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Homework Help: Two Theoretical Questions about Force (have answers but confused)

  1. Nov 8, 2005 #1
    These are from a test, I missed them both. Could someone explain it to me?

    1. Consider the two statements:

    A) Near the surface of the Earth the force of gravity causes a 2
    kg rock which is in freefall to experience twice the magnitude of acceleration as a 1 kg rock.
    B) Near the surface of the Earth, the force of gravity on a 2 kg rock is twice the force of gravity on a 1 kg rock.
    I put that B was true but A was false...but A is true and B was false.

    If the force of gravity is F=ma, wouldnt twice the mass make the force of gravity of the 2 kg rock twice the force of the 1 kg

    And I thought the magnitude of acceleration doesn't change near the surface of the Earth, that it's 9.8 m/s^2 for objects of all masses. Huh? Is my thinking wrong?

    One more:

    Compare the force needed to throw a 1 kg object horizontally with a given speed on the Moon and on the Earth.

    I put that it would take greater force on Earth, but the answer is that it is the same for both.

    If the acceleration due to gravity is higher on Earth than on the moon, then wouldn't it be harder to throw the ball?

    Because on the Moon you could throw it and it would just sort of gently fly out of your hand and hover forward?

    Sorry, I realize these questions are pretty basic but I want to make sure I understand this for my comprehensive semester test. :yuck:
  2. jcsd
  3. Nov 8, 2005 #2

    Chi Meson

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    You are correct, A is completely wrong. B is correct. THere is not even the slightest doubt about it. Who said A was correct?
    This time "they" are correct. THe trick is the "horizontally." Since gravity acts only vertically, only the inertia of the object matters with horizontal acceleration. THere would be the "real" factor of having to exert more force to hold it up while accelerating it, but ignore hta for now.
  4. Nov 8, 2005 #3
    Gotcha, thanks Chi. I see the reason for the 2nd now.

    The first one..Maybe he graded it wrong. I thought it was right too thats why I was so confused. I'll have to ask my prof. tomorrow.
    anyway, thanks again for the help
  5. Nov 9, 2005 #4

    Chi Meson

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    I want to know how this one ends up. If your Prof said anything other than "Oh no, what a stupid mistake for me to make!" I would doubt anything he has to say for the rest of the class.
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