Balanced and Unbalanced Forces in Motion

By the end of this lesson, you will be able to analyze how balanced and unbalanced forces change or keep an object's state of motion, using Newton's First Law of Motion to explain what you see.

A force is a push or a pull on an object. Every object around you has forces acting on it right now, even when it looks like nothing is happening. When two or more forces act on an object, we can add them up to see their total effect. If the forces cancel each other out, we call them balanced forces. If one force is stronger than the others, we call them unbalanced forces, and there is some leftover force pushing or pulling in one direction.

Newton's First Law of Motion says that an object at rest stays at rest, and an object in motion keeps moving at the same speed in the same direction, unless an unbalanced force acts on it. This tendency of an object to resist a change in its motion is called inertia. So the law tells us something simple: balanced forces do not change motion, and unbalanced forces do.

Think about a football sitting on a Friday night field in Texas before the kickoff. Gravity pulls it down, and the ground pushes it up with the same strength. Those forces are balanced, so the ball stays at rest. Then the kicker's foot hits it. That kick is an unbalanced force, so the ball changes from resting to flying through the air. In the air, gravity and air resistance are unbalanced against nothing pushing it forward, so the ball slows, curves down, and eventually stops when it hits the ground. Every change you saw, from rest to motion to stopping, happened because of an unbalanced force.

To analyze any situation, ask two questions. First, is the object speeding up, slowing down, turning, or starting or stopping? If yes, the forces must be unbalanced. Second, is the object sitting still or moving at a steady speed in a straight line? If yes, the forces must be balanced.

Try it: A skateboard rolls at a steady speed on flat pavement, then begins to slow down, so are the forces on it balanced or unbalanced while it slows, and which force is doing the slowing?