Force and Laws of Motion – MCQs with Answers and Explanations
CBSE Class 9 — Physics
Chapter 9: Force and Laws of Motion — 50 MCQs with Answers & Clear Explanations (NCERT-aligned)
Content Bank — Key formulas & reminders
- F = m·a — net force equals mass times acceleration.
- p = m·v — momentum; Impulse = Δp = F·Δt.
- Friction: static (f_s ≤ μ_s N) and kinetic (f_k ≈ μ_k N).
- Action–reaction pairs act on different bodies (Newton III).
Instructions: Choose one correct option per question. On selecting an option the correct answer and a clear explanation will appear immediately.
Basics & Definitions (Q1–Q8)
Q1.
Which of the following is the SI unit of force?
Answer: B — kg·m/s² (Newton). The SI unit of force is newton (N). 1 N = 1 kg·m/s². Option A is momentum unit, C is impulse (N·s), D is pressure.
Q2.
Momentum of a body depends on:
Answer: C — mass and velocity. Momentum p = m·v. It depends on both mass and velocity; acceleration relates to change in velocity but not directly to instantaneous momentum.
Q3.
Impulse is defined as:
Answer: B — change in momentum. Impulse J = F·Δt equals the change in momentum Δp. It has unit N·s (equivalent to kg·m/s).
Q4.
Inertia of a body is directly related to:
Answer: C — mass. Inertia is the resistance to change in motion and is quantified by mass. Greater mass → greater inertia.
Q5.
Which of the following best describes weight?
Answer: B — gravitational force on a body. Weight W = m·g and is a force; mass is intrinsic and density is mass per unit volume.
Q6.
Which quantity is conserved in an isolated system when no external force acts?
Answer: A — momentum. Linear momentum is conserved in absence of external forces. Kinetic energy is conserved only in elastic collisions, not always.
Q7.
Which of the following is a vector quantity?
Answer: C — displacement. Displacement is a vector (magnitude + direction). Speed, mass, temperature are scalars.
Q8.
If the net force on an object is zero, which statement is correct?
Answer: C — either at rest or moving with constant velocity. Net force zero implies zero acceleration (Newton II), so motion is uniform or the object is at rest.
Newton's Laws (Q9–Q17)
Q9.
Newton's first law is also known as the law of:
Answer: B — inertia. Newton I states that a body resists change in its state of motion — this is inertia.
Q10.
Newton's second law gives the relationship between:
Answer: C — force, mass and acceleration. F = m·a describes this relationship; acceleration in direction of net force.
Q11.
Action and reaction forces act on:
Answer: B — two different bodies. Newton's third law pairs act on different bodies; therefore they do not cancel each other on the same object.
Q12.
Which law explains why passengers lurch forward when a car stops suddenly?
Answer: A — Newton's first law. Due to inertia, passengers tend to maintain their motion when the car decelerates suddenly.
Q13.
If action force on body A by B is 10 N to the right, the reaction on B by A is:
Answer: B — 10 N to the left on B. Action and reaction are equal in magnitude and opposite in direction; the reaction acts on B opposite to the action.
Q14.
Which statement is true about Newton’s second law?
Answer: C — both A and B. The general form is F = dp/dt; for constant mass it reduces to F = m·a.
Q15.
Which physical quantity indicates the 'amount of motion' of a body?
Answer: B — momentum. Momentum (p = m·v) quantifies how much motion a body has.
Q16.
Which of the following best illustrates Newton's third law?
Answer: B — recoil of a gun when fired. The bullet (action) gets forward momentum while the gun (reaction) recoils backward; forces are equal and opposite.
Q17.
Which is NOT a consequence of Newton's laws?
Answer: C — existence of fictitious forces in accelerating frames. Fictitious forces arise in non-inertial (accelerating) frames to apply Newton's laws there; they are not direct consequences but are introduced to use Newtonian mechanics in such frames.
Force & Acceleration (Q18–Q26)
Q18.
If net force on a 2 kg body is 10 N, its acceleration is:
Answer: B — 5 m/s². a = F/m = 10 / 2 = 5 m/s².
Q19.
When a constant force acts on an object for longer time, its change in momentum will:
Answer: C — increase. Δp = F·Δt, so for constant F, larger Δt → larger change in momentum.
Q20.
A 5 kg object is moving at 4 m/s. What is its momentum?
Answer: C — 20 kg·m/s. p = m·v = 5 × 4 = 20 kg·m/s.
Q21.
Doubling the mass of an object while applying the same net force will:
Answer: B — halve its acceleration. a = F/m, so doubling m halves a for the same F.
Q22.
If a car's engine provides a constant thrust and the car's mass decreases (fuel burnt), its acceleration will:
Answer: B — increase. For same thrust (force), a = F/m increases as mass m decreases.
Q23.
A 10 N and 6 N force act on a body in opposite directions. The net force is:
Answer: B — 4 N. Net = 10 − 6 = 4 N in direction of larger force.
Q24.
Which expression gives stopping distance s when initial speed is v and deceleration is a?
Answer: B — s = v² / (2a). Using v² = u² + 2as with final velocity zero (u = v) gives s = v²/(2a).
Q25.
A body of mass m experiences zero net force. Which of the following must be true?
Answer: B — a = 0. Zero net force implies zero acceleration. Velocity may be zero or constant non-zero.
Q26.
For a body in circular motion at constant speed, which statement is correct?
