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B About gravitational attraction

  1. Dec 7, 2016 #1
    say i were standing next to something very massive, like a battleship in drydock, and i were to hold a feather (not massive) very close to it. Would i witness any gravitational attraction between the feather and the ship? If not, why not? What if i did the same with a single filament of spider silk? (This is a serious question!, please answer.) thank you!
  2. jcsd
  3. Dec 7, 2016 #2
    Have you calculated the expected forces using Newton's law of gravity?
  4. Dec 7, 2016 #3


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    One of the early experiments to measure the mass of the Earth involved measuring the gravitational attraction of the mountain Schiehallion in Scotland:

  5. Dec 7, 2016 #4


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    Well, the entire mass of the Earth is barely enough to bend the feather a little bit, and the battleship is far less massive. Throw in the movement of the air and there's no way you're going to notice the gravity of the battleship.
  6. Dec 7, 2016 #5
    The magnitude of the force on the test object (feather, spider silk) is proportional to the mass of the object by the law of gravitation. The reaction of the test object to the force is proportional to its mass by Newton's 2nd law F=ma. If you are thinking you can detect gravity by the motion of the test object, then since both the size of the force and the reaction to the force are proportional to mass, the mass doesn't matter. Using a smaller lighter object does not make it easier to detect. All that does is make it easier to be disturbed by something else.

    Now if you want to measure the force directly with a spring or similar, then size does matter. Bigger is bigger, and once again a feather or spider silk is a poor choice. What you want is a lead brick.

    As to whether you can measure the force of gravity between a battleship and, for example, a lead brick, yes, absolutely. However, don't forget that distance matters much more than mass, so it's just as easy to measure with a much smaller mass in very close proximity.

    The classic experiment to measure the force of gravity is the Cavendish balance. Two fair sized (baseball sized perhaps) lead weights are at opposite ends of a short beam (30cm perhaps). The beam is supported horizontally from a long ribbon wire. The elastic resistance of the wire to being twisted is known (torsion spring). Two other similar weights on a similar beam are mounted on a pivot below the first beam so that they can be rotated to a position close to touching the first weights in such a way that gravity creates a torque on the torsion spring. Later the weights on the pivot can be rotated almost 180 degrees putting them on the other side of the balance weights and nearly reversing the torque due to gravity. A mirror on the balance beam provides a way to reflect a beam of light onto a distant wall. The equilibrium position of the beam of light on the wall is noted for each of the two positions of the pivot, and from the change in angle of the torsion balance the force of gravity between the weights can be determined.
  7. Dec 7, 2016 #6


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    Do you know how to calculate the gravitational force between two objects?

    (This is a serious question.)

  8. Dec 7, 2016 #7
    William; The force of gravity doesn't depend upon the weight of the object dropped. Watch what happens when a feather and a hammer are dropped by an astronaut on the moon (no wind or air resistance).:
  9. Dec 7, 2016 #8


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    The force of gravity certainly does. The acceleration of an object by gravity does not.

    Edit: To be clear, weight itself is already a force. The weight of an object changes with mass, but the acceleration does not.
  10. Dec 12, 2016 #9
    F=MA? No, i'm wrong. It is the use of the universal gravitational equation:
    F = GMm/R2
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