## Gravity and the Normal Force

• There are many different kinds of forces in the world. Some common ones are gravity, friction, magnetism.
• Gravity is a very simple force: it causes an attraction between all bodies which contain mass. The strength of the attraction between two bodies is given by
```                     G * m1 * m2
F  =  -------------
r*r
```
where G is the gravitational constant, m1 and m2 are the masses of the bodies, and r is the separation between them.
• The direction of the gravitational force between two objects lies along the line connecting them.
• As forces go, gravity is pretty weak: even a puny human can overcome the gravitational pull of the entire earth.
• The strength of the gravitational force between the Earth and an object of mass m on its surface can be written as
```                     G * M(earth) * m      G * M(earth)
F  =  -----------------  =  -------------- * m
R(earth)^2            R(earth)^2

=        g        * m
```
where g, the gravitational acceleration at the surface of the earth, has a value of about 9.8 meters per second squared, downwards.

• Gravity is a long-range force; other forces only occur when objects come into contact. One example of a contact force is the normal force.
• The normal force is just the component of a contact force which is perpendicular to the plane of contact. It prevents objects from moving through each other.
• The other component of a contact force, parallel to the plane of contact, is called friction.
• The normal force often acts in a vertical direction, since objects on the earth's surface are usually resting on some surface.

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