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GRAVITY
Law of Gravitation
Centripetal Force
Law of Gravitation
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States that any two masses exert an attractive force on
each other.
This attractive force depends on the mass of the two
objects and it increases as the mass of either of the
objects increase.
You can’t feel the attraction between you and your
notebook because the force is too weak.
Only the Earth is close enough and has a large enough
mass so that you can feel its gravitational pull.
The Sun has a larger mass than the Earth but it is too far
away for you to experience its gravitational pull.
Gravity
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It is one of the four basic forces.
You may think that a star in another galaxy is
too far away to exert a gravitational force on
you, but no matter how far away something is
the gravitational attraction never disappears.
All stars in a galaxy exert a gravitational force
on each other. This is what helps give the
galaxies their shape.
Gravitational Acceleration
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All things near Earth’s surface fall at an acceleration of
9.8m/s2.
You can still use Newton’s second law to find out the
force of the object that is falling.
 F=ma
And if you are talking about a falling
object the formula would be F=m*9.8m/s2
The force of Earth’s gravity is always downward.
When an object is influenced only by the force of
gravity, it is said to be in free fall.
No matter how large or small an objects mass is, they
will fall with the same acceleration. If I was to drop a
marble and a bowling ball at the same time, they will
hit the ground at the same time.
Weight
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You can also use Newton’s second law to calculate
weight.
Weight is the gravitational force exerted on an object.
𝑔𝑟𝑎𝑣𝑖𝑡𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑓𝑜𝑟𝑐𝑒 = 𝑚𝑎𝑠𝑠 ∗
𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑑𝑢𝑒 𝑡𝑜 𝑔𝑟𝑎𝑣𝑖𝑡𝑦
So on Earth you would use 𝑊 = 𝑚 ∗ 9.8𝑚/𝑠 2
Example Problem:
 What does a person with a mass of 70kg weigh on
Earth?
Weight
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If you traveled away from Earth, the
gravitational pull would become weaker
making you weigh less.
Weight and mass are NOT the same thing.
Weight is a force and mass is a measure of the
amount of matter an object contains.
They are related though. The more mass an
object has, the more it will weigh due to the
force of gravity.
Weightlessness
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Astronauts are not actually weightless in space.
They would have to be a long way from Earth
to be free from the effects of gravity.
They are actually experiencing the same thing
you feel when you are in an elevator that is
going down. The shuttle is actually falling back
toward Earth so that is why they appear to
float.
Projectile Motion
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Anything that is thrown or shot through the air is called a
projectile.
These projectiles travel in a curved path due to their
own inertia and Earth’s gravitational pull.
They have horizontal and vertical velocities.
Centripetal Force
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Centripetal acceleration is the acceleration toward the
center of a curved or circular path.
Centripetal force is the force that is pulling the object to
the middle while it is turning. This force could come from
a wall that causes a ball to turn or from the friction
between the tires and the road as a car makes a turn.
If the road were icy, the tires would not have this
friction and the car would continue to go straight.
The Earth’s gravity exerts a centripetal force on the
Moon that keeps it orbiting in a circular path.