Physics MCQs for NEET — Practice Questions with Answers

The text concludes, 'From the above discussion it follows that the total time taken from a point on the object to the corresponding point on the image is the same measured along any ray.'

The text explains, 'As the pulse arrives at the wall, it exerts a force on the wall. By Newton’s Third Law, the wall exerts an equal and opposite force on the string generating a reflected pulse that differs by a phase of $\pi$.'

According to the NCERT text, 'Every force encountered in mechanics does not have an associated potential energy. For example, work done by friction over a closed path is not zero and no potential energy can be associated with friction.' Gravitational, spring, and electrostatic forces are all conservative forces and thus have associated potential energy functions.

The NCERT text states: 'If the two forces on the body consist of a conservative force $F_c$ and a non-conservative force $F_{nc}$, the conservation of mechanical energy formula will have to be modified. By the WE theorem $(F_c + F_{nc}) \Delta x = \Delta K$. But $F_c \Delta x = - \Delta V$. Hence, $\Delta(K + V) = F_{nc} \Delta x$, which means $\Delta E = F_{nc} \Delta x$. Over the path this assumes the form $E_f - E_i = W_{nc}$ where $W_{nc}$ is the total work done by the non-conservative forces over the path.' Therefore, the total mechanical energy changes by an amount equal to the work done by the non-conservative force.

The NCERT text states that for a conservative force, 'The work done by the conservative force depends only on the end points.' It also implies that if the work done or kinetic energy did depend on factors like path taken, 'the force would be called non-conservative.' Thus, for a non-conservative force, the work done depends on the path taken.

The NCERT text points out that for conservative forces, 'the work done by this force in a closed path is zero.' Conversely, for non-conservative forces like friction, 'work done by friction over a closed path is not zero and no potential energy can be associated with friction.' This directly indicates that the work done by a non-conservative force over a closed path is generally non-zero.

The NCERT exercise asks about 'work done by the resistive force of air on a vibrating pendulum in bringing it to rest,' implying it does negative work, causing energy loss. The text explicitly states 'work done by friction or viscous force on a moving body is negative.' When a non-conservative force like air resistance does negative work, it causes a reduction in the system's total mechanical energy, leading to damping and eventually bringing the pendulum to rest.

The general form of the Work-Energy Theorem states $\Delta K = W_{net}$. The NCERT text elaborates: 'By the WE theorem ($F_c + F_{nc}) \Delta x = \Delta K$.' Here, ($F_c + F_{nc}$) represents the net force, so $\Delta K$ is equal to the total work done by all forces (conservative and non-conservative).

The NCERT exercises ask to state the sign of work done by various forces. One option is 'work done by friction on a body sliding down an inclined plane'. In such a scenario, friction acts opposite to the direction of motion, thus doing negative work. The text also generally states 'The work done by the friction or viscous force on a moving body is negative.' Gravity, spring force, and electrostatic force are conservative forces and can do positive or negative work, but friction is explicitly non-conservative and its work is often negative during motion.

The NCERT text states: 'The total mechanical energy of a system is conserved if the forces, doing work on it, are conservative.' Conversely, if mechanical energy is not conserved, it implies that non-conservative forces are doing work on the system. The equation $\Delta E = W_{nc}$ directly shows that a change in total mechanical energy ($\Delta E$) is due to the work done by non-conservative forces ($W_{nc}$). While conservative forces may still be present, it's the non-conservative ones that cause non-conservation of mechanical energy.