GRAVITATIONAL FORCE
LECTURE NOTES
Gravity is the weakest of the universal forces.
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Gravitational force is an attractive force that acts between any two masses.
NEWTON’S LAW OF UNIVERSAL GRAVITATION: every object in the universe attracts every other object.
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Objects with relatively small masses do not exert strong gravitational forces.
An object with a huge mass (the earth) exert a large gravitational force.
Gravitational force depends upon mass and distance.
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Increasing mass increases the gravitational force an object can exert upon another.
Increasing distance, significantly reduces the gravitational force and object exerts.
Gravity acts over long distances
The gravitational force between two objects is proportional to their masses and decreases rapidly as the
distance between the two masses increases.
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The greater the mass of the objects, the greater is the gravitational force.
Gravitational force decreases with the square of the distance between the objects.
o If the distance between two masses doubles, the force of gravity is only one fourth as
strong. Examples A and C from the figure below show this relationship.
Gravitational force depends upon mass and distance. (See figure above)
A. Two masses, X and Y, attract each other.
B. The larger mass of X results in a larger gravitational force.
C. Increasing the distance between the masses significantly reduces the gravitational force.
Gravity is the weakest universal force, but it is the most effective force over long distances.
GRAVITATIONAL FORCE
LECTURE NOTES
The Earth, Moon, and Tides
The moon has inertia, so according to Newton’s first law, it should continue to move along a straight
path until acted upon by a force.
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That force is Earth’s gravitational force, which acts continuously to pull the moon toward it, as
shown in the figure below.
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Earth’s gravitational attraction keeps the moon in an elliptical orbit around the earth.
This is similar to if you tie a string to an eraser and twirl it in a circle above your head.
o As you twirl the eraser, the string exerts a centripetal force on the eraser.
o Centripetal force is a center-directed force that continuously changes the direction of
an object to make it move around the center.
Tides
The gravitational pull from the moon produces two bulges in Earth’s oceans.
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One bulge is on the side of Earth closest to the moon
The other bulge is on the side of Earth farthest from the moon.
Earth rotates once per day beneath these two bulges and this rotation results in two high and
two low tides per day on earth.