* Net Force: A force only causes motion if it's the net force acting on an object. This means the overall force acting on the object after considering all forces (including opposing forces like friction).

* Balanced Forces: If forces are balanced, they cancel each other out. The net force is zero, and the object remains at rest or continues moving at a constant speed in a straight line. For example, a book sitting on a table experiences a downward force due to gravity and an upward force from the table, resulting in no motion.

* Inertia: Objects at rest tend to stay at rest, and objects in motion tend to stay in motion at a constant velocity (Newton's First Law of Motion). A force is needed to overcome this tendency and cause a change in motion.

Examples:

* Pushing a heavy box: If you push a heavy box with enough force to overcome friction, it will move. However, if you push with a force that's less than the friction, the box won't move.

* Holding a weight: You exert a force upwards to hold a weight, but it doesn't move because the force of gravity pulling it down is balanced by your upward force.

In summary, a force is needed to cause motion, but the motion only occurs if the force is unbalanced and overcomes any opposing forces.