Example of forces

Normal force: is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book.

Weight force: is the force that acts on a mass due to gravity.

Gravitational force: is an attractive force that occurs between two masses. The Universal Law of Gravitation is usually stated as an equation:

F_gravity = G M₁M₂/r²

where F_gravity is the attractive gravitational force between two objects of mass M₁ and M₂ separated by a distance r. The constant G in the equation is called the Universal Constant of Gravitation. The value of G is:

G = 6.67 X 10^-11 meters³ kilograms^-1 seconds^-2