The inability of a body to change its state of rest or of uniform motion in a straight line by itself is called which physical property?

Introduction / Context:
This question is based on Newtonian mechanics and tests your understanding of a very fundamental concept: why objects tend to remain at rest or in uniform straight-line motion unless acted upon by an external force. The property that describes this tendency is central to Newton's first law of motion and appears frequently in school physics and competitive exams.

Given Data / Assumptions:

• The body may either be at rest or moving with uniform velocity in a straight line.
• The phrase “inability to change its state” means the body cannot change this state on its own.
• No external unbalanced force is acting on the body.
• We must choose the correct name for this property from the options.

Concept / Approach:
Newton's first law states that a body continues to be in its state of rest or of uniform motion in a straight line unless compelled to change that state by an external unbalanced force. The tendency of a body to resist any change in its state of motion is called inertia. Momentum, velocity and acceleration are all related quantities in mechanics, but they do not describe this “resistance to change” property itself. The correct approach is to recall that inertia is defined as the natural tendency of objects to oppose changes in their state of motion.

Step-by-Step Solution:
Step 1: Identify the key phrase in the question: “inability of a body to change its state of rest or uniform motion.” Step 2: Recall Newton's first law of motion, which explains that this behaviour is due to inertia. Step 3: Compare the definitions of the options: momentum (mass * velocity), velocity (rate of change of displacement), acceleration (rate of change of velocity). Step 4: Notice that none of these quantities expresses resistance to change of motion; they are measures of motion, not of reluctance to change. Step 5: Conclude that the correct term for this property is inertia.

Verification / Alternative check:
Textbooks usually define inertia as “the property of a body by virtue of which it cannot change its state of rest or of uniform motion in a straight line by itself.” This wording closely matches the language in the question. Furthermore, examples such as passengers lurching forward when a moving bus stops or backward when it starts are all explained using inertia. Momentum, velocity and acceleration appear in separate formulas and laws (like p = m * v or a = F / m) but do not stand for the property that resists changes in motion.

Why Other Options Are Wrong:
Momentum: This is the product of mass and velocity and measures the quantity of motion, not resistance to change of motion.
Velocity: This is the rate of change of displacement, describing how fast and in which direction a body moves, not the inability to change state.
Acceleration: This is the rate of change of velocity and is itself a measure of change, not resistance to change.

Common Pitfalls:
Some students confuse inertia with momentum because both involve mass, or believe that a high velocity means a body “resists change” more. Although a body with higher mass will indeed have more inertia, the correct term for the property itself remains inertia, while momentum is a separate quantity. Carefully distinguishing these definitions is essential for solving conceptual mechanics questions accurately.

Final Answer:
The inability of a body to change its state of rest or uniform motion in a straight line by itself is called Inertia.