1. Newton’s Law of Universal Gravitation
Gravitational Force
Newton’s Law of Universal Gravitation

2. Gravitational Force Gravity is the weakest universal force
Gravity pulls objects towards the center of the Earth
Gravity affects all objects in the universe
It is the same force on a falling apple and the force that keeps the moon in it’s orbit

3. Gravitational Force
Gravitational force is an attractive force that acts between any two masses
Newton’s law of universal gravitation states that every object in the universe attracts every other object

4. Effects of Gravitational Force
In the example of the falling apple, the Earth exerts a force on the apple and the apple exerts an equal force on the Earth
You also exert a force on your textbook and your textbook exerts a force back on you
Your mass and the mass of the textbook is so small that you do not notice the force of gravity pulling your textbook towards you

5. Mass Matters
There needs to be a large mass like the Earth for a large gravitational force
A larger mass has a larger gravitational force
Gravitational force is also affected by the distance between the two objects

6. Gravity and Distance
The gravitational force between two objects is proportional to their masses
Gravitational force decreases as the distance between masses increases, it will decrease with the inverse square of the distance between objects
For example if the distance between two objects doubles (x2) then the gravity will be ¼ (1/22) as strong

7. Gravity
Gravity is the weakest universal force, but very effective over long distances
Gravity keeps us on the Earth, keeps the moon, planets and stars in orbit
The sun’s mass is about 300,000 times the mass of the Earth so the sun’s gravitational force is much stronger than the Earth’s

8. Earth, Moon and Tides
Earth’s gravitational force keeps the moon in a nearly circular orbit around the Earth
The earth’s gravitational force is a centripetal force that keeps the moon in its path
The gravitational pull of the moon causes two bulges in Earth’s oceans
The moon causes two high and two low tides per day