# 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 M_{1}M_{2}/r^{2}

where F_{gravity} is the attractive gravitational force between two objects of mass M_{1} and M_{2} 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} meters^{3} kilograms^{-1} seconds^{-2}