1
With two equal opposite pulls the block —
💡 Hint:
The resultant is zero.
Explanation:
The forces are balanced, so the block does not move.
Class 9 · Science · Chapter 9 · बिहार बोर्ड (BSEB)CBSE · NCERT 2026-27
Force and Laws of Motion
Force and Laws of Motion
How to use this page:
1. Read — 9.1 carom · 9.2 coin · 9.3 tray · 9.4 carts · 9.5 balloon · 9.6 cork, diagram, worked example, board tip
2. Check — each lesson has its own questions; the number follows the lesson
3. Mastery ★ — all of that lesson correct. Redo the wrong ones
4. The memory figure shows two trolleys before and after a collision, with momentum arrows, when no outside unbalanced force acts.
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- 1 Balanced and unbalanced forces
- 2 The first law — Activity 9.1
- 3 Inertia and mass — Activities 9.2 and 9.3
- 4 Momentum and the second law — F = ma
- 5 The third law — Activity 9.4
- 6 Conservation of momentum — Activities 9.5 and 9.6
- 7 A longer time means a smaller force
- Chapter winner — every lesson at mastery ★
🎬 Shorts
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📄 PDF download — notes + answer key
Balanced and unbalanced forces
संतुलित और असंतुलित बल · Section 9.1 · friction
A force changes the state of motionNotes
A push, a pull or a hit is the everyday idea of a force. A force can start an object, stop it, change its direction, or change its shape. A spring stretches and a rubber ball becomes oblong when it is pressed.
If strings X and Y on opposite faces of a block are pulled equally, the block does not move. Such forces are balanced. If the pull on one side is larger, the block moves that way. That force is unbalanced.
Only an unbalanced force changes rest or uniform motion. On a rough floor a small push is cancelled by friction, so the box does not slide. The moment the push exceeds friction, the box starts. Stop pedalling a bicycle and friction slows it. If friction were removed, the object would keep the speed it had already gained.
| Case | Forces | Result |
|---|---|---|
| X and Y equal | Balanced | Block stays |
| Larger pull one side | Unbalanced | Motion to that side |
| Push = friction | Balanced | Box stays put |
Worked exampleExample
Question: Two children push a box from opposite sides with 40 N and 40 N. Why does the box not move?
Answer: The two forces are equal and opposite, so the resultant is zero. The forces are balanced. The state of motion does not change.
10-second revision
- Balanced forces: resultant zero, state unchanged
- Unbalanced force: speed or direction changes
- Friction acts opposite to the push
Board tip · BSEBBoard tip
Write that balanced forces give no acceleration. Only “a force acts” stays incomplete.
Board tip · CBSEBoard tip
Remember figure 9.4: a small push is balanced, a larger push is unbalanced.
Check your understandingall correct = mastery ★
2
On a rough floor a small push can be balanced by friction.
Explanation:
True — then the box does not slide.
3
The force that changes the state of motion is called ______.
💡 Hint:
Balanced forces do not do this job.
Explanation:
An unbalanced force.
4
A bicycle slows when pedalling stops because —
💡 Hint:
Between the floor and the wheels.
Explanation:
Friction is opposite to the motion, so the speed falls.
5
Define a balanced force and an unbalanced force in one line each.
Model answer (board format)
Balanced forces have zero resultant and do not change the state of motion. An unbalanced force has a non-zero resultant and changes speed or direction.
Check your answer:
The first law — Activity 9.1
प्रथम नियम — क्रियाकलाप 9.1 · Section 9.2 · Activity 9.1
From Galileo to the first law of NewtonNotes
Galileo watched a marble on an inclined plane. Rolling down, its speed rises. Rolling up, its speed falls. If the two planes are level and there is no friction, the marble does not change its speed.
An object stays at rest, or in uniform motion in a straight line, until an unbalanced force compels a change. That is the first law. The resistance to a change of state is inertia, so the law is also called the law of inertia.
If a bus stops suddenly the body wants to keep moving forward, so the passenger leans forward. A seat belt puts a force on the body and slows that forward motion. If the bus starts suddenly the feet move forward with the floor and the rest of the body stays back by inertia. On a sharp turn the body wants to keep going straight.
Activity 9.1 — a pile of carom coinsActivity
Make a pile of similar carom coins on a table. Give the bottom coin a sharp horizontal hit with the striker. If the hit is fast enough, only the bottom coin moves out. The coins above fall straight down onto the table because of their inertia. They do not run off horizontally with the striker.
Worked exampleExample
Question: Why does a passenger fall forward when a moving bus brakes?
