Newton's 3rd Law

Definition

Newton's 3rd law, also known as the law of action and reaction, states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another object, the second object exerts an equal and opposite force on the first object.

5 Must Know Facts For Your Next Test

1. Newton's 3rd law applies to all types of forces, including contact forces (e.g., friction, tension, normal force) and field forces (e.g., gravitational, electrical, magnetic).
2. The action and reaction forces always act on different objects, never on the same object.
3. The action and reaction forces are equal in magnitude but opposite in direction, meaning they have the same numerical value but point in opposite directions.
4. Newton's 3rd law is essential for understanding the motion of objects, as it helps explain how forces interact and cause changes in an object's velocity or acceleration.
5. The concept of action and reaction forces is crucial in the analysis of various physical systems, such as the motion of vehicles, the operation of mechanical devices, and the dynamics of celestial bodies.

Review Questions

• Explain how Newton's 3rd law applies to the forces involved in a person walking across a room.
  • When a person walks across a room, Newton's 3rd law can be observed in the interaction between the person's feet and the floor. As the person pushes backward on the floor with their feet (the action force), the floor pushes forward on the person's feet with an equal and opposite reaction force. This reaction force is what allows the person to move forward and walk across the room. The action and reaction forces are equal in magnitude and opposite in direction, resulting in the person's forward motion.
• Describe how Newton's 3rd law can be used to explain the motion of a rocket during launch.
  • During a rocket launch, the rocket's engines produce a large force that propels the rocket upward (the action force). According to Newton's 3rd law, the rocket's engines also exert an equal and opposite reaction force on the surrounding air and exhaust gases, causing them to be expelled downward. This reaction force is what generates the upward thrust that accelerates the rocket into the sky. The action and reaction forces act on different objects (the rocket and the exhaust gases), resulting in the rocket's motion.
• Analyze how Newton's 3rd law relates to the concept of equilibrium in the context of 5.6 Common Forces.
  • $$\text{In the context of 5.6 Common Forces, Newton's 3rd law is closely related to the concept of equilibrium. When the action and reaction forces acting on an object are equal in magnitude and opposite in direction, the object is in a state of equilibrium, with no net force acting on it. This is essential for understanding the behavior of objects under the influence of common forces, such as friction, tension, and normal forces. For example, when a book rests on a table, the normal force exerted by the table on the book is equal to the gravitational force exerted by the Earth on the book, resulting in a state of equilibrium and the book remaining stationary.}$$

Related terms

Action Force: The force exerted by one object on another object.

Reaction Force: The force exerted by the second object on the first object, which is equal in magnitude and opposite in direction to the action force.

Equilibrium: A state of balance where the action and reaction forces are equal in magnitude and opposite in direction, resulting in no net force on the objects.