Understanding Newton's Laws of Motion

Newton's laws of motion are fundamental principles in physics that describe the relationship between a body and the forces acting upon it, and its motion in response to those forces. There are three laws that cover different aspects of motion and forces.

Identifying the Law of Equal and Opposite Forces

The question describes a principle where the forces between two interacting bodies are always equal in magnitude and opposite in direction. Let's examine how each of Newton's laws relates to this concept.

Newton's First Law of Motion (Law of Inertia)

This law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. It primarily deals with the state of motion in the absence of a net force or when the net force is zero. It does not specifically describe the interaction forces between two different bodies.

Newton's Second Law of Motion

This law relates the net force acting on an object to its mass and acceleration. It is often expressed by the formula \(\vec{F}_{net} = m\vec{a}\). It describes how a force causes a change in motion (acceleration) but doesn't directly state that forces between *two different* bodies are equal and opposite.

Newton's Third Law of Motion (Law of Action and Reaction)

This law states that for every action, there is an equal and opposite reaction. When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body. These forces act on different bodies. This perfectly matches the description given in the question: "The forces between two bodies are always equal and opposite."

Therefore, the idea that forces between two bodies are always equal and opposite is stated in the form of Newton's third law of motion.

Comparing the Options

Let's look at the provided options in the context of Newton's laws:

• Option 1: second law of motion - Incorrect, as the second law relates force, mass, and acceleration for a single body or system, not the equal and opposite interaction forces between two bodies.
• Option 2: third law of motion - Correct, as this law explicitly states the principle of equal and opposite forces between two interacting bodies.
• Option 3: first and second law of motion - Incorrect, neither the first nor the second law, individually or combined, states the equal and opposite nature of interaction forces between two bodies.
• Option 4: first law of motion - Incorrect, as the first law deals with inertia and the state of motion under zero net force, not the interaction forces between two bodies.

Conclusion

The statement that forces between two bodies are always equal and opposite is the definition of the action-reaction principle, which is the core of Newton's third law of motion.

Revision Table: Newton's Laws and Forces

Additional Information: Understanding Action-Reaction Pairs

It is important to understand that the equal and opposite forces described by Newton's third law always act on different bodies. For example, when you push a wall, you exert a force on the wall (action). The wall simultaneously exerts an equal and opposite force back on you (reaction). These forces do not cancel each other out because they are applied to different objects. This principle explains phenomena like walking (feet push backward on ground, ground pushes forward on feet) and birds flying (wings push air down, air pushes wings up).