Newton's Third Law

• Newton’s Third Law states that for every action, there is an equal and opposite reaction. This means that any force exerted on a body will create a force of equal magnitude but in the opposite direction on the object that exerted the first force.

• It’s relevant conceptually in a variety of physical phenomena: launching a rocket, walking (pushing the ground backwards to propel ourselves forward), or in an air balloon expelling air downwards to ascend.

• Action and reaction forces are part of the same interaction and they occur at the same time.

• These forces always act in pairs. If object A exerts a force on object B, then object B also exerts an equal and opposite force on object A.

• The action and reaction forces are equal in size but opposite in direction. Therefore, they don’t need to have the same effect. Other factors, like mass and acceleration as in the Second Law of motion, determines the net result of these forces.

• Despite acting on different bodies, these pairs of forces are often confused as acting on the same body, hence causing a net force of zero. This is not the case. Each force in a pair acts on a different body; therefore they don’t cancel each other out.

• Newton’s Cradle is a common demonstration of this law. When one ball is lifted and released (action), the final ball in the series is forced to swing out at a comparable height (reaction), showing that the force has been transferred through the balls.

• Newton’s third law is integral in understanding how forces work and interact, not only in simple mechanics but also in more complex systems like gravitational force between celestial bodies.

• Misconception to avoid: ‘Action’ and ‘reaction’ in this law doesn’t mean ‘cause’ and ‘effect’. They occur simultaneously, not sequentially.

• Remember, this law applies whether the bodies in question are at rest or in motion.

• Friction is often involved in action-reaction pairs, such as pushing off the ground to walk or run. The ground pushes back due to the force of friction, allowing movement.

By understanding Newton’s Third Law and other laws of motion, you can create a conceptual model of why and how objects move.