Answer: The brakes reduce the speed of the bus. The body of the passenger keeps its earlier motion because of inertia, so the passenger leans forward.
10-second revision
- First law = law of inertia
- Only an unbalanced force changes the state
- In carom the upper coins fall straight down
Board tip · BSEBBoard tip
In the definition write both rest and uniform straight motion. Only rest stays incomplete.
Board tip · CBSEBoard tip
Write the two bus cases apart: stopping throws you forward, a sudden start throws you back.
Check your understandingall correct = mastery ★
1
In Activity 9.1 the upper coins —
💡 Hint:
Only the bottom coin leaves.
Explanation:
The upper coins keep their rest by inertia and fall straight down.
2
The first law is also called the law of inertia.
Explanation:
True — inertia is the resistance to a change of state.
3
In a bus that stops suddenly the passenger leans ______.
💡 Hint:
The body keeps the old motion.
Explanation:
Forward.
4
A marble moving on a frictionless level surface —
💡 Hint:
The conclusion of Galileo.
Explanation:
With no unbalanced force the speed does not change.
5
A seat belt exerts a force to slow the forward motion of the body.
Explanation:
True — to reduce injury when the vehicle stops suddenly.
6
State the first law of motion of Newton and explain it with one bus example.
Model answer (board format)
An object stays at rest or in uniform motion in a straight line until an unbalanced force changes that state. If a moving bus stops suddenly, the body keeps moving forward, so the passenger leans forward.
Check your answer:
Inertia and mass — Activities 9.2 and 9.3
जड़त्व और द्रव्यमान — क्रियाकलाप 9.2 और 9.3 · Section 9.3 · Activities 9.2 · 9.3
Activities 9.2 and 9.3Activity
9.2 Cover an empty glass with a stiff card and place a five-rupee coin on the card. Flick the card sharply sideways. The card flies off and the coin falls straight into the glass because of inertia. It does not run off with the card.
9.3 Place a water-filled tumbler on a tray and turn around fast with the tray. The water spills, because the water resists the change of state. A groove in a saucer keeps the cup from toppling in a sudden jerk.
Mass is the measure of inertiaNotes
Inertia is not the same in every object. An empty box is easy to push. A box full of books is hard. A football of a given size flies off. The same force on a stone of that size hardly moves the stone, and the foot can be hurt.
Mass is the measure of inertia. The SI unit is the kg. A heavier object has more inertia. A train has more inertia than a bicycle, so a force that gives a small cart a large speed makes almost no change in the train. Shake a branch hard and some leaves come off, because the branch moves and the leaves want to stay where they were.
Worked exampleExample
Question: A rubber ball and a stone have the same size. Which has more inertia?
Answer: The stone has more mass, so it has more inertia. Mass is the measure of inertia.
10-second revision
- In 9.2 the coin falls straight into the glass
- In 9.3 a fast turn spills the water
- More mass, more inertia
Board tip · BSEBBoard tip
With the definition of inertia, also write the unit kg.
Board tip · CBSEBoard tip
A five-rupee coin is heavier than a one-rupee coin, so its inertia is larger.
Check your understandingall correct = mastery ★
1
The greatest inertia is in —
💡 Hint:
Look at the mass.
Explanation:
The train has the greatest mass, so it has the greatest inertia.
2
In Activity 9.2 the coin flies off horizontally with the card.
Explanation:
False — the coin falls straight into the glass by inertia.
3
The measure of inertia of an object is its ______.
💡 Hint:
The SI unit is the kg.
Explanation:
Mass.
4
Why do leaves come off when a branch is shaken? How does the water of Activity 9.3 match this?
Model answer (board format)
The branch moves. The leaves want to keep their state of rest by inertia, so some leaves come off. When the tray turns fast, the water also resists the change of state and spills.
Check your answer:
Momentum and the second law — F = ma
संवेग और द्वितीय नियम — F = ma · Section 9.4 · formulas 9.1 to 9.5
Momentum and its changeNotes
A table-tennis ball does not hurt. A fast cricket ball can. A parked truck looks harmless. A truck moving even slowly can be dangerous. The effect depends on both mass and velocity.
Momentum is p = mv. Its direction is the direction of the velocity. The SI unit is kg m s⁻¹. An unbalanced force changes momentum.
The second law: the rate of change of momentum is proportional to the applied unbalanced force, in the direction of the force. For mass m, initial velocity u, final velocity v and time t, the change is m(v − u). The rate is m(v − u)/t = ma. In SI the constant of proportion is chosen as 1, so F = ma. 1 N = 1 kg m s⁻². If F = 0, then v stays equal to u. That is the mathematical form of the first law.