Answer: B — net force is towards centre. Centripetal force directed to centre causes centripetal acceleration despite constant speed, because velocity direction changes.
Friction (Q27–Q33)
Q27.
Static friction acts when:
Answer: C — tendency to move but no relative motion. Static friction prevents motion up to a maximum value; if no tendency to move then static friction may be zero.
Q28.
Which of these reduces friction between two surfaces?
Answer: A — polishing and lubrication. These reduce asperity contact and allow smoother sliding; increasing roughness or normal force typically increases friction; sand increases traction in some contexts but increases friction.
Q29.
Which coefficient is usually larger?
Answer: B — μ_s (static). Static friction coefficient is typically larger than kinetic because starting motion requires breaking microscopic contacts.
Q30.
Maximum static friction is proportional to:
Answer: A — normal force. f_s(max) = μ_s N; surface area (for many practical surfaces) does not significantly affect friction if pressure distribution consistent.
Q31.
Which situation increases stopping distance the most?
Answer: B — wet/icy road (lower μ). Lower friction (μ) reduces braking force → lower deceleration → larger stopping distance.
Q32.
Work done against friction is mainly converted into:
Answer: C — thermal energy. Friction dissipates mechanical work into heat and sometimes sound; it causes wear and heating of surfaces.
Q33.
Rolling friction is usually:
Answer: B — much smaller than sliding friction. Rolling friction (like ball bearings) is usually much less than sliding friction, making rolling preferred for transport to reduce energy loss.
Momentum & Impulse (Q34–Q41)
Q34.
A ball with momentum 10 kg·m/s is brought to rest in 0.2 s. The average force applied is:
Answer: B — −50 N (direction opposite to motion). Δp = −10 kg·m/s; F_avg = Δp/Δt = −10 / 0.2 = −50 N. Negative indicates opposite to initial momentum direction.
Q35.
Which design feature reduces force received by passengers during a crash?
Answer: B — crumple zones and airbags. These increase Δt for stopping, reducing average force (F = Δp/Δt) on occupants; thin belts or rigid surfaces increase peak forces.
Q36.
Total momentum before collision equals total momentum after collision if:
Answer: C — both A and B. Momentum is conserved when external forces are negligible during collision; internal forces are those between colliding bodies.
Q37.
A 0.2 kg ball moving at 15 m/s hits a wall and rebounds with speed 10 m/s. If collision time is 0.05 s, average force magnitude on the ball is:
Answer: A — 100 N. Δp = m(v_f − v_i) = 0.2(−10 − 15) = 0.2(−25) = −5 kg·m/s (sign indicates reversal). |F| = |Δp|/Δt = 5 / 0.05 = 100 N.
Q38.
Which of the following statements is true about perfectly inelastic collisions?
Answer: B — momentum conserved, kinetic energy not conserved. In perfectly inelastic collisions bodies stick together; momentum conserved but some kinetic energy converted to other forms.
Q39.
Which engineering feature increases the collision time to reduce forces?
Answer: B — crumple zones and padded interiors. They deform to absorb energy and increase Δt, thereby reducing average forces on occupants.
Q40.
Impulse has the same units as:
Answer: B — momentum. Impulse (N·s) is equivalent to kg·m/s, same as momentum.
Q41.
To decrease the force experienced during a ball catch, the fielder should:
Answer: B — increase catching time by moving hands backward. Increasing Δt reduces average force (F = Δp/Δt). Stiff arms reduce time and increase force.
Action–Reaction & Applications (Q42–Q47)
Q42.
When a book rests on a table, the book exerts a force on the table equal to:
Answer: B — its weight (downward). The book pushes down on table with force equal to its weight; table reacts upward with equal normal force.
Q43.
How does a rocket move forward in space where there is vacuum?
Answer: B — expels gas backward; reaction pushes rocket forward. Rocket propulsion works by action–reaction and conservation of momentum, not by pushing against air.
Q44.
A swimmer moves forward by:
Answer: A — pushing water backward. The action is pushing water backward; reaction from water pushes swimmer forward (third law).
Q45.
Why do rockets have staged boosters?
Answer: B — to discard empty mass and improve acceleration. Jettisoning spent stages reduces mass, so for the same thrust acceleration increases (a = F/m), improving efficiency.
Q46.
Which is true about action–reaction forces?
Answer: B — act on different objects and do not cancel on each object. They are equal/opposite but affect each body separately; one cannot cancel the other on same body.
Q47.
Which of the following is an example of action–reaction?
Answer: B — person standing on ground and ground pushing back. Person exerts weight on ground (action); ground exerts equal normal reaction upward (reaction) on person.
Problem Solving, Equilibrium & Miscellaneous (Q48–Q50)
Q48.
Which is the correct first step while solving a force problem?
Answer: B — draw a free-body diagram. Free-body diagrams show all forces and directions, making equations and sign conventions clear before calculation.
Q49.
A body is in equilibrium when:
Answer: C — both net force and net torque zero. For full static equilibrium (no linear or rotational acceleration), both conditions must be satisfied.
Q50.
Which precaution is NOT useful for reducing injuries in collisions?
Answer: D — removing crumple zones is NOT useful. Crumple zones increase impact time and absorb energy; removing them increases forces on occupants and injury risk.
These MCQs are strictly aligned to NCERT Class 9 Physics Chapter 9 — Force and Laws of Motion, and designed for clear concept understanding and CBSE board exam practice.