Lesson 11.3: Forces act in pairs

Lesson Focus

Ever wonder how rockets launch or jellyfish swim? This lesson explores Newton's Third Law, revealing that for every action, there is an equal and opposite reaction. We'll see how forces always work in pairs to create motion.

Learning Objectives

• Learn how Newton's third law explains the relationship between action and reaction force pairs.
• Discover how all three of Newton's laws of motion work together to describe and predict an object's movement.
• Compare action and reaction forces, and understand how they differ from balanced forces acting on a single object.

This is Newton's third law. When you push a wall (the action force), the wall pushes back on you with an equal and opposite force (the reaction force). This happens because forces always occur in pairs. These forces act on different objects, so they don't cancel each other out.

A jellyfish demonstrates Newton's third law to move. By squeezing its body, it applies a downward action force to the water. In response, the water exerts an equal and opposite upward reaction force on the jellyfish, propelling it. This shows how an object can move forward by pushing something backward.

Don't confuse action-reaction forces with balanced forces. Balanced forces act on a single object and can cancel out, resulting in no motion. Action-reaction forces act on two different objects. For example, your foot pushes the ground backward, and the ground pushes your foot forward, making you walk.

Imagine a skater pushing another skater. The initial push is the action force that moves the second person. Even if the second person does nothing, their body exerts an equal and opposite reaction force back on the first skater. As a result, both skaters will move away from each other.

Newton's first, second, and third laws work together to explain any object's motion. For a kangaroo to jump, it must use force to overcome its inertia (1st Law), its acceleration depends on its mass (2nd Law), and its legs push the ground backward to launch it forward (3rd Law).