Worked exampleExample
Question: A force acts for 2 s on an object of 5 kg. The velocity rises from 3 m s⁻¹ to 7 m s⁻¹. Find the magnitude of the force.
Formula: F = m(v − u)/t
Substitution: F = 5 kg × (7 − 3) m s⁻¹ / 2 s = 5 × 4 / 2
Answer: F = 10 N.
If this same 10 N acts for 5 s and u = 3 m s⁻¹, then v = u + Ft/m = 3 + (10 × 5)/5 = 13 m s⁻¹.
10-second revision
- p = mv, unit kg m s⁻¹
- F = ma, unit newton N
- If F = 0 the velocity does not change
Board tip · BSEBBoard tip
In a numerical write the formula, the values and the unit. Writing only 10 stays incomplete.
Board tip · CBSEBoard tip
Remember the definition of 1 N: an acceleration of 1 m s⁻² in 1 kg.
Check your understandingall correct = mastery ★
1
The SI unit of momentum is —
💡 Hint:
p = mv.
Explanation:
kg times m s⁻¹, that is kg m s⁻¹. The newton is the unit of force.
2
In SI the second law has the form F = ______.
💡 Hint:
k = 1.
Explanation:
F = ma.
3
Momentum is a scalar and has no direction.
Explanation:
False — the direction of momentum is the direction of the velocity.
4
The greater force is needed to accelerate —
💡 Hint:
Find both from F = ma.
Explanation:
F1 = 2 × 5 = 10 N and F2 = 4 × 2 = 8 N. The first force is larger.
5
A force of 5 N gives m1 an acceleration of 10 m s⁻² and m2 an acceleration of 20 m s⁻². What acceleration results if both masses are tied together? Write the formula and the unit.
Model answer (board format)
Formula m = F/a. m1 = 5/10 = 0.50 kg. m2 = 5/20 = 0.25 kg. Total m = 0.75 kg. a = F/m = 5 N / 0.75 kg = 6.67 m s⁻².
Check your answer:
The third law — Activity 9.4
तृतीय नियम — क्रियाकलाप 9.4 · Section 9.5 · Activity 9.4
Two objects, two forcesNotes
Every action has an equal and opposite reaction, and the two forces act on two different objects. Join two spring balances. Fix one end to a wall. Whatever one balance pulls, the other shows the same amount the opposite way.
To walk, you push the road backward. The road pushes your foot forward. When a sailor jumps forward from a boat, the boat moves back. A gun pushes the bullet forward. The bullet pushes the gun backward with an equal force, so the gun recoils. The bullet has a small mass, so its acceleration is very large. The forces are equal. The accelerations need not be equal.
Activity 9.4 — a sand bag on two cartsActivity
Let two children stand on two separate carts. Give them a bag of sand and ask them to play catch. Throwing the bag is the action. The child who throws gets an immediate opposite reaction and that cart moves back. A white line on the wheels makes the motion easy to see.
Then place two children on one cart and one child on the other. The accelerations differ even though the force is of the same kind. That is the second law, because the masses differ. The cart can be plywood of about 50 cm × 100 cm with ball-bearing wheels. A skateboard makes a straight line hard to keep.
Worked exampleExample
Question: A massive truck parked by the road does not move when you push it. Did action and reaction cancel on one object?
Answer: No. Your push is on the truck and the push of the truck is on you. They are on two objects, so they do not cancel each other to zero. The huge mass of the truck makes the acceleration very small.
10-second revision
- FAB = −FBA, two different objects
- Forces equal, acceleration depends on mass
- In 9.4 throwing the bag moves the cart back
Board tip · BSEBBoard tip
Always write that action and reaction do not act on the same object.
Board tip · CBSEBoard tip
Give the gun and bullet pair with a sketch. Acceleration is inverse to the mass.
Check your understandingall correct = mastery ★
1
The force that moves you forward while walking is —
💡 Hint:
You push the road backward.
Explanation:
The road exerts an equal forward reaction on the foot.
2
Action and reaction act on the same object and cancel each other.
Explanation:
False — they act on two different objects.
3
When the sailor jumps forward the boat moves ______.
💡 Hint:
Figure 9.12.
Explanation:
Backward.
4
The acceleration of the bullet is much larger than that of the gun because —
💡 Hint:
F is equal, and a = F/m.
Explanation:
The forces are equal. The bullet has a small m, so a is large.
5
In Activity 9.4, two children on one cart and one on the other can show different accelerations.
Explanation:
True — even for the same kind of force, different masses give different accelerations.
6
State the third law of motion. Use the recoil of a gun to explain why the accelerations are not equal.
