Universal Forces

Gravity (A Universal Force)

Isaac Newton

Isaac Newton had a lot of brilliant ideas in his lifetime and a massive one was universal gravitation. Universal gravitation is not just the fact that objects on earth fall to the ground. His big idea was that all objects in the universe attract all other objects.

For example, not only does the Earth use its gravity to pull the moon towards it, the moon also pulls on the Earth causing it to be slightly displaced.

Newton's Law of Universal Gravitation

Any object with mass is attracted to any other object with mass.

The amount of force depends on distance and the amount of mass of the object.

As the mass of one object increases, the gravitational force does as well.

This is why larger planets have a larger gravitational pull.

As the distance between two objects increases, the gravitational force between them decreases.

This is why large objects like the sun, that are very far away, don't pull you into them.

Newton realized that both the apple and moon are really falling. The only difference is that the moon has inertia, and the apple doesn't.

What is inertia?

Inertia is the resistance of any physical object to any change in its velocity

Inertia

While the apple is falling from a still position in the tree, the moon has a constant velocity.

Normally the moon would move forward in a straight line at a constant velocity, through space. But because of gravity, the moon is attracted to the earth, making it fall towards it. This inertia, or the moon's resistance to being pulled into the Earth, is what keeps it orbiting.

The moon acts like a projectile with its inertia taking it forward as it also falls to the earth. This combined action keeps the moon in orbit around the earth.

This is why an object on earth falls to the ground while the moon continues in its circular path.