What Is the Difference Between Balanced and Unbalanced Forces?

Oliver Clarke/CC-BY-2.0

Balanced forces are forces that produce no change in motion, while unbalanced forces produce some type of acceleration. With balanced forces, any forces in a given direction are exactly matched by forces in the opposite direction. With unbalanced forces, the forces in one direction are greater than those in the opposite direction, resulting in a net force that produces acceleration.

Literally any object that is not moving is influenced by balanced forces, as there is nothing that is not influenced by at least gravity. A rock does not move under the influence of most winds, because the force exerted by friction with the ground exactly matches the force exerted by the wind. Similarly, it does not sink through the earth because the force of its weight is exactly matched by the resistance of the materials beneath it. Objects in a constant state of motion, that is, motion at a constant speed along a straight line, are also under balanced forces.

In any other case, when an object speeds up, slows down or changes direction, it is under the influence of unbalanced forces. Speeding up means that the net force is in the direction of motion, while slowing down means the force is in the opposite direction. Turning means that the net force is at an angle to the original direction of motion.