Model answer (board format)
Every action has an equal and opposite reaction, and they act on two different objects. The forces on the gun and the bullet are equal. The bullet has a small mass, so by a = F/m its acceleration is very large and the gun moves back slowly.
Check your answer:
Conservation of momentum — Activities 9.5 and 9.6
संवेग संरक्षण — क्रियाकलाप 9.5 और 9.6 · Section 9.6 · Activities 9.5 · 9.6
With no outside force the sum does not changeNotes
Two balls A and B move on one straight line with velocities uA and uB, and uA is larger. During the short collision time t, A exerts FAB on B and B exerts FBA on A. By the third law, FAB = −FBA.
From this, mA uA + mB uB = mA vA + mB vB, provided no external unbalanced force acts. The total momentum before the collision equals the total momentum after it. This is conservation of momentum. A conservation law is checked by experiment. It is not treated as proved by one experiment.
Activities 9.5 and 9.6Activity
9.5 Inflate a large balloon and tie the neck. Tape a straw on the surface and pass a thread through the straw. Hold both ends. Open the neck. The escaping air goes one way and the straw moves the opposite way.
9.6 This is a teacher demonstration. Put a little water in a good glass test tube and fit a cork. Hang the tube horizontally on two strings. Heat it until steam blows the cork out. The tube recoils opposite to the cork. The cork is light, so its speed looks larger than the backward speed of the tube. Do not point the mouth toward a face.
Worked exampleExample
Question: A bullet of 20 g moves at 150 m s⁻¹. The pistol has mass 2 kg. Both start at rest. Find the recoil velocity of the pistol.
Formula: m1 v1 + m2 v2 = 0, because the total momentum was zero before.
Substitution: 0.02 kg × 150 m s⁻¹ + 2 kg × v = 0, so 3 + 2v = 0.
Answer: v = −1.5 m s⁻¹. The negative sign means the direction opposite to the bullet.
10-second revision
- No external force means total momentum stays the same
- In 9.5 air leaves and the balloon goes the other way
- For 20 g and 2 kg the recoil is −1.5 m s⁻¹
Board tip · BSEBBoard tip
Write the direction of the negative sign in a sentence. Only −1.5 stays incomplete.
Board tip · CBSEBoard tip
In both 9.5 and 9.6 the light part moves fast and the heavy part moves slowly.
Check your understandingall correct = mastery ★
1
Conservation of momentum needs —
💡 Hint:
The system is isolated.
Explanation:
Total momentum does not change only when there is no external unbalanced force.
2
In Activity 9.5 the balloon moves in the same direction as the escaping air.
Explanation:
False — the balloon moves opposite to the air.
3
The magnitude of the pistol recoil is ______ m s⁻¹ when the bullet is 0.02 kg × 150 m s⁻¹ and the pistol is 2 kg.
💡 Hint:
3 + 2v = 0.
Explanation:
1.5 m s⁻¹, opposite to the bullet.
4
A girl of 40 kg jumps at 5 m s⁻¹ onto a stationary cart of 3 kg. Find their common velocity. Write the formula, the substitution and the unit. Take the external horizontal force as zero.
Model answer (board format)
Formula: 40 × 5 + 3 × 0 = (40 + 3) v. Substitution: 200 = 43 v. v = 200/43 = 4.65 m s⁻¹, in the direction of the jump.
Check your answer:
A longer time means a smaller force
समय बढ़ाओ तो बल घटता है · Second law · everyday cases
The same change of momentum, a different forceNotes
F = (mv − mu)/t. If the change of momentum is fixed, a larger time t means a smaller force.
While catching a fast ball, a fielder draws the hands back with the ball. The ball gets more time to stop, the acceleration falls, and the force of the impact on the hands falls. In a high jump a cushion or sand also increases the time and decreases the force.
A negative sign means the force is opposite to the motion. The force of the brakes is opposite to the velocity of the car.
Worked exampleExample
Question: A car of 1000 kg moves at 108 km/h and stops in 4 s after the brakes are applied. Find the force of the brakes.
Formula: F = m(v − u)/t, and 108 km/h = 108 × 1000/3600 = 30 m s⁻¹.
Substitution: F = 1000 kg × (0 − 30) m s⁻¹ / 4 s = −30000/4.
Answer: F = −7500 N. The negative sign means the force is opposite to the motion.
10-second revision
- F = Δp / t
- Drawing the hands back increases the time
- 108 km/h = 30 m s⁻¹, brakes −7500 N
Board tip · BSEBBoard tip
Change km/h into m/s by multiplying by 5/18, then use the formula.
Board tip · CBSEBoard tip
Write a negative force in words as “opposite to the motion”.
Check your understandingall correct = mastery ★
1
Hands are drawn back while catching a fast ball so that —
💡 Hint:
F = Δp / t.
Explanation:
A longer time makes the same change of momentum need a smaller force.
2
108 km/h is equal to 30 m s⁻¹.
Explanation:
True — 108 × 5/18 = 30.
3
The brake force on the car above is ______ N.
💡 Hint:
1000 × (0 − 30) / 4.
Explanation:
−7500 N, opposite to the motion.
4
Sand or a cushion is used in a high jump so that —
💡 Hint:
The same idea as in a catch.
Explanation:
The stopping time increases, so the force decreases.
5
A stone of 1 kg is thrown at 20 m s⁻¹ on ice and stops after 50 m. Find the friction force. Write v² = u² + 2as and F = ma.
Model answer (board format)
0 = 20² + 2a(50). 0 = 400 + 100a. a = −4 m s⁻². F = ma = 1 × (−4) = −4 N. Friction is 4 N opposite to the motion.
Check your answer:
❓ Full question bank — with answers and explanations — 67 questions
Pick a type. The 35 lesson checks are separate — each lesson has as many as its topic needs. All correct earns mastery ★.
BSEB modelpractice · not annual60 tried · 0 correctMultiple choiceMCQ140 tried · 0 correctTrue / falseTrue / false80 tried · 0 correctFill in the blanksFill80 tried · 0 correctMatchMatch20 tried · 0 correctAssertion–reasonAssertion–reason50 tried · 0 correctBalancingBalancing40 tried · 0 correctIdentifyIdentify20 tried · 0 correctShort and longVSA, SA, LA120 tried · 0 correctCBSE-stylenot a PYQ60 tried · 0 correct
No question is marked as a verified past paper. The BSEB set is a model for practice. CBSE items are CBSE-style, not a copy of any year’s paper.
Multiple choice
0/14Pick one option. A wrong try brings a hint.
1
If equal opposite forces act on a block, it —
💡 Hint:
The resultant is zero.
Explanation:
The forces are balanced, so the state does not change.
2
The measure of inertia is —
💡 Hint:
A heavier object resists more.
Explanation:
Mass is the measure of inertia. The unit is the kg.
3
A standing passenger in a bus that starts suddenly —
💡 Hint:
The feet go forward, the body stays back.
Explanation:
By inertia the body stays back.
4
In p = mv the direction of v —
💡 Hint:
Momentum is a vector.
Explanation:
The direction of momentum is the direction of velocity.
5
1 newton is equal to —
💡 Hint:
F = ma.
Explanation:
1 N = 1 kg × 1 m s⁻².
6
If F = 0, then —
💡 Hint:
The first law from the second.
Explanation:
If the force is zero the velocity does not change.
7
The force 2 kg × 5 m s⁻² is —
💡 Hint:
F = ma.
Explanation:
F = 10 N.
8
A gun recoils because —
💡 Hint:
The third law.
Explanation:
The bullet pushes the gun backward.
9
When the sand bag is thrown, the cart —
💡 Hint:
Activity 9.4.
Explanation:
The throw is the action. The cart moves back by the reaction.
10
The recoil speed for 0.02 kg × 150 m s⁻¹ and a 2 kg pistol is —
💡 Hint:
3 + 2v = 0.
Explanation:
v = −1.5 m s⁻¹.
11
A 40 kg girl jumps at 5 m s⁻¹ onto a 3 kg cart. The common speed is about —
💡 Hint:
200 = 43v.
Explanation:
v = 200/43 = 4.65 m s⁻¹.
12
Drawing the hands back in a catch —
💡 Hint:
F = Δp/t.
Explanation:
Larger time, smaller force.
13
A car at 30 m s⁻¹ stops in 4 s, m = 1000 kg. The brake force is —
💡 Hint:
m(v−u)/t.
Explanation:
F = 1000 × (0−30)/4 = −7500 N.
14
Leaves come off a shaken branch because —
💡 Hint:
The branch moves.
Explanation:
The leaves want to keep their state of rest.
True or false
0/81
Balanced forces give an object an acceleration.
Explanation:
False — the resultant is zero, so there is no acceleration.
2
Mass is the measure of inertia.
Explanation:
True — the SI unit is the kg.
3
The direction of momentum is fixed opposite to the velocity.
Explanation:
False — the direction is the direction of the velocity.
4
1 N is the force that gives 1 kg an acceleration of 1 m s⁻².
Explanation:
True — the unit is chosen from F = ma.
5
Action and reaction act on the same object.
