Learn › Homi Bhabha Class 9 · Physics › Laws of Motion & Force
Laws of Motion & Force — Homi Bhabha Class 9 Physics MCQs with Solutions
Free Homi Bhabha Class 9 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.
▶ Practise Laws of Motion & Force online (free)
Subtopics
Sample questions with solutions
Q1 — Newton's Laws of Motion · easy · theory
Newton's first law of motion is also commonly known as the:
A. law of momentum
B. law of action and reaction
C. law of acceleration
D. law of inertia ✓ Correct
Solution: The first law states that a body stays at rest or in uniform motion unless acted on by a net external force, which is exactly the idea of inertia, so it is called the law of inertia.
Q2 — Inertia · easy · theory
The inertia of a body depends only on its:
A. mass ✓ Correct
B. colour
C. shape
D. speed
Solution: Inertia is the resistance to a change in the state of motion and is measured by mass; it does not depend on speed, shape or colour.
Q3 — Inertia · easy · theory
When a bus at rest suddenly starts moving forward, the passengers tend to fall backward. This is due to:
A. inertia of motion
B. inertia of direction
C. friction between feet and floor
D. inertia of rest ✓ Correct
Solution: The lower body moves with the bus while the upper body, by inertia of rest, tends to stay in place, so the passenger falls backward.
Q4 — Momentum & Impulse · easy · numerical
The momentum of a body of mass 2 kg moving with a velocity of 5 m/s is:
A. 25 kg m/s
B. 10 kg m/s ✓ Correct
C. 2.5 kg m/s
D. 7 kg m/s
Solution: Momentum p = mass × velocity = 2 × 5 = 10 kg m/s.
Q5 — Friction · easy · theory
The force of friction between two surfaces in contact always acts:
A. vertically downward
B. opposite to the direction of relative motion ✓ Correct
C. in the direction of motion
D. perpendicular to the surfaces
Solution: Friction is a resistive force; it always opposes the relative motion (or tendency of motion) between the surfaces in contact.
Q6 — Friction · easy · theory
Which of the following is the best way to reduce friction between two moving machine parts?
A. pressing the surfaces harder together
B. making the surfaces rougher
C. increasing the load on the parts
D. applying a lubricant such as oil ✓ Correct
Solution: A lubricant forms a thin layer that separates the surfaces and fills the tiny grooves, greatly reducing friction; the other options all increase friction.
Q7 — Conservation of Momentum · easy · theory
When no net external force acts on a system, its total momentum:
A. keeps decreasing steadily
B. remains constant ✓ Correct
C. keeps increasing steadily
D. is always zero
Solution: The law of conservation of momentum states that in the absence of a net external force the total momentum of a system stays constant.
Q8 — 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 $F = ma$.
Q9 — 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.
Q10 — 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.
Q11 — 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.
Q12 — 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 ($F = \Delta p / \Delta t$).
Q13 — 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.
Q14 — 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 $m_1 = 4m_2$, $v_1 : v_2 = 1 : 4$.
Q15 — 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.
Q16 — 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.
Q17 — 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.
Q18 — 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).
Q19 — 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.
Q20 — 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.
Q21 — Newton's Laws of Motion · hard · theory
A horse pulls a cart and both accelerate forward, even though the cart pulls back on the horse with an equal and opposite force. This is possible because:
A. the action and reaction cancel out, so no force is really needed to move
B. the horse’s pull on the cart is larger than the cart’s pull on the horse
C. the action and reaction act on different bodies, and the forward push of the ground on the horse’s hooves exceeds the backward resistance on the cart ✓ Correct
D. the reaction force acts a moment later than the action force
Solution: Action and reaction act on different bodies and never cancel on the same body. The horse pushes the ground back and the ground pushes it forward; when this exceeds the resistance on the cart, the whole system accelerates.
Q22 — Newton's Laws of Motion · hard · numerical
A body of mass 2 kg moving at 6 m/s is brought to rest in 3 s by a constant force. The magnitude of the force is:
A. 4 N ✓ Correct
B. 3 N
C. 2 N
D. 12 N
Solution: Acceleration a = (0 - 6)/3 = -2 m/s²; force magnitude F = m|a| = 2 × 2 = 4 N.
Q23 — Inertia · hard · theory
Assertion (A): A body of larger mass needs a larger force to give it the same acceleration as a lighter body. Reason (R): The inertia of a body is directly proportional to its mass.
A. A is false but R is true
B. A is true but R is false
C. Both A and R are true but R is NOT the correct explanation of A
D. Both A and R are true and R is the correct explanation of A ✓ Correct
Solution: From F = ma, a fixed acceleration needs force proportional to mass, and mass is the measure of inertia; so R (inertia is proportional to mass) correctly explains A.
Q24 — Momentum & Impulse · hard · numerical
A constant force of 5 N acts for 4 s on a 2 kg body starting from rest. The final velocity of the body is:
A. 2.5 m/s
B. 10 m/s ✓ Correct
C. 5 m/s
D. 40 m/s
Solution: Impulse = F × t = 5 × 4 = 20 kg m/s equals the change in momentum. Starting from rest, m × v = 20, so v = 20/2 = 10 m/s.
Q25 — Friction · hard · theory
Ball bearings are fitted between the moving parts of many machines mainly to:
A. increase the friction so the parts grip better
B. replace sliding friction with the much smaller rolling friction ✓ Correct
C. remove friction completely from the machine
D. increase the area of contact between the parts
Solution: The balls roll instead of slide, converting sliding friction into rolling friction, which is far smaller; this reduces wear and energy loss. Friction can be reduced but never made exactly zero.
Q26 — Conservation of Momentum · hard · theory
A rocket rises by ejecting hot gases downward at high speed. The forward motion of the rocket is best explained by the principle of:
A. conservation of momentum ✓ Correct
B. inertia of rest
C. conservation of energy
D. the rocket pushing against the surrounding air
Solution: The gases carry momentum downward, so the rocket must gain an equal amount of momentum upward to keep the total constant; this is conservation of momentum, which is why rockets also work in the vacuum of space where there is no air to push against.
Q27 — 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.
Q28 — 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.
Q29 — 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.
Q30 — 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.