Newton's Laws of Motion: Third Law

Newton’s Laws of Motion: Third Law

Newton’s Third Law of Motion

  • Newton’s Third Law of Motion states that for every action, there is an equal and opposite reaction.

  • This principle explains that forces always come in pairs. The size of the forces in the pair will be equal, and the direction of the forces will be opposite.

  • An example of Newton’s Third Law is a swimmer pushing off a pool wall. The push force exerted by the swimmer on the wall (action) leads the wall to push back on the swimmer with an equal force in the opposite direction (reaction).

  • This law applies to all types of forces and situations in day-to-day life and the universe at large.

Identifying Action-Reaction Pairs

  • The biggest challenge in understanding Newton’s Third Law is correctly identifying the action-reaction pairs, as they are not always visible or intuitive.

  • Remember, action-reaction pairs do not cancel each other out because they act on different objects. For instance, the force of the swimmer on the wall and the force of the wall on the swimmer act on different bodies, so they don’t cancel each other out.

Rocket Propulsion

  • Rocket propulsion is a classical example of Newton’s Third Law. The rocket pushes gases downward (action), and the gases push the rocket upwards (reaction), enabling it to lift off.

  • In this context, it’s important to highlight that even in the absence of a physical object to push against, ejection of matter (in this case, gases) can create a force that propels an object in the opposite direction.

Implications and Use

  • Newton’s Third Law has significant implications in analysing and predicting the behaviour of objects under the influence of different forces.

  • It’s important to consider both the action and reaction forces while solving problems related to forces and motion.

  • Newton’s Third Law is also useful in building devices and systems, from vehicles to sports equipment, that make good use of action-reaction forces.