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Laws of Motion & Force — Homi Bhabha Exam Physics MCQs with Solutions
Free Homi Bhabha Exam Physics Laws of Motion & Force MCQs with step-by-step solutions covering Newton's Laws of Motion, Inertia, Momentum & Impulse, Friction, Conservation of Momentum. Practise online on Prepizo — no login needed.
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Sample questions with solutions
Q1 — Newton's Laws of Motion · hard · theory
Newton's second law of motion states that the net force on a body equals:
A. the rate of change of its momentum ✓ Correct
B. the product of its mass and velocity
C. its change in momentum
D. its mass times its speed
Solution: $F = \dfrac{\Delta p}{\Delta t}$; for constant mass this reduces to $F = ma$.
Q2 — Newton's Laws of Motion · hard · numerical
A force of 10 N acts to the right and a force of 4 N acts to the left on a 2 kg body. Its acceleration is:
A. 7 m/s² to the right
B. 3 m/s² to the right ✓ Correct
C. 5 m/s² to the right
D. 2 m/s² to the left
Solution: Net force $= 10 - 4 = 6$ N; $a = \dfrac{F}{m} = \dfrac{6}{2} = 3$ m/s² in the direction of the larger force.
Q3 — Inertia · hard · theory
Which of the following bodies, moving at the same speed, has the greatest inertia?
A. A bicycle
B. A car
C. A football
D. A loaded truck ✓ Correct
Solution: Inertia increases with mass; the loaded truck has the largest mass and hence the greatest inertia.
Q4 — Inertia · hard · theory
Assertion (A): A heavy stone is harder to start moving than a light ball. Reason (R): Greater mass means greater inertia.
A. A is false but R is true
B. Both A and R are true but R is NOT the correct explanation of A
C. A is true but R is false
D. Both A and R are true and R is the correct explanation of A ✓ Correct
Solution: More mass means more inertia, so more force is needed to change its state of rest — R correctly explains A.
Q5 — Momentum & Impulse · hard · theory
A cricketer moves his hands backward while catching a fast ball to:
A. decrease the time of impact
B. increase the force on his hands
C. increase the time of impact and so reduce the force ✓ Correct
D. increase the momentum of the ball
Solution: Since impulse (change in momentum) is fixed, increasing the contact time reduces the force ($F = \Delta p / \Delta t$).
Q6 — Momentum & Impulse · hard · numerical
The momentum of a body changes by 20 kg·m/s in 4 s. The average force acting on it is:
A. 5 N ✓ Correct
B. 24 N
C. 80 N
D. 16 N
Solution: $F = \dfrac{\Delta p}{\Delta t} = \dfrac{20}{4} = 5$ N.
Q7 — Momentum & Impulse · hard · numerical
Two bodies have the same momentum. If the mass of the first is 4 times that of the second, the ratio of their velocities (first : second) is:
A. 4 : 1
B. 1 : 4 ✓ Correct
C. 1 : 1
D. 2 : 1
Solution: For equal momentum $m_1 v_1 = m_2 v_2$; with $m_1 = 4m_2$, $v_1 : v_2 = 1 : 4$.
Q8 — Friction · hard · theory
For two given surfaces, which statement is correct?
A. Friction is zero once motion starts
B. Kinetic friction is greater than static friction
C. Limiting (maximum static) friction is greater than kinetic friction ✓ Correct
D. Both are always equal
Solution: It takes a larger force to start motion than to keep it going, so limiting static friction exceeds kinetic friction.
Q9 — Friction · hard · numerical
A block presses on a surface with a normal force of 50 N. If the coefficient of friction is 0.2, the frictional force is:
A. 50.2 N
B. 10 N ✓ Correct
C. 250 N
D. 2.5 N
Solution: Friction $= \mu N = 0.2 \times 50 = 10$ N.
Q10 — Friction · hard · theory
Assertion (A): Friction is called a "necessary evil". Reason (R): Friction wastes energy as heat but is also needed for walking and braking.
A. Both A and R are true but R is NOT the correct explanation of A
B. Both A and R are true and R is the correct explanation of A ✓ Correct
C. A is false but R is true
D. A is true but R is false
Solution: Friction both wastes energy (evil) and is essential for walking/gripping (necessary) — R correctly explains why it is a necessary evil.
Q11 — Conservation of Momentum · hard · numerical
A gun of mass 4 kg fires a bullet of mass 0.02 kg at 100 m/s. The recoil velocity of the gun is:
A. 5 m/s
B. 2 m/s
C. 0.5 m/s ✓ Correct
D. 0.05 m/s
Solution: By conservation of momentum $0 = m_g v_g + m_b v_b \Rightarrow v_g = \dfrac{0.02 \times 100}{4} = 0.5$ m/s (opposite to the bullet).
Q12 — Conservation of Momentum · hard · numerical
A 2 kg trolley moving at 3 m/s collides and sticks to a stationary 1 kg trolley. Their common velocity after collision is:
A. 3 m/s
B. 6 m/s
C. 2 m/s ✓ Correct
D. 1.5 m/s
Solution: Momentum conserved: $2 \times 3 = (2+1)v \Rightarrow v = \dfrac{6}{3} = 2$ m/s.
Q13 — Conservation of Momentum · hard · numerical
A stationary bomb of mass 6 kg explodes into two equal pieces. If one piece moves at 4 m/s, the other moves at:
A. 4 m/s in the opposite direction ✓ Correct
B. 8 m/s in the opposite direction
C. 4 m/s in the same direction
D. 2 m/s in the opposite direction
Solution: Initial momentum is zero; with equal masses the two pieces must move with equal speed in opposite directions, so 4 m/s oppositely.