Explanation:
False — they act on two different objects.
6
With no external unbalanced force, total momentum does not change in a collision.
Explanation:
True — this is conservation of momentum.
7
In Activity 9.6 the test tube recoils in the direction of the cork.
Explanation:
False — the tube recoils opposite to the cork.
8
Increasing the time in a catch increases the force on the hands.
Explanation:
False — by F = Δp/t the force decreases.
Fill in the blanks
0/81
Without an unbalanced force an object does not change its ______.
💡 Hint:
Its state.
Explanation:
The first law.
2
Momentum p = ______.
💡 Hint:
mv.
Explanation:
The direction is that of the velocity.
3
The SI unit of force is the ______.
💡 Hint:
The newton.
Explanation:
kg m s⁻².
4
In F = ma, a is acceleration, unit ______.
💡 Hint:
m s⁻².
Explanation:
The rate of change of velocity.
5
When you jump forward from a boat, the boat moves ______.
💡 Hint:
Backward.
Explanation:
Figure 9.12.
6
The conservation equation is mA uA + mB uB = ______.
💡 Hint:
mA vA + mB vB.
Explanation:
The later velocities are v.
7
20 g written in kg is ______ kg.
💡 Hint:
0.02 kg.
Explanation:
1000 g = 1 kg.
8
A 1000 kg car, u = 30 m s⁻¹, v = 0, t = 4 s, so |F| = ______ N.
💡 Hint:
7500 N.
Explanation:
The sign is negative.
Match
0/21
Match the quantity with its unit or form.
Column B: A. kg · B. kg m s⁻² · C. kg m s⁻¹ · D. The unit N from F = ma
1. Momentum
2. Force
3. Measure of inertia
4. 1 newton
Explanation:
Momentum is kg m s⁻¹, force uses the newton, mass is the kg, and 1 N = 1 kg m s⁻².
2
Match the activity with what you observe.
Column B: A. Coin falls into the glass · B. Motion opposite the air · C. Tube opposite the cork · D. Upper coins fall straight down
1. 9.1 carom pile
2. 9.2 coin and card
3. 9.5 balloon
4. 9.6 cork
Explanation:
9.1 is the straight fall, 9.2 the glass, 9.5 opposite the air, 9.6 opposite the cork.
Assertion–reason
0/5Check both statements, then see whether the reason explains the assertion.
1
Assertion (A): An unbalanced force can change the speed of an object.
Reason (R): An unbalanced force gives an acceleration.
Explanation:
Both are true and R explains A.
2
Assertion (A): Mass is the measure of inertia.
Reason (R): The unit of momentum is the newton.
Explanation:
A is true. R is false — the unit of momentum is kg m s⁻¹, not the newton.
3
Assertion (A): Drawing the hands back while catching a fast ball reduces the hurt.
Reason (R): A shorter time also decreases the force.
Explanation:
A is true. R is false — the force falls when the time increases, not when it decreases.
4
Assertion (A): Action and reaction cancel on the same object.
Reason (R): These forces act on two different objects.
Explanation:
A is false. R is true.
5
Assertion (A): With no external force the total momentum of two balls does not change in a collision.
Reason (R): During the collision the action and the reaction are equal and opposite.
Explanation:
Both are true and R explains this conservation.
Coefficient practice
0/4These equations are not a result of this chapter. This is only coefficient practice for H2 + O2 → H2O.
1
This is coefficient practice 1, not a result of this chapter. Force and laws of motion has no chemical equation. Fill the coefficients of H2 + O2 → H2O.
H2 + O2 → H2O
Explanation: 2H2 + O2 → 2H2O — coefficient practice for water. Do not treat it as a result of this chapter.
2
This is coefficient practice 2, not a result of this chapter. Force and laws of motion has no chemical equation. Fill the coefficients of H2 + O2 → H2O.
H2 + O2 → H2O
Explanation: 2H2 + O2 → 2H2O — coefficient practice for water. Do not treat it as a result of this chapter.
3
This is coefficient practice 3, not a result of this chapter. Force and laws of motion has no chemical equation. Fill the coefficients of H2 + O2 → H2O.
H2 + O2 → H2O
Explanation: 2H2 + O2 → 2H2O — coefficient practice for water. Do not treat it as a result of this chapter.
4
This is coefficient practice 4, not a result of this chapter. Force and laws of motion has no chemical equation. Fill the coefficients of H2 + O2 → H2O.
H2 + O2 → H2O
Explanation: 2H2 + O2 → 2H2O — coefficient practice for water. Do not treat it as a result of this chapter.
Classify
0/21
Place each case as a balanced or an unbalanced force.
