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Movement on foreign planets

  1. Oct 22, 2006 #1
    A colonist on a distant planet with a radius 0.92 times the earth's is pulling on a box of mass 12 kg across the floor. The coefficient of static friction between the box and the floor is 0.70. He has to pull with a force of greater than 73N to start the box moving.
    a) Determine the mass of the planet.
    b) Determine the acceleration due to gravity of an object in free fall near the surface of the planet.
    c) On the earth, if the colonist were to throw the box directly upward as hard as he could, it would reach a maximum height of 6.3m. How high would the box go if he threw it on this planet?
    d) What is the total time this box would be in the air? How does this compare to the time in the air if thrown upwards on the surface of the earth?

    Here are my answers, but I am not sure about all of the answers.
    Not sure here
    Really not sure here
    d)2.269 and for the second part i have no idea what's flying.

    Thanks for all your help in advance
  2. jcsd
  3. Oct 22, 2006 #2


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    I did not check a) because I don't remember the mass of the earth and I don't know what you did to get it. b) looks OK. c) does not. If b) is correct it should go higher on the foreign planet. For d) think about it a bit more after you get c) You should be able to find how long it takes an object to reach maximum height when you know the maximum height.

    By the way, all of your answers are wrong until you supply the units.
  4. Oct 23, 2006 #3
    I know I have the units and thanks for spotting the stupid 5.6 mistake. I'll get back to you tonight to tel if I got it.

    By the way mass of planet is gotten from Fg=GMm/r^2
    Where in this caser I will know Gmr and Fg
    here is 73/0.70
    and in the end M should come out to what I said.

    Thanks again
  5. Oct 23, 2006 #4


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    That's fine. You could also deduce the ratio of the mass of the planet to the mass of the earth from the ratio of the normal force on the planet to the normal force on earth since you know the ratio of the radii. There is often more than one way to get there.
  6. Oct 29, 2006 #5
    ok new issue with this question my time comes out to be 2.556 seconds, it seems that the man is throwing it so it moves parabolically, some of my friends have different answers so if u could help that'd be great
  7. Oct 30, 2006 #6


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    The problem says straight up. I don't think parabolic motion is an issue. What may be an issue is interpretation of "as hard as he can". One might assume that means the same initial velocity, but I suspect that is not what it means. If the man can apply a certain amount of force for a certain distance on earth, he should be able to do the same on the planet. I would assume he does the same amount of work on the object in either case. It then becomes a question of energy, which is fortunate becaue you don't know how high the release point is anyway so knowing the release velocity would not be enough to solve the problem.
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