Lesson 11.1: Forces change motion

Lesson Focus

Forces are the pushes and pulls that change how things move. We will explore what forces are, how they combine, and how Newton’s first law helps us predict an object's motion when forces act on it.

Learning Objectives

• Define what a force is, recognizing it as a simple push or a pull that can change an object's motion.
• Explain how unbalanced forces, or the net force, are required to change an object’s speed or direction of motion.
• Use Newton's first law (the law of inertia) to predict why objects resist changes in their state of motion.

A force is a push or a pull that changes an object's motion. Forces, like contact forces, gravity, and friction, are vectors, meaning they have both a size and a direction. To make a pencil write or to stop a rolling ball, a force must be applied. All changes in motion begin with a force.

Often, multiple forces act on an object simultaneously. The net force is the total combination of all forces. When the net force is zero, forces are balanced, and motion does not change. For example, in a tied tug-of-war, the balanced forces result in no movement. The overall force dictates the outcome.

Only an unbalanced force can change an object’s motion, because the net force is not zero. To speed up, slow down, or turn, the forces must be unequal. Pedaling a bike harder creates an unbalanced force that overcomes friction and increases your speed. Motion only changes when one force overpowers another.

Newton's first law states that objects resist changes to their motion. An object at rest stays at rest, and a moving object maintains its velocity, unless acted upon by an unbalanced force. A rolling hockey puck only stops because of friction. This law explains why things don't spontaneously move or stop on their own.

The tendency of an object to resist a change in its motion is called inertia. This concept is the core of Newton's first law. Inertia is directly related to mass; more mass means more inertia. This is why it's much harder to push a full car than an empty one. Mass is a measure of inertia.