Q14 — Newton's Laws of Motion · medium · numerical
A force acts on a body of mass 5 kg producing an acceleration of 3 m/s². The magnitude of the force is:
A. 1.67 N
B. 8 N
C. 15 N ✓ Correct
D. 45 N
Solution: By Newton's second law $F = ma = 5 \times 3 = 15$ N.
Q15 — Newton's Laws of Motion · medium · theory
When a moving bus stops suddenly, a standing passenger lurches forward. This is best explained by:
A. Newton's first law (inertia of motion) ✓ Correct
B. Newton's third law
C. the law of conservation of momentum
D. Newton's second law
Solution: The lower body stops with the bus while the upper body tends to continue moving forward due to inertia of motion.
Q16 — Newton's Laws of Motion · medium · theory
Assertion (A): A body in uniform motion continues to move uniformly unless an external force acts on it. Reason (R): This is a statement of Newton's first law (law of inertia).
A. Both A and R are true but R is NOT the correct explanation of A
B. A is false but R is true
C. A is true but R is false
D. Both A and R are true and R is the correct explanation of A ✓ Correct
Solution: A is exactly the content of the first law, which R names — so R correctly explains A.
Q17 — Newton's Laws of Motion · medium · numerical
A body of mass 4 kg experiences a net force of 20 N. Its acceleration is:
A. 80 m/s²
B. 5 m/s² ✓ Correct
C. 0.2 m/s²
D. 16 m/s²
Solution: $a = \dfrac{F}{m} = \dfrac{20}{4} = 5$ m/s².
Q18 — Inertia · medium · theory
The inertia of a body depends on its:
A. mass ✓ Correct
B. acceleration
C. velocity
D. shape
Solution: Inertia is the tendency to resist a change in the state of motion, and it is measured by (and increases with) mass.
Q19 — Inertia · medium · theory
Dust particles are removed when a carpet is beaten with a stick. This is due to:
A. inertia of rest ✓ Correct
B. friction
C. inertia of direction
D. inertia of motion
Solution: Beating sets the carpet in motion while the dust, by inertia of rest, tends to stay in place and so separates.
Q20 — Inertia · medium · theory
When a playing card on a glass is flicked sharply, the coin on it drops into the glass. This illustrates:
A. conservation of energy
B. inertia of motion
C. inertia of rest ✓ Correct
D. Newton's third law
Solution: The card moves out quickly, but the coin, by inertia of rest, stays and falls into the glass.
Q21 — Inertia · medium · theory
A passenger in a car falls sideways when the car takes a sharp turn. This is due to:
A. gravity
B. inertia of direction ✓ Correct
C. inertia of motion
D. inertia of rest
Solution: The body tends to keep moving in its original straight-line direction, i.e. inertia of direction.
Q22 — Momentum & Impulse · medium · numerical
The momentum of a body of mass 2 kg moving at 5 m/s is:
A. 2.5 kg·m/s
B. 7 kg·m/s
C. 10 kg·m/s ✓ Correct
D. 25 kg·m/s
Solution: Momentum $p = mv = 2 \times 5 = 10$ kg·m/s.
Q23 — Momentum & Impulse · medium · numerical
A 3 kg body is accelerated from rest to 4 m/s. The impulse given to it is:
A. 12 N·s ✓ Correct
B. 1.33 N·s
C. 7 N·s
D. 0.75 N·s
Solution: Impulse = change in momentum $= m(v - u) = 3(4 - 0) = 12$ N·s.
Q24 — Momentum & Impulse · medium · theory
The SI unit of momentum is:
A. kg·m/s²
B. kg·m/s ✓ Correct
C. N·m
D. kg/m·s
Solution: Momentum = mass × velocity, so its unit is kg·m/s (equivalently N·s).
Q25 — Friction · medium · theory
The force of friction between two surfaces always acts:
A. opposite to the direction of relative motion ✓ Correct
B. downward
C. in the direction of motion
D. perpendicular to the surface
Solution: Friction opposes (or tends to oppose) the relative motion between the surfaces in contact.
Q26 — Friction · medium · theory
Ball bearings are used in machines because:
A. rolling friction is larger than sliding friction
B. they eliminate friction completely
C. rolling friction is much smaller than sliding friction ✓ Correct
D. they increase friction
Solution: Converting sliding into rolling greatly reduces friction, since rolling friction is much smaller.
Q27 — Friction · medium · theory
The treads (grooves) on vehicle tyres are provided to:
A. increase friction (grip) with the road ✓ Correct
B. reduce friction with the road
C. make the tyre look attractive
D. reduce the weight of the tyre
Solution: Treads increase the grip (friction) between tyre and road, improving traction and braking.
Q28 — Conservation of Momentum · medium · theory
The recoil of a gun and the forward motion of a rocket are both explained by:
A. conservation of energy
B. the law of friction
C. conservation of momentum ✓ Correct
D. inertia of rest
Solution: In each case, the momentum gained by the ejected mass equals the momentum gained by the system in the opposite direction.
Q29 — Conservation of Momentum · medium · theory
The total momentum of a system is conserved only when:
A. no net external force acts on the system ✓ Correct
B. the bodies have equal mass
C. friction is present
D. the system is at rest
Solution: Momentum is conserved for an isolated system, i.e. when the net external force is zero.
Q30 — Conservation of Momentum · medium · numerical
Two bodies of masses 2 kg and 3 kg move towards each other with momenta of equal magnitude 6 kg·m/s. The total momentum of the system is:
A. 0 kg·m/s ✓ Correct
B. 6 kg·m/s
C. 12 kg·m/s
D. 5 kg·m/s
Solution: Equal and opposite momenta cancel, giving a total system momentum of zero.