Third law of motion

Definition

The third law of motion states that for every action, there is an equal and opposite reaction. This means that whenever one object exerts a force on a second object, the second object exerts a force of equal magnitude but in the opposite direction on the first object. This principle explains how forces interact in pairs and is fundamental to understanding motion and dynamics in various physical systems.

5 Must Know Facts For Your Next Test

1. The third law highlights the mutual interactions between objects, making it essential for understanding how forces work together in different scenarios.
2. This law can be observed in everyday situations, such as when walking; as your foot pushes down on the ground, the ground pushes back up with equal force.
3. In a rocket launch, the engines expel gas downward (action), which propels the rocket upward (reaction) due to the third law.
4. The concept of action and reaction helps explain why objects can exert forces on each other without changing their own state of motion unless acted upon by a net force.
5. The third law is crucial for understanding equilibrium in static systems, as well as dynamics in moving systems where forces are balanced or unbalanced.

Review Questions

• How does the third law of motion explain the interaction between two colliding objects?
  • When two objects collide, they exert forces on each other simultaneously. According to the third law of motion, the force exerted by Object A on Object B is equal in magnitude and opposite in direction to the force exerted by Object B on Object A. This interaction can be observed in sports like billiards; when one ball strikes another, both balls experience equal and opposite forces, leading to changes in their respective motions.
• Discuss how the third law of motion applies to the operation of a jet engine.
  • In a jet engine, air is compressed and mixed with fuel before being ignited. The combustion creates high-pressure gases that are expelled backward at high speed (action). According to the third law of motion, this action generates an equal and opposite reaction that propels the jet forward. The effectiveness of a jet engine relies on this principle, as it converts fuel energy into thrust through the expulsion of gases.
• Evaluate the implications of the third law of motion in designing safety features for vehicles.
  • Understanding the third law of motion is critical in vehicle safety design. For instance, during a collision, as a car comes to an abrupt stop (action), the passengers inside experience an equal and opposite reaction force due to inertia. Safety features like seatbelts and airbags are designed to mitigate these forces and protect passengers by reducing acceleration forces acting on their bodies. This application underscores the importance of considering action-reaction pairs when developing effective safety measures.

Related terms

Force: A push or pull acting upon an object resulting from its interaction with another object.

Action-Reaction Pair: The pair of forces that occur simultaneously when two objects interact, where each force is equal in magnitude and opposite in direction.

Inertia: The tendency of an object to resist changes to its state of motion, directly related to its mass.