I used the same equation U=mgh, solving for h gives: U/mg=h.
    Since the gravitational acceleration is GM/R2 the acceleration on the new planet is 0.28m/s2. I just plugged it in to give me a jump height of 35m.
  10. Jul 31, 2012 #9
    You can check about Apollo astronauts jumping on the surface of the moon.
    Conservation of energy applies everywhere.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook