Newtonian Mechanics – Identify the Statement\nThe total linear momentum of a system of masses in any direction remains constant unless acted upon by an external force in that direction. This statement is known as:

Introduction / Context:
Conservation laws form the backbone of mechanics. Momentum conservation is especially important for collisions, rockets, and systems with internal interactions but negligible external influence.

Given Data / Assumptions:

• Closed system or net external force along the direction considered is zero.
• Classical (Newtonian) mechanics regime.

Concept / Approach:
Newton's second law in vector form is ΣF = d( p )/dt. If ΣF = 0 in a given direction, then dp/dt = 0, implying momentum p is constant in that direction. This is the formal basis of the principle of conservation of momentum.

Step-by-Step Solution:
Start: ΣF_dir = d( p_dir )/dt. If ΣF_dir = 0 ⇒ d( p_dir )/dt = 0. Therefore p_dir = constant over time. This is termed the conservation of momentum in that direction.

Verification / Alternative check:
Applies to multi-body systems with internal forces obeying action–reaction; internal forces cancel in total momentum change, leaving only external forces to alter system momentum.

Why Other Options Are Wrong:
First law: inertia principle (no change in state without net force), not a conservation statement for a system’s momentum. Second law: defines relation between force and rate of change of momentum. Energy conservation: different physical quantity. D’Alembert’s principle: transforms dynamics into statics with inertial forces, not a conservation law.

Common Pitfalls:
Confusing energy and momentum conservation; they are distinct and context-dependent.

Final Answer:
Principle of conservation of momentum