According to Newton's first law of motion, an object remains in the same state of motion unless a resultant force acts on it. If the resultant force on an object is zero, this means:
Two forces can be added together to find a resultant force. The resultant force is a single force that has the same effect as two or more forces acting together.
The resultant force of two forces that act in the same direction can easily be calculated - to do this, just add the magnitudes of the two forces together.
Two forces, 3 newtons (N) and 2 newtons (N), act to the right. Calculate the resultant force.
3 N + 2 N = 5 N to the right
Two forces that act in opposite directions produce a resultant force that is smaller than either individual force. It is often easiest to subtract the magnitude of the smaller force from the magnitude of the larger force.
A force of 5 N acts to the right, and a force of 3 N acts to the left. Calculate the resultant force.
5 N - 3 N = 2 N to the right
The tendency of an object to continue in its current state (at rest or in uniform motion) is called inertia. Inertial mass is defined as the ratio of force over acceleration. Inertial mass is a measure of how difficult it is to change the velocity of an object.