Normal force

In physics, the normal force is the perpendicular component of the contact force exerted on an object by the surface on which it sits. The parallel component of the 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, $40kg*9.81m/s^{2}=392.4$ newtons where $9.81m/s^{2}$ is equal to the acceleration due to gravity (near the Earth's surface).

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.