SSC Stenographer 2026 PYQ — Question 14

Solution: The correct answer is In an isolated system, the total momentum before and after a collision remains constant, regardless of the internal forces.
Explanation:
☛ The Law of Conservation of Linear Momentum states that if no external net force acts on a system (an isolated system), the total linear momentum of that system remains constant over time.
☛ During any collision within an isolated system, internal forces act between colliding bodies in equal and opposite pairs according to Newton's Third Law of Motion.
☛ Because these internal forces cancel out in pairs, they cannot change the net momentum of the system; thus, the total momentum before collision equals the total momentum after collision.
Extra Exam-Relevant Facts:
☛ Linear Momentum (p): Defined as the product of mass and velocity (p = m × v). It is a vector quantity with the SI unit kg·m/s (or N·s).
☛ Derivation: The conservation of momentum is a direct consequence of Newton's Second Law (F = dp/dt) and Newton's Third Law (action and reaction). When external force F = 0, dp/dt = 0, meaning momentum (p) is constant.
☛ Elastic vs Inelastic Collisions: Linear momentum is conserved in both elastic and inelastic collisions, whereas kinetic energy is conserved only in perfectly elastic collisions.
☛ Rocket Propulsion: Operates on the principle of conservation of linear momentum as high-velocity exhaust gases ejected backward propel the rocket forward.
Important One-Liners for Upcoming Exams:
☛ What is the dimensional formula of linear momentum? → [M¹ L¹ T⁻¹]
☛ Which law of motion directly gives the mathematical definition of force? → Newton's Second Law of Motion
☛ What happens to the kinetic energy in a perfectly inelastic collision? → It is not conserved (maximum kinetic energy is lost as heat/sound)
☛ On which conservation law does the recoil of a gun depend? → Conservation of Linear Momentum
☛ What quantity is defined as the rate of change of linear momentum? → Force