Class 9 Physics - Chapter 2: Force and Laws of Motion

Balanced and Unbalanced Forces

Q1. Can an object remain at rest even if multiple forces are acting on it?

Yes, if the net force is zero (balanced), the object can remain at rest.

Q2. A moving car hits a wall and stops. Which force caused the stop?

The wall applied an unbalanced force (reaction), changing its motion.

Q3. Can a body experience unbalanced force and still move at constant speed?

No, unbalanced forces always change speed/direction.

Q4. Can a net force be zero even with multiple forces?

Yes, if all forces cancel out (equal and opposite forces).

Q5. A satellite moves at constant speed around Earth. Are forces balanced?

No. It's in circular motion, direction changes → acceleration exists.

Q6. What happens when two unequal forces act on a body in opposite directions?

The body moves in the direction of the larger force.

Q7. Why does a hanging object stretch a spring?

Its weight (unbalanced) stretches the spring until balanced by spring force.

Q8. Is friction an unbalanced force?

Yes, it acts opposite to motion and causes deceleration.

First Law of Motion

Q1. Why do passengers fall forward when a bus suddenly stops?

Due to inertia, upper body keeps moving.

Q2. Why is it dangerous to jump from a moving vehicle?

Due to inertia, body continues moving forward after landing.

Q3. Why do dust particles come off a hanging carpet when beaten?

Dust resists motion (inertia), remains at rest while carpet moves.

Q4. In space, will a ball thrown continue to move forever?

Yes, no external force → inertia keeps it moving.

Q5. What property resists change in motion?

Inertia.

Q6. Why is a heavy object harder to stop than a light one?

More mass → more inertia.

Q7. Is inertia a force?

No. It’s a property of matter.

Q8. Why do we tie luggage on roof of moving bus?

To prevent inertia from throwing it off on sudden stops.

Q9. If no force acts, will a moving body ever stop?

No, unless acted by friction or resistance.

Q10. Can a body have zero inertia?

No. All mass has inertia.

Inertia and Mass

Q1. Which has more inertia – iron ball or wooden ball of same size?

Iron ball → more mass.

Q2. Can a massless body have inertia?

No. Inertia depends on mass.

Q3. Why is a train hard to start and hard to stop?

Large mass → large inertia.

Q4. Does shape affect inertia?

No. Only mass matters.

Q5. Does inertia act in zero gravity?

Yes. It's independent of gravity.

Second Law of Motion

Q1. Two people apply equal force on a light and heavy object. Which accelerates more?

Lighter object.

Q2. Why does a small bullet cause big damage?

Small mass, high speed → large momentum → high force.

Q3. How do athletes increase time while landing long jump?

More time → less impact force.

Q4. Why are modern cars designed to crumple in crashes?

To increase impact time, reduce force on passengers.

Q5. Can a constant force produce uniform motion?

No. It produces uniform acceleration.

Q6. Is motion possible without force?

Yes. Constant motion. But not acceleration.

Q7. A box is pushed and moves. Why does it stop after a while?

Friction opposes motion → unbalanced force.

Q8. If mass doubles and force same, what happens to acceleration?

It becomes half.

Q9. Can force be zero and acceleration non-zero?

No. a = F/m. If F = 0, then a = 0.

Q10. What happens when force acts opposite to motion?

Object slows down.

Mathematical Form of Second Law

Q1. Define force in terms of rate of momentum change.

F = (mv - mu)/t

Q2. What does F = 0 mean for momentum?

Momentum remains constant.

Q3. When is F = ma used instead of momentum formula?

When mass is constant.

Q4. If time of impact doubles, what happens to force?

Force becomes half.

Q5. Can an object have momentum but no acceleration?

Yes, if moving with uniform velocity.

Q6. Can force be negative?

Yes, it just means opposite direction.

Q7. Is change in direction a form of acceleration?

Yes. Acceleration includes direction change.

Q8. Does high momentum always mean high speed?

Not always. Heavier mass can also contribute.

Q9. Why does a steel ball hurt more than a sponge ball of same size?

More mass → more momentum.

Q10. Force = 0, but object is moving. Is this possible?

Yes, with uniform velocity (no acceleration).

Third Law of Motion

Q1. Why can we not walk on ice easily?

Lack of reaction force from slippery ground.

Q2. Why does a boat move backward when we jump forward?

Action-reaction pair.

Q3. Why do rockets work in space where there's no air?

Action: gas expelled backward. Reaction: rocket moves forward.

Q4. If Earth pulls us, do we pull Earth too?

Yes, with equal force. But Earth doesn't move due to huge mass.

Q5. How does swimming involve Newton’s third law?

We push water backward, water pushes us forward.

Q6. Can action and reaction cancel each other?

No. They act on different bodies.

Q7. Is there a delay between action and reaction?

No. They occur simultaneously.

Q8. Why does a balloon move in opposite direction when air is released?

Air rushing out is action; balloon moves opposite as reaction.

Q9. Why does a fish push water backwards?

To get forward push from water.

Q10. While rowing a boat, how do oars help motion?

They push water backward → reaction moves boat forward.

Law of Conservation of Momentum

Q1. Two skaters push off each other. Why do they move in opposite directions?

To conserve total momentum.

Q2. Why do guns recoil after firing?

To conserve momentum. Bullet forward, gun backward.

Q3. Can momentum be created or destroyed?

No. It only transfers.

Q4. What condition must exist for momentum to be conserved?

No external force.

Q5. A person jumps from boat. Why does boat move backward?

Opposite momentum → conservation.

Q6. Is momentum scalar or vector?

Vector. Has direction.

Q7. Does zero net momentum mean no motion?

No. Two equal and opposite momenta can cancel.

Q8. If two objects stick after collision, is momentum still conserved?

Yes.

Q9. What if external force acts during a collision?

Then momentum is not conserved.

Q10. Can an explosion conserve momentum?

Yes, if no external force.