2. The Law of Gravitation
• You exert an attractive force on everything
around you and everything is exerting an
attractive force on you.
• This attractive force is called gravity.
• Anything that has mass is attracted by the
force of gravity.
• The Law of Gravitation states that any two
masses exert an attractive force on each
other.
3. The Law of Gravitation
• Gravitation force depends on two things:
– The mass of the two objects
– The distance between the two objects
• Why do you suppose the Earth exerts a force
on you that you can feel, but you can’t feel
the force the desk is exerting on you?
4. Gravity – a basic force
• There are four basic forces:
–Gravity
–Electromagnetic force
–Nuclear force = Strong
–Nuclear force = Weak
Apply to
an atom
only
Caused by electricity
and magnetism
5. Gravitational Acceleration
• When objects fall, the
gravitational pull is 9.8 m/s2
• When a falling object is only
affected by gravity it is said to
be in free fall.
6. Gravitational Acceleration
• If you drop a bowling ball and a
marble off a bridge, which would
hit the water first?
• Things to consider…
7. Gravitational Acceleration
– Force of gravity is greater on the bowling ball
because of its larger mass.
– The larger mass means it has a larger inertia so
more force is needed to change its velocity.
– Gravitational force on the marble is smaller
because it has a smaller mass
– The inertia on the marble is less and less force is
needed to change the velocity.
– Therefore, all objects fall with the same
acceleration!
8. Weight
• Even if you are standing still and you have zero
acceleration, the force of gravity is still present.
• Weight is equal to the support force, which under
normal circumstances is also equal to the force
exerted on an object by gravity.
• Capital W stands for weight.
• Fg = mass x (acceleration due to gravity)
9. Weight
• Because the gravitational force is
the same as the weight and the
acceleration due to gravity, the
equation can also be written as:
• W = (m)(9.8 m/s2
)
• …if the object is being supported!
10. Weight & Mass
• Weight and mass are not the same
thing.
• Weight is a force
• Mass is a measure of the amount of
matter an object contains.
• The greater the weight, the greater
the attraction between the object
and Earth.
11. Weightlessness
• To be nearly weightless, you
would have to be far from the
Earth.
• Astronauts experience this
feeling when they are in space.
• Why does the space station
spin?
12. Gravity Factors
• The acceleration due to gravity
is directly proportional to the
mass of the objects and
inversely related to the
distance squared.
13. Inverse Square Law
When a quantity varies as
the inverse square of its
distance from its source.
14. Law of Universal Gravitation
Every object attracts every
other object with a force that
for any two objects is
directly proportional to the
mass of each object.
15. Universal Gravitation
• To find the Fg
between any two
objects in the
universe.
• G is the
gravitational
constant of
proportionality
• G = 6.67 x 10-11 N m2
/kg2
• F = G m1m2/r2
