When some body rests on a surface, the contact force that the surface exerts on the object may always be shown as components of force parallel and perpendicular to the surface upon which the object rests. It is the perpendicular component of the contact force that we commonly call the normal force. The other, parallel force, is known as the frictional force .
In a simple case such as a 40kg object resting upon a table, the normal force from the table to the object is equal but in opposite direction to the force applied to the table by the object (i.e. weight of the object). In this case the normal force is given by, 40kg * 9.81m / s2 = 392.4 newtons where 9.81m / s2 is equal to the force of gravity.
Note that the sign of the calculated value will be either positive or negative depending upon whether the positive Y-axis is taken to be either positive or negative.
