All 24 MCQs of 9th Class Physics Chapter 3, Dynamics, with answers and the reason each option is right or wrong: the 9 textbook exercise MCQs plus 15 more board-style MCQs. Punjab Board new syllabus.
Try each one first: tap an option to check it, then read the reasoning and why the others are wrong.
A note about the questions
The wording follows published solved exercises and every answer was re-checked by hand, so the wording or the order of options may differ slightly from your print.
Answered 0 of 24 · Correct 0
Set A: Textbook exercise 9 MCQs
The multiple-choice questions at the end of Chapter 3 in your book, in the book's option order.
Q3.1Newton's Laws of Motion
When we kick a stone, we get hurt. This is due to:
Answer(d) reaction
Step by step
The foot exerts a force (action) on the stone.
By Newton's third law, the stone exerts an equal and opposite force (reaction) on the foot.
That reaction force on the foot is what hurts.
Why the others are wrong
(a) inertia explains why the stone resists starting to move, not why the foot is pushed back.
(b) velocity is not a force and cannot push on the foot.
(c) momentum changes in the collision, but the pain comes from the force on the foot.
Remember
It hurts because the stone pushes back: reaction.
Q3.2Newton's Laws of Motion
An object will continue its motion with constant acceleration until:
Answer(b) the resultant force on it is zero
Step by step
By F = ma, a body accelerates only while a resultant force acts on it.
When the resultant force becomes zero, the acceleration becomes zero.
The body then keeps moving with constant velocity, as Newton's first law says.
Why the others are wrong
(a) a decreasing force would change the acceleration too, but the book's point is the first law: acceleration stops when the resultant force is zero. One answer key marks (a); (b) matches the explanation both keys give.
(c) an increasing force would increase the acceleration, not end it.
(d) a force at right angles to the velocity turns the body in a circle; it does not stop it accelerating.
Remember
No resultant force, no acceleration.
Q3.3Force and Types of Force
Which of the following is a non-contact force?
Answer(c) electrostatic force
Step by step
A contact force needs the bodies to touch; a non-contact force acts across a gap.
Friction, air resistance and tension all need contact.
Charged bodies attract or repel without touching, so electrostatic force is non-contact.
Why the others are wrong
(a) friction acts only between surfaces in contact.
(b) air resistance needs the air to touch the moving body.
(d) tension acts through a string attached to the body.
A ball with momentum p hits a solid wall and bounces back with the same speed. Its momentum p′ after the collision is:
Answer(b) p' = −p
Step by step
Momentum p = mv is a vector: it has a direction.
The ball comes back at the same speed, so the size of its momentum is unchanged.
Its direction is reversed, so p′ = −p. (The change in momentum is −2p.)
Why the others are wrong
(a) this ignores that the direction has reversed.
(c) the speed did not double.
(d) −2p is the change in momentum, not the final momentum.
Remember
Bounces back at the same speed: same size, opposite sign.
Q3.5Conservation of Momentum
A particle of mass m moving with velocity v collides head-on with an identical particle at rest. In an elastic collision, the velocity of the first particle after the collision is:
Answer(c) 0
Step by step
Momentum before = mv + 0 = mv.
In an elastic head-on collision of equal masses the two simply swap velocities.
So the first particle stops (velocity 0) and the second moves off with v; momentum after = 0 + mv.
Why the others are wrong
(a) the first particle cannot keep its full velocity after hitting the other one.
(b) bouncing straight back at v would need the second particle to be much heavier, like a wall.
(d) −v/2 does not conserve momentum here. (If the particles stuck together they would move at +v/2.) The book prints this option as "−1/2", a misprint.
Remember
Equal masses, elastic, one at rest: they swap velocities (think of Newton's cradle).
Q3.6Conservation of Momentum
Conservation of linear momentum is equivalent to:
Answer(c) Newton's third law
Step by step
During a collision, body 1 pushes body 2 and body 2 pushes back equally (third law).
Both forces act for the same time, so the impulses, and hence the momentum changes, are equal and opposite.
They cancel, so the total momentum is conserved: the conservation law comes from the third law.
Why the others are wrong
(a) the first law is about a single body with no resultant force.
(b) the second law links force and the rate of change of momentum of one body; conservation needs the action-reaction pair.
(d) it is linked to the third law.
Remember
Equal and opposite forces → equal and opposite momentum changes → total conserved.
Q3.7Momentum and Impulse
A 5 kg object moving at 10 m s⁻¹ is acted on by a constant force for 5 s, after which it moves at 2 m s⁻¹ in the opposite direction. The force is:
Answer(c) −12 N
Step by step
Take the first direction as positive: vᵢ = +10 m s⁻¹ and vf = −2 m s⁻¹.
