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Homework Help: Newtons third law/law of gravitation

  1. Nov 19, 2012 #1
    1. The problem statement, all variables and given/known data

    A comet is affected by a gravitational force due to the presence of the Earth. Later, the comet is ten times farther from the Earth. What is the size of the new gravitational force acting on the comet?

    a) One hundred times larger than before.
    b) Ten times larger than before.
    c) One tenth as large as before.
    d) One hundredth as large as before.

    Based on what I'm given, gravitational force is inversely proportional to the square of the distance (r). So if the distance is 10, don't you just square 10 to get 100 times larger? I'm kinda stuck between a or d I don't know why =p

    Also for the question on the attachment (sorry for the horrendous quality) I can't seem to narrow down the choices given that I don't understand what each choice means. All I know is that newtons third law is equal and opposite force but I can't find a corresponding answer that upholds the statement.

    2. Relevant equations
    Fgrav= GM1M2/r^2

    Attached Files:

  2. jcsd
  3. Nov 19, 2012 #2


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    Do you understand what inversely proportional proportional means? If y is inversely proportional to x then y is proportional to 1/x. So if you double x, what happens to y?
    The equation you quote tells you how strong an attractive force a mass m1 will feel from a mass m2, right?
    The first option observes that if you swap m1 and m2 in the equation you get the same answer. Do you agree with that? What does that imply about the force each mass experiences? Is that what Newton's 3rd law says?
    The second option says the larger mass feels a stronger pull, but because it has more mass that cancels out that the pull is stronger. Do you agree with the first part of that? If so, do you agree with the second part? If so, is that what Newton's 3rd law is saying?
    The third option is the same as the second except that it claims the smaller mass feels the stronger pull.
    The fourth option says that if the masses are different then the two masses will feel different forces.
  4. Nov 19, 2012 #3
    Thanks for clarifying all the answer choices. Crystal clear to me now =]
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