NEET Work Energy and Power MCQs & PYQs

Work done by conservative forces in a closed loop is zero. For all forces, it depends on energy change and non-conservative forces. Force is required to move a body, especially to overcome inertia or friction. Therefore, both Assertion (A) and Reason (R) are false.

Assertion is true by definition of a conservative force, which does zero work over any closed path.

Reason is also true as displacement is zero for a closed path, i.e., \(\Delta \vec{r} = 0\). However, the reason is not the correct explanation for why work by a conservative force is zero; this is due to path independence and \(\Delta U = 0\) for a closed path.

Thus, both are true, but R is not the correct explanation of A.

Assertion is true: Non-conservative forces like friction or air resistance dissipate mechanical energy into other forms (e.g., heat).

Reason is false: The work done by a non-conservative force is not always negative. For example, an applied external force can be non-conservative and do positive work on a system.

Thus, A is true but R is false.

Assertion is false: Mechanical energy \(E = K + U\) is conserved when only conservative forces do work. External conservative forces (e.g., gravity) can act and do work, yet mechanical energy can be conserved.

Reason is false: If no non-conservative forces act, then \(\Delta K + \Delta U = 0\), meaning mechanical energy is conserved. In this scenario, the work done by external forces \(W_{ext}\) is not necessarily zero. For instance, gravity does work when an object falls, but \(E\) is conserved.

Therefore, both Assertion and Reason are false.

Assertion is true: An athlete pushes backward on the ground, and the ground exerts a forward static friction force on the athlete, causing acceleration.

Reason is true: The static friction force acts in the direction of the athlete's motion. Therefore, it does positive work, which directly increases the athlete's kinetic energy \(K\) according to the work-energy theorem.

Both A and R are true, and R provides the correct explanation for A.

Work done by internal forces depends on relative displacement (\(\Delta \vec{r}\)) which is frame-independent, so (A) is true.

The value of a force is generally not independent of the reference frame, especially if non-inertial frames are considered, so (R) is false.

Work done (\(W = \vec{F} \cdot \vec{d}\)) depends on displacement (\(\vec{d}\)) which is frame-dependent in different inertial frames. Therefore, work is not always the same and is not an invariant quantity. Both Assertion and Reason are false.

For a bound system, the potential energy (\(U\)) is negative and its magnitude is greater than the kinetic energy (\(K\)). Since total energy is \(E = K + U\), \(E\) must be negative. Both Assertion and Reason are true, and Reason correctly explains Assertion.

Work is \(W = \vec{F} \cdot \vec{d} = Fd cos\theta\). If \(theta = 0\), \(W\) is positive, so (A) is true. Frictional force always opposes motion, so work done by it is negative or zero, never positive. So (R) is true. However, (R) does not explain (A).

Work done by non-conservative forces can be positive (e.g., applied force) or negative (e.g., friction), so (A) is false.

Non-conservative forces can dissipate (friction) or add (engine thrust) energy. So (R) is also false.