Change in momentum = m(vf − vᵢ) = 5 × (−2 − 10) = −60 kg m s⁻¹.
F = change in momentum ÷ time = −60 ÷ 5 = −12 N.
Why the others are wrong
(a) 5 N does not come from these numbers.
(b) −10 N uses only the stopping part (10 → 0) and forgets the reversal to −2.
(d) −15 N does not come from these numbers.
Remember
Reversing direction means the final velocity gets a minus sign.
Q3.8Momentum and Impulse
When a large force acts on an object for a very short time, which is easiest to determine?
Answer(c) the product of force and time
Step by step
In a hit or a kick the force is huge and the contact time very short, so both are hard to measure.
Impulse = force × time = change in momentum.
The change in momentum is found from mass and the velocities before and after, so the product F × t is easy to determine.
Why the others are wrong
(a) the force changes during the very short contact and is hard to measure.
(b) the contact time is too short to measure easily.
(d) the impulse can be found.
Remember
Impulse = F × t = Δp.
Q3.9Friction
A lubricant is put between two surfaces because it:
Answer(c) prevents direct contact between the surfaces
Step by step
Friction comes from the tiny bumps of two surfaces catching on each other.
A lubricant forms a thin layer between the surfaces.
The surfaces no longer touch directly, so friction is much smaller.
Why the others are wrong
(a) less friction does mean less heating, but that is a result, not the reason.
(b) ball bearings are a separate way of reducing friction (rolling).
(d) a lubricant does not turn sliding into rolling.
Remember
Lubricants keep surfaces apart.
Set B: More MCQs from this chapter 15 MCQs
Extra questions on the same ideas, of the kind set in board papers and class tests.
QB1Force and Types of Force
The SI unit of force, the newton, is equal to:
Answer(b) kg m s⁻²
Step by step
F = m × a.
Units: kg × m s⁻² = kg m s⁻², which is named the newton.
Why the others are wrong
(a) kg m s⁻¹ is the unit of momentum.
(c) kg m² s⁻² is the joule, the unit of energy.
(d) kg s⁻² has no metre in it.
Remember
1 N gives 1 kg an acceleration of 1 m s⁻².
QB2Mass, Weight and Inertia
The inertia of a body depends on its:
Answer(a) mass
Step by step
Inertia: the tendency of a body to keep its state of rest or of uniform motion.
A heavier (more massive) body is harder to start or stop, so inertia depends on mass.
Why the others are wrong
(b) a body at rest has inertia too.
(c) shape does not change how hard a body is to accelerate.
(d) colour has nothing to do with inertia.
Remember
More mass, more inertia.
QB3Mass, Weight and Inertia
The weight of a 5 kg bag on the Earth (g = 10 m s⁻²) is:
Answer(d) 50 N
Step by step
Weight is the force of gravity on a body: W = mg.
W = 5 kg × 10 m s⁻² = 50 N.
Why the others are wrong
(a) 0.5 N divides by g instead of multiplying.
(b) 5 is the mass in kg, not the weight in newtons.
(c) 15 N adds instead of multiplying.
Remember
Weight in newtons ≈ mass in kg × 10.
QB4Mass, Weight and Inertia
When a body is taken from the Earth to the Moon, its mass:
Answer(b) stays the same
Step by step
Mass is the quantity of matter in a body.
Moving the body does not add or remove matter, so its mass is the same.
Its weight W = mg falls, because g on the Moon is about 1.6 m s⁻².
Why the others are wrong
(a) it is the weight that becomes about one-sixth.
(c) neither mass nor weight increases on the Moon.
(d) matter does not disappear.
Remember
Mass stays; weight changes.
QB5Newton's Laws of Motion
A force of 20 N acts on a 4 kg body. Its acceleration is:
Answer(c) 5 m s⁻²
Step by step
From F = ma, a = F ÷ m.
a = 20 ÷ 4 = 5 m s⁻².
Why the others are wrong
(a) 80 multiplies instead of dividing.
(b) 0.2 is m ÷ F, upside down.
(d) 24 adds the numbers.
Remember
Acceleration = force ÷ mass.
QB6Momentum and Impulse
The momentum of a 2 kg ball moving at 3 m s⁻¹ is:
Answer(a) 6 kg m s⁻¹
Step by step
Momentum p = mass × velocity.
p = 2 × 3 = 6 kg m s⁻¹.
Why the others are wrong
(b) 1.5 divides instead of multiplying.
(c) 5 adds instead of multiplying.
(d) 9 does not come from p = mv here.
Remember
Momentum = mass × velocity.
