Sometimes the forces on an object are balanced. In a free body diagram, balanced forces are drawn as arrows of equal length pointing in opposite directions. When forces are balanced, they cancel out and the resultant force is zero.
A book resting on a table is an example. Its weight acts downwards, and the table pushes up on it with a contact force of the same size. The two arrows are equal and opposite, so the resultant force is zero and the book stays at rest.
A zero resultant force does not mean an object has to be still. A car moving at a steady speed along a straight road has a driving force that is equal to the drag force. The forces are balanced, so the car does not accelerate and keeps moving at a constant velocity. An object only changes its speed or direction when the resultant force is not zero.
