According to Newtonian mechanics, which of the following physical quantities never occurs singly in nature but always appears as an interaction between two bodies?

Introduction / Context:

In classical mechanics, forces describe interactions between bodies. Newton third law states that every action has an equal and opposite reaction. This question asks which quantity from the list is never isolated in nature but always appears as part of such an interaction pair.

Given Data / Assumptions:

• We consider the standard definitions of inertia, momentum, force, and velocity.
• Newton laws of motion apply.
• We focus on whether the quantity can be associated with a single body alone or must involve at least two interacting bodies.

Concept / Approach:

Force is defined as an interaction that can change the motion of a body. Newton third law tells us that forces always come in pairs: if body A exerts a force on body B, then body B exerts an equal and opposite force on body A. Thus the concept of force inherently involves at least two bodies. Inertia, momentum, and velocity, on the other hand, are properties that can be assigned to a single body without reference to another body.

Step-by-Step Solution:

Verification / Alternative check:

Consider an object at rest on a table. The table exerts an upward normal force on the object, while the object exerts an equal downward force on the table. These are two forces forming an interaction pair. There is no way to have one of these forces without the corresponding other force, which illustrates that a single, isolated force is not observed in nature.

Why Other Options Are Wrong:

Option A: Inertia is a scalar property of a body related to its mass and does not require another body to be defined.

Option B: Momentum is defined as mass times velocity and can be specified for a single particle or body.

Option D: Velocity is a vector that gives the speed and direction of motion of a body and does not require a second body to exist.

Common Pitfalls:

Some students think of force as something that a single body possesses, like mass. In reality, force describes the interaction between bodies. Remembering the phrase forces always occur in action reaction pairs can help keep this idea clear. It is a core part of Newtonian mechanics and frequently tested in conceptual questions.

Final Answer:

The quantity that never occurs singly in nature is force.