QB7Momentum and Impulse
The unit of momentum, kg m s⁻¹, is the same as:
Answer(d) N s
Step by step
1 N = 1 kg m s⁻².
N × s = kg m s⁻² × s = kg m s⁻¹, the unit of momentum.
Why the others are wrong
(a) N m is the unit of work (the joule) or torque.
(b) the joule is the unit of energy.
(c) N s⁻¹ is force per second.
Remember
Impulse (N s) = change in momentum (kg m s⁻¹).
QB8Newton's Laws of Motion
Action and reaction forces:
Answer(b) act on different bodies
Step by step
When A pushes B, B pushes A back equally and oppositely.
One force acts on B, the other on A: different bodies.
Forces on different bodies cannot cancel, so each body can still accelerate.
Why the others are wrong
(a) if they acted on the same body they would cancel, and nothing could ever accelerate.
(c) they act on different bodies, so they do not cancel.
(d) they are always opposite in direction.
Remember
Action and reaction: equal, opposite, on different bodies.
QB9Momentum and Impulse
Air bags and seat belts reduce injuries in a crash because they:
Answer(c) increase the time over which the momentum changes
Step by step
In a crash the passenger's momentum changes from mv to 0, whatever happens.
F = Δp ÷ t, so a longer stopping time means a smaller force.
Air bags and belts stretch out the stopping time, so the force on the body is smaller.
Why the others are wrong
(a) they reduce the force; that is the point.
(b) the mass does not change.
(d) the passenger still stops, so the change in momentum is the same.
Remember
Same Δp, more time, less force.
QB10Friction
The force of friction always acts:
Answer(a) opposite to the motion (or attempted motion)
Step by step
Friction acts along the surfaces in contact.
It always opposes sliding, or the attempt to slide.
Why the others are wrong
(b) friction never helps the sliding motion.
(c) the force at right angles to the surfaces is the normal reaction.
(d) friction acts along the surface, not upward.
Remember
Friction always fights the motion.
QB11Friction
Compared with sliding friction, rolling friction is:
Answer(c) much smaller
Step by step
In sliding, the whole contact surface rubs along.
A wheel rolls: each part touches the ground briefly and lifts off without rubbing.
So rolling friction is much smaller, which is why wheels and ball bearings are used.
Why the others are wrong
(a) it is the other way round.
(b) rolling friction is much smaller.
(d) it is small, but not zero.
Remember
Wheels and ball bearings: rolling beats sliding.
QB12Force and Types of Force
Which of the following is a contact force?
Answer(d) normal reaction
Step by step
A contact force needs the bodies to touch.
A table pushes up on a book only where they touch: the normal reaction.
Why the others are wrong
(a) gravity acts across empty space.
(b) magnets attract without touching.
(c) charges attract or repel without touching.
Remember
Gravitational, magnetic and electrostatic forces act at a distance.
QB13Air Resistance and Terminal Velocity
A skydiver is falling at terminal velocity. The net force on the skydiver is:
Answer(c) zero
Step by step
As the skydiver speeds up, air resistance grows.
At terminal velocity air resistance has grown to equal the weight.
The forces balance: net force 0, so acceleration 0 and constant velocity.
Why the others are wrong
(a) that would be the net force only at the very start, before air resistance builds up.
(b) the net force can never exceed the weight while falling.
(d) the forces balance, so the net force is zero.
Remember
Terminal velocity: forces balanced, no acceleration.
QB14Newton's Laws of Motion
A rocket moves forward because:
Answer(a) it pushes hot gases backward and the gases push the rocket forward
Step by step
The engine forces hot gases out backward at high speed (action).
The gases push the rocket forward with an equal force (reaction).
No air is needed, which is why rockets work in space.
Why the others are wrong
(b) rockets work in space, where there is no air to push against.
(c) gravity pulls the rocket back down, not forward.
(d) air friction opposes the rocket's motion.
Remember
Rockets push on their own exhaust, not on the air.
QB15Conservation of Momentum
A 4 kg gun fires a 20 g bullet at 200 m s⁻¹. The recoil speed of the gun is:
Answer(b) 1 m s⁻¹
Step by step
Before firing, the total momentum is 0.
After: m₁v₁ + m₂v₂ = 0, with m₁ = 0.02 kg, v₁ = 200 m s⁻¹, m₂ = 4 kg.
0.02 × 200 + 4 × v₂ = 0, so v₂ = −4 ÷ 4 = −1 m s⁻¹ (backward).
Why the others are wrong
(a) 0.5 m s⁻¹ is half the correct value.
(c) 2 m s⁻¹ is double the correct value.
(d) 4 is the bullet's momentum in kg m s⁻¹, not the gun's speed.
