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course Phy 121

4.20.11 at 9:45pm

027. Newton's Law of Universal Gravitation

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Question: `q001. Note that this assignment contains 8 questions.

Masses attract each other. The forces of attraction are equal and opposite: The force

exerted by one small concentrated mass on another is equal in magnitude but in the opposite

direction from the force exerted on it by the other. Greater masses exert greater

attractions on one another.

If two such objects remain separated by the same distance while one object increases to 10

times its original mass while the other remains the same, there will be 10 times the

original force.

If both objects increase to 10 times their original masses, there will be 100 times the

original force.

The force of attraction is inversely proportional to the square of the distance between

the objects. That means that if the objects move twice as far apart, the force becomes 1 /

2^2 = 1/4 as great; if they move 10 times as far apart, the force becomes 1 / 10^2 = 1/100

as great.

The same statements hold for spherical objects which have mass distributions which are

symmetric about their centers, provided we regard the distance between the objects as the

distance between their centers.

Suppose a planet exerts a force of 10,000 Newtons on a certain object (perhaps a

satellite) when that object is 8000 kilometers from the center of the planet. How much

force does the satellite exert on the planet?

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Your solution:

-10,000

confidence rating #$&*:

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Given Solution:

The gravitational forces exerted by the planet and the object are equal and opposite, and

are both forces of attraction, so that the object must be exerting a force of 10,000

Newtons on the planet. The object is pulled toward the planet, and the planet is pulled

toward the object.

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Self-critique (if necessary):

Would it be negative, as I wrote? Or is it safer just to say 10,000?

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Self-critique rating: