Q.A lighter object A and a heavier object B are initially at rest. Both are imparted the same linear momentum. Which will start with greater kinetic energy: A or B or both will start with the same energy?

The work-energy theorem states that the work done on a body by a net force equals the resulting change in that body's kinetic energy. For a body decelerated from speed u to speed v by a constant opposing conservative force F over a distance s, algebraic manipulation of the kinematic equation $v^2=u^2-2as$ shows $\frac{1}{2}mu^2-\frac{1}{2}mv^2=Fs$ -- the decrease in kinetic energy exactly equals the work done by the opposing force.

When a non-conservative force (like air resistance) also acts, the theorem generalises to: decrease in potential energy = increase in kinetic energy + work done against the non-conservative force. This theorem is the essential bridge connecting the mechanics of force and motion (kinematics/dynamics) to the language of energy, and underlies why, for instance, a dropped ball's landing speed is always somewhat less than the ideal frictionless prediction once air resistance is accounted for.

The work-energy theorem is a foundational result from the NCERT Class 11 Physics chapter on Work, Energy and Power, and it ranks among the most frequently asked important questions in CBSE board papers as well as JEE Main and NEET. Searches like "work energy theorem derivation class 11" or "work energy theorem numericals" typically lead back to exactly this NCERT-aligned proof.