GRAVITY

Chapter 3Section 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.

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?

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

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.

Gravitational Acceleration

• If you drop a bowling ball and a marble off a bridge, which would hit the water first?

• Things to consider…

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!

Weight

• Even if you are standing still and you have zero acceleration, the force of gravity is still present.

• Weight is the gravitational force exerted on an object.

• Capital W stands for weight.• Gravitational force = mass x

(acceleration due to gravity)

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)

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.

Weightlessness

• To be nearly weightless, you would have to be far from the Earth.

• Astronauts experience this feeling when they are in space.