Laws of Motion MCQs for NEET — Physics Questions with Answers

As per Newton's First Law, uniform velocity (constant velocity) means that the acceleration is zero. According to Newton's Second Law ($F=ma$), if $a=0$, then the net external force $F$ must be zero. The NCERT text also states: 'To summarise, if the net external force is zero, ... a body in motion continues to move with a uniform velocity.'

The NCERT text refers to 'Aristotle’s fallacy' and states, 'It is incorrect to assume that a net force is needed to keep a body in uniform motion.' This implies Aristotle believed a continuous force was necessary to maintain motion, which Galileo and Newton later disproved.

The text states: 'However, if there were no friction, the ball would continue to move with a constant velocity on the horizontal plane.' A perfectly smooth surface implies no friction, and if air resistance is also ignored (which is often implied in 'perfectly smooth' or idealized scenarios), the ball would maintain constant velocity due to inertia.

If a block slides down a rough inclined plane at constant velocity, its acceleration is zero. According to Newton's First Law and Second Law ($F=ma$), if $a=0$, then the net external force on the block must be zero. The forces (gravity, normal force, friction) balance each other out during uniform motion.

The NCERT text explicitly states: 'The second law is consistent with the First Law ($F = 0$ implies $a = 0$).' This means if the net external force is zero, the acceleration is zero, which is the core statement of the First Law.

According to the NCERT text (Chapter: LAWS_OF_MOTION, 4.8 EQUILIBRIUM OF A PARTICLE), 'Equilibrium of a particle in mechanics refers to the situation when the net external force on the particle is zero.'. Rotational equilibrium conditions do not apply to a particle.

As stated in the NCERT text (Chapter: LAWS_OF_MOTION, 4.8 EQUILIBRIUM OF A PARTICLE), 'Equilibrium under three concurrent forces $F_1$, $F_2$ and $F_3$ requires that the vector sum of the three forces is zero. $F_1 + F_2 + F_3 = 0$'. This ensures translational equilibrium.

The NCERT text (Chapter: LAWS_OF_MOTION) states, 'A particle is in equilibrium under the action of forces $F_1$, $F_2,... F_n$ if they can be represented by the sides of a closed n-sided polygon with arrows directed in the same sense.'

From the NCERT text (Chapter: LAWS_OF_MOTION, after Example 4.6), 'Equation (4.11) implies that $F_{1x} + F_{2x} + F_{3x} = 0$, $F_{1y} + F_{2y} + F_{3y} = 0$, $F_{1z} + F_{2z} + F_{3z} = 0$'. This means the sum of force components along each of the x, y, and z axes must be zero for equilibrium.

The NCERT text (Chapter: LAWS_OF_MOTION, 4.8 EQUILIBRIUM OF A PARTICLE) explicitly states, 'According to the first law, this means that, the particle is either at rest or in uniform motion.'