Strings X and Y pulled equally
The push exceeds friction
Brakes bring a car to rest
A book lies at rest on a table
Explanation:
Equal pulls and a resting book are balanced. A push past friction, and the brakes, are unbalanced.
2
Place each example on the right law.
The carom pile
F = ma
Recoil of a gun
Total p in a two-ball collision
Explanation:
Carom is inertia, F = ma is the second law, the gun is the third law, and total p is conservation.
Very short answer
0/51
Define inertia in one line.
Model answer (board format)
Inertia is the tendency of an object to resist a change in its state of rest or of uniform motion.
Check your answer:
2
Write the definition, direction and SI unit of momentum.
Model answer (board format)
Momentum is the product of mass and velocity, p = mv. Its direction is the direction of the velocity. The SI unit is kg m s⁻¹.
Check your answer:
3
Define 1 newton.
Model answer (board format)
1 newton is the force that produces an acceleration of 1 m s⁻² in a mass of 1 kg.
Check your answer:
4
Why is luggage on a roof tied with a rope?
Model answer (board format)
If the bus stops, starts or turns suddenly, the luggage wants to keep its state by inertia and can fall. The rope holds it by an unbalanced force.
Check your answer:
5
Write one condition for conservation of momentum.
Model answer (board format)
No external unbalanced force should act on the system.
Check your answer:
Short answer
0/41
A truck starts from rest and rolls down a hill, covering 400 m in 20 s. Find the acceleration. If the mass is 7 tonnes, find the force. 1 tonne = 1000 kg.
Model answer (board format)
u = 0, s = ut + ½at². 400 = ½ a × 400. a = 2 m s⁻². m = 7000 kg. F = ma = 7000 × 2 = 14000 N.
Check your answer:
2
A hockey ball of 200 g arrives at 10 m s⁻¹ and the stick sends it back along the same line at 5 m s⁻¹. Find the change of momentum.
Model answer (board format)
m = 0.2 kg. Take the forward direction as positive. u = 10, v = −5. Δp = m(v − u) = 0.2(−5 − 10) = −3 kg m s⁻¹. The magnitude is 3 kg m s⁻¹, opposite to the initial velocity.
Check your answer:
3
A vehicle of 1500 kg is to be stopped with a negative acceleration of 1.7 m s⁻². What force is needed between the road and the vehicle?
Model answer (board format)
F = ma = 1500 × (−1.7) = −2550 N. The magnitude is 2550 N, opposite to the motion.
Check your answer:
4
Two objects, each of mass 1.5 kg, approach on one line from opposite directions at 2.5 m s⁻¹ and stick together. Find the common velocity.
Model answer (board format)
Take one direction as positive. Total p = 1.5×2.5 + 1.5×(−2.5) = 0. After they stick, the total momentum is still 0, so the common velocity is 0.
Check your answer:
Long answer
0/31
State the first, second and third laws in one sentence each. Then show how the first law follows when F = 0, and why action and reaction do not cancel on one object.
Model answer (board format)
First: without an unbalanced force an object stays at rest or in uniform straight motion. Second: the rate of change of momentum is proportional to the unbalanced force. Third: action and reaction are equal, opposite and on two objects. Put F = 0 in F = m(v−u)/t and v = u. The action is on one object and the reaction on the other, so they do not form a cancelling pair on one object.
Check your answer:
2
A player of 60 kg at +5 m s⁻¹ and a player of 55 kg at −6 m s⁻¹ collide on one line and stick. Find the common velocity. Write the formula, the substitution, the direction and the unit.
Model answer (board format)
Total momentum before = 60×5 + 55×(−6) = 300 − 330 = −30 kg m s⁻¹. After, (60+55)v = 115v. 115v = −30. v = −30/115 = −0.26 m s⁻¹. The direction is from right to left, the initial direction of the second player.
Check your answer:
3
Check the three student comments about an insect hitting the windscreen of a fast car. Which force and which change of momentum act on both?
Model answer (board format)
The forces on the insect and the car are equal and opposite, because they are an action-reaction pair. The changes of momentum also have equal magnitude. The velocity of the insect changes a lot because its mass is small, so it is killed. It is wrong to say that the change of momentum of the insect is larger, or that the car exerted a larger force because its speed was larger.
Check your answer:
BSEB model paper · practice
0/6This model set is for practice. It is not a question from any year’s annual examination.
1
The net external unbalanced force on an object at rest is zero. The object —
💡 Hint:
The first law.
Explanation:
If the force is zero, rest continues. A uniform velocity is also possible if it was already moving.
2
Dust comes out when a carpet is beaten with a stick because —
💡 Hint:
The carpet moves.
