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Question about Gravitation

  1. Oct 11, 2011 #1
    Ok so my physics teacher was telling us about the gravitation and he gave us an example of two people standing and he said that these two people are attracting each other. Then he calculated the gravitation force between them. It was a clear thing for me until this.
    but then he said that they both don't move from their places because they are pulling each other with an equal force and these both forces cancel the effect of each other. I am saying that they don't cancel out as they are attracting each other. But this force between them is so small that they are overcome by other forces like force of friction,air resistance etc. That's why they don't move from their places. because the same thing happens when we jump from a building and we are attracted towards the centre of the earth.
    Can anybody tell me who is right and who is wrong?
    And if i am right, do give some examples also.
  2. jcsd
  3. Oct 11, 2011 #2

    Doc Al

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    You were right and he was wrong. Very wrong. (Actually, a bit hard to believe he really said that.)

    When things exert forces on each other (per Newton's 3rd law) those force act on different bodies and never directly 'cancel' each other. The acceleration of an object is determined by all the force on the object and Newton's 2nd law. Were the two people in space, where no other forces (such as friction) acted, then they would accelerate towards each other.
  4. Oct 11, 2011 #3


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    You are smarter and more sensible than your teacher, it seems.

  5. Oct 12, 2011 #4
    were the 2 people touching each other?
  6. Oct 12, 2011 #5
    Honestly it does sound like something a teacher would say. I've had techers spew some crazily wrong information.
  7. Oct 12, 2011 #6


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    achoo, welcome to Physics Forums!
    Yes, you are right, those two folks do not move towards one another because the gravitational attraction between them is so tiny.

    Newton's law of universal gravitation states that every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

    Just memorize this and "stick to your guns" by relying on the Laws. And Good Luck to you!
  8. Oct 12, 2011 #7


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    The equal & opposite forces just means that the combined center of mass of the two masses doesn't accelerate. The individual masses will of course accelerate towards each other.
  9. Oct 12, 2011 #8
    Welcome! :smile:
    Perhaps he forgot to tell, but more likely you did not recall, that people who are standing are withheld from accelerating towards each other by the ground on which they stand. That force (indeed from friction) is equal to the force with which the person is attracted, so that that person keeps standing at the same place. Those forces "cancel" out in the sense that the person does not accelerate. Perhaps it's better to say that they compensate each other.

    Note that air resistance only can slow down the motion, it cannot stop it - as your example also illustrates!
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