Conceptual Q1

Q: Why does a ball thrown upward slow down? Analyse using the work-energy theorem.

A: As the ball rises, gravity (downward) acts opposite to displacement (upward). W_gravity = −mgh (negative). By W_net = $\Delta K$E: $\Delta K$E = −mgh < 0. Hence KE decreases, speed decreases. At maximum height: all KE converted to PE.

Conceptual Q2

Q: A man carries a heavy load and walks horizontally. He says he "did a lot of work." Is he correct in the physics sense?

A: No. In the physics sense, the force (upward, to support the load) is perpendicular to displacement (horizontal). W = 0. Physiologically he tires, but that is internal biochemical energy expenditure, not mechanical work done on the load.

Conceptual Q3

Q: Can the kinetic energy of a system increase even if no external force acts on it?

A: Yes! In an explosion, internal chemical energy converts to KE. The total momentum remains zero (no external force), but total KE increases from zero. Internal forces do work on individual fragments.

Conceptual Q4

Q: In a head-on elastic collision between equal masses, prove that if one was at rest, the other stops completely.

A: Use elastic formulae: v_{1} = (m_{1}−m_{2})u_{1}/(m_{1}+m_{2}). With m_{1} = m_{2} = m: v_{1} = 0. The moving ball stops. v_{2} = 2m_{1}u_{1}/(m_{1}+m_{2}) = u_{1}. The stationary ball moves with the initial velocity.