Download 11-2 Gravity

11-2: Gravity
Monday, November 23, 2015
3:20 PM
Key Terms:
1. free fall – the motion of a body when only the force of gravity is acting on the body.
2. terminal velocity – the constant velocity of a falling object when the force of air resistance is equal
in magnitude and opposite in direction to the force of gravity.
3. projectile motion – the curved path that an object follows when thrown, launched, or otherwise
projected near the surface of Earth; the motion of objects that are moving in two dimensions
under the influence of gravity.
I. Law of Universal Gravitation
a. All matter is affected by gravity
i. All objects exert gravitational forces on one another, no matter how small they are.
ii. Dependent on two things:
1) Size of the masses
2) Distance between the objects
b. Gravitational force increases as mass increases
i. The gravitational force between most objects around you is relatively very small.
c. Gravitational force decreases as distance increases
i. If the distance between two objects is doubled, the gravitational force between them
decreases to one-fourth its original value.
ii. If the distance is tripled, the force decreases to one-ninth its original value.
iii. Gravitational force is weaker than other types of forces, even though it holds the
planets, stars, and galaxies together.
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II. Free Fall and Weight
When gravity is the only force acting on an object, the object is said to be in free fall.
a. Free-fall acceleration near Earth's surface is constant
i. In the absence of air resistance, all objects near Earth's surface accelerate at the same
rate, regardless of their mass.
ii. Objects near Earth's surface accelerate at 9.8 m/s2
iii. Heavier objects experience greater gravitational force than lighter objects, but heavier
objects are also harder to accelerate. The extra mass of heavy objects compensates
for the additional gravitational force.
b. Weight is equal to mass times free-fall acceleration
i. Weight is a force, so the SI unit for weight is the newton.
ii.
iii. Objects in free-fall, like astronauts in the space shuttle, experience apparent
weightlessness.
c. Weight is different from mass
i. Although mass and weight are directly proportional to one another, they are not the
same.
ii. Mass is a measure of the amount of matter in an object.
iii. Weight is the gravitational force an object experiences because of its mass.
d. Weight influences shape
i. Large land animals must have strong skeletons to support their mass against the force
of gravity.
ii. Organisms that live in water do not need strong skeletons, because gravity is offset by
the upward force of the water.
e. Velocity is constant when air resistance balances weight
i. A falling object stops accelerating when the force of the air resistance becomes equal
to the gravitational force on the object.
ii. This causes objects to reach a maximum velocity, known as terminal velocity.
III. Free Fall and Motion
Free fall occurs when gravity is the only force acting on a falling object.
Objects in space are in free fall, because there is no air resistance in space.
a. Orbiting objects are in free fall
i. Objects in space are not weightless in space; they are in free fall.
ii. Even though an astronaut may travel far away from Earth, there is still a gravitational
force between the astronaut and Earth.
iii. If an astronaut could travel across the universe, far away from any stars or planets,
the astronaut would still have weight. It would be incredibly miniscule, but it would
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the astronaut would still have weight. It would be incredibly miniscule, but it would
exist.
b. Two motions combine to cause orbiting
i. An object is said to be orbiting when it is traveling in a circular or nearly circular path
around another object.
ii. Objects in orbit have forward motion in addition to being in free fall.
IV. Projectile Motion and Gravity
Projectile motion has two independent components--horizontal and vertical. The
combination of these two components form a curved path.
a. Projectile motion has some horizontal motion
Horizontal motion is motion that is perpendicular to Earth's gravitational field.
b. Projectile motion also has some vertical motion
i. Vertical motion is motion in the direction in which the force of Earth's gravity acts.
ii. In the absence of air resistance, all objects, regardless of mass, near Earth's surface
have a gravitational acceleration of 9.8m/s2.
Quiz: Choose the word that completes each sentence correctly.
1. The force of gravity is inversely proportional to mass/distance squared.
2. Disregarding air resistance, free-fall acceleration near Earth's surface does/does not depend on
mass.
3. The mass/weight of an astronaut on the moon is the same as it is on Earth.
4. Terminal velocity is reached when the forces on a skydiver become balanced/unbalanced.
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