Explanation:
The dust wants to keep its state.
3
A wooden cabinet is moved at constant velocity by a horizontal force of 200 N. The friction force is ______ N.
💡 Hint:
200 N, in the opposite direction.
Explanation:
Constant velocity means zero acceleration.
4
If the net external unbalanced force is zero, can the object travel with a non-zero velocity?
Model answer (board format)
Yes. The velocity can be non-zero, but both its magnitude and its direction must stay constant. That is uniform straight-line motion.
Check your answer:
5
A bullet of 10 g at 150 m s⁻¹ enters wood and stops in 0.03 s. Find the force of the wood and the distance of penetration.
Model answer (board format)
m = 0.01 kg. a = (0 − 150)/0.03 = −5000 m s⁻². F = ma = 0.01 × (−5000) = −50 N. The magnitude is 50 N. Distance s = ut + ½at² = 150×0.03 + ½(−5000)(0.03)² = 4.5 − 2.25 = 2.25 m.
Check your answer:
6
An 8000 kg engine pulls five wagons of 2000 kg each. The engine exerts 40000 N and the track friction is 5000 N. Find the total mass, the net force and the acceleration.
Model answer (board format)
Mass of the wagons = 5×2000 = 10000 kg. Total mass with the engine = 18000 kg. Net force = 40000 − 5000 = 35000 N. a = F/m = 35000/18000 = 1.94 m s⁻².
Check your answer:
CBSE-style questions
0/6These are competency-based practice questions. They are not copies of a CBSE paper.
1
A box on a rough floor does not move under a small push. The best reason is —
💡 Hint:
Figure 9.4.
Explanation:
Friction balances the push.
2
The velocity of a 100 kg object rises from 5 m s⁻¹ to 8 m s⁻¹ in 6 s. The magnitude of the force is —
💡 Hint:
F = m(v−u)/t.
Explanation:
F = 100×(8−5)/6 = 50 N.
3
Assertion (A): A heavy truck parked on the road does not move under a small push.
Reason (R): Action and reaction cancel on the same object.
Explanation:
A is true. R is false — the two forces are on different objects. The large mass of the truck makes the acceleration negligible.
4
Assertion (A): When air leaves a balloon, the balloon moves in the opposite direction.
Reason (R): If the external horizontal force is negligible, total momentum stays conserved.
Explanation:
Both are true and R explains A.
5
A student says a ball rolling on level ground stops because every object wants to stop if no force acts. Correct this.
Model answer (board format)
If no unbalanced force acts, the ball keeps a uniform velocity. It stops because friction opposes the motion. Rest is not a special natural state.
Check your answer:
6
An object of 1 kg at 10 m s⁻¹ sticks to a stationary block of 5 kg. Find the total momentum just before and just after the impact, and the common velocity.
Model answer (board format)
Before, p = 1×10 + 5×0 = 10 kg m s⁻¹. With no external force the total momentum after is also 10 kg m s⁻¹. (1+5)v = 10. v = 10/6 = 1.67 m s⁻¹.
Check your answer:
🏛️ Board exam corner — Bihar Board (BSEB)— CBSE
Switch board with BSEB | CBSE above. The lessons follow the same NCERT chapter.
BSEB model · practiceBoard tip
This page has no verified annual-exam question, because no source page has been added. The model set below is practice in the board pattern.
🏛️ Model questions on one page →
The verified label will be used only when a source page for the question is available.
CBSE-style · competency-based
These are case and assertion-reason practice items. Do not treat them as past CBSE questions.
🔁 Spaced review — today’s questions
Wrong questions return soon; correct ones return after a few days.
🧠 What you learned + equation sheet
What you learned
- Balanced forces do not change the state of motion. An unbalanced force gives acceleration.
- The first law is the law of inertia. Mass is the measure of inertia.
- Momentum is p = mv. In SI, force is F = ma, and 1 N = 1 kg m s⁻².
- F = (mv − mu)/t. A larger time means the same change of momentum needs a smaller force.
- Action and reaction are equal, opposite, and on two different objects.
- With no external unbalanced force, total momentum stays conserved in a collision.
| What | Keep this |
|---|---|
| Momentum | p = mv, unit kg m s⁻¹ |
| Force | F = ma, unit newton N |
| 1 newton | 1 kg × 1 m s⁻² |
| First law | F = 0 means v stays u |
| Third law | FAB = −FBA, two bodies |
| Conservation | mA uA + mB uB = mA vA + mB vB |
The notes are original writing. The textbook was used only for activity order and numbers. “Verified” will be used only on a question that has a source page.