Physics MCQs for NEET — Practice Questions with Answers

The NCERT states: 'Thus if $p_b$ is the momentum of the bullet after firing and $p_g$ is the recoil momentum of the gun, $p_g = -p_b$ i.e. $p_b + p_g = 0$. That is, the total momentum of the (bullet + gun) system is conserved.' This implies that the initial total momentum (zero) equals the final total momentum.

The NCERT text explains: 'In an isolated system (i.e. a system with no external force), mutual forces between pairs of particles in the system can cause momentum change in individual particles, but since the mutual forces for each pair are equal and opposite, the momentum changes cancel in pairs and the total momentum remains unchanged.'

The NCERT clearly states: 'The total momentum of an isolated system of particles is conserved.' An isolated system is one with no external force.

The NCERT states under 'POINTS TO PONDER': 'During a collision: (a) the total linear momentum is conserved at each instant of the collision; (b) the kinetic energy conservation (even if the collision is elastic) applies after the collision is over and does not hold at every instant of the collision.'

The question asks 'Is momentum conserved in the collision?' If we consider just the molecule, its momentum changes in direction, so its momentum isn't conserved. However, for the system of the molecule and the wall, momentum is conserved. The NCERT implicitly refers to the 'system' being conserved. When a molecule hits a wall and rebounds, the wall experiences an impulse equal and opposite to the impulse on the molecule, ensuring conservation of momentum for the molecule-wall system. The text for the example problem 5.14 doesn't explicitly state the answer for the question posed within the problem statement, but general principle implies system momentum conservation.

The NCERT text clearly states: 'The second and third laws of motion lead to an important consequence: the law of conservation of momentum.'

According to the NCERT text, 'In forward bias, the current first increases very slowly, almost negligibly, till the voltage across the diode crosses a certain value. After the characteristic voltage, the diode current increases significantly (exponentially), even for a very small increase in the diode bias voltage.'

The NCERT states, 'This voltage is called the threshold voltage or cut-in voltage (~0.2V for germanium diode and ~0.7 V for silicon diode).'

The NCERT text mentions, 'For the diode in reverse bias, the current is very small (~μA) and almost remains constant with change in bias. It is called reverse saturation current.'

As per the NCERT, 'For diodes, we define a quantity called dynamic resistance as the ratio of small change in voltage ΔV to a small change in current ΔI: $r_d = \Delta V / \Delta I$ (14.6)'