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Forces on an object falling to earth Question

  1. Oct 24, 2007 #1
    1. The problem statement, all variables and given/known data

    As an object falls toward earth

    a. the object does not exert a force on earth
    b. the object exerts a downward force on earth
    c. Newton's third law does not apply
    d. the upward acceleration of earth is negligible because of its large mass

    2. Relevant equations


    3. The attempt at a solution

    I chose A, but the answer for this question is d. Can someone explain how earth accelerates?
  2. jcsd
  3. Oct 24, 2007 #2
    Two bodies in space always exert a gravitational force on each other, F = (G*m1*m2)/r^2 which pulls each body towards the other... this is newton's third law, the postulate of equal/opposite forces... if the earth exerts on a gravitational force on a falling object, that same object must reciprocate and pull on the earth as well. this force is just far too weak for the earth to accelerate appreciably
  4. Oct 24, 2007 #3


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    Newton's third law. The object exerts a force on the earth equal and opposite to the force the earth exerts on the object. Now reconsider d). Why doesn't the earth jump up to meet the object?
  5. Oct 24, 2007 #4
    ok, I understand now. I forgot gravity also had an opposite force.

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