Laws of Motion Class 9 ICSE — Chapter 3

This ICSE chapter, Laws of Motion Class 9 ICSE, is Chapter 3 of the Class 9 Physics book. It explains why objects move, stop, or stay still. You will study the three laws of motion, momentum, the units of force, and gravity. You will also learn free fall and the difference between mass and weight. Each section below follows the book’s own order and numbering.

Download Laws of Motion Class 9 ICSE Physics Chapter 3 PDF

Download the Laws of Motion Class 9 ICSE Physics chapter PDF to revise offline or print for practice. It is the full chapter as printed in the book — 24 pages, 18 sections, 115 questions.

What is inside Chapter 3

24 pages · 18 sections · 6 worked examples · 115 exercise questions

• EXERCISE 3(A): 7 questions
• Multiple Choice Type: 2 questions
• EXERCISE 3(B): 21 questions
• Multiple choice type :: 3 questions
• Multiple choice type :: 18 questions
• EXERCISE 3(D): 14 questions
• Multiple choice type :: 4 questions
• EXERCISE 3(E): 22 questions
• Multiple choice type :: 24 questions

3.1 FORCE

Force is the physical cause that changes, or tends to change, the state of rest, the state of motion, or the size and shape of a body. This definition covers every effect a push or pull can have.

Force has two main effects. It can start, stop, speed up, slow down, or turn a body. A goalkeeper stopping a shot uses force. It can also change dimensions. Pressing a lump of clay flattens it into a disc.

The effect depends on the body. A rigid object, like a steel rod, changes only its motion under force. A non-rigid object, like a sponge, also changes shape, because its particles shift spacing.

This leads to the two broad kinds of force: contact and non-contact. Sections 3.2 and 3.3 explain them.

3.2 CONTACT FORCES

Contact forces act only when two bodies touch. The chapter lists five of them.

• Frictional force opposes sliding or rolling along a surface. A book pushed across a rough desk slows because friction acts opposite to its motion.
• Normal reaction force is the surface’s upward push, perpendicular to the surface, balancing the body’s weight.
• Tension is the pull a stretched string exerts on a hanging body.
• Restoring force of a spring pushes or pulls a body back toward its natural length.
• Collision forces act when two bodies strike each other, equal and opposite at the instant of impact.

The spring force is special. Its magnitude is directly proportional to the elongation or compression. That is why a spring balance works — the reading grows steadily as the spring stretches.

3.3 NON-CONTACT FORCES

Non-contact forces act at a distance, without any touch. Three kinds exist: gravitational, electrostatic, and magnetic.

Gravitational force pulls every mass toward every other mass. It is always attractive. Electrostatic and magnetic forces can pull or push, because like charges repel and unlike charges attract.

All three share one rule. The force varies inversely as the square of the separation. If you double the gap between two bodies, the force falls to one-fourth. This inverse-square behaviour returns in Section 3.14, where you meet the universal law of gravitation.

3.4 NEWTON’S FIRST LAW OF MOTION

Before Galileo, scientists thought a force had to keep acting to keep a body moving. A cycle stops when you stop pedalling, so the idea seemed true.

Galileo showed it was wrong. A body keeps moving at constant speed once set in motion, if no friction opposes it. The cycle only stops because friction acts. This is the law of inertia.

Newton’s first law states: a body at rest stays at rest, and a moving body keeps moving with the same speed in the same direction, unless an external force acts on it.

The law has two parts. It defines inertia, the property by which a body cannot change its own state. It also defines force as the external cause that changes that state. Force is a vector. Two equal and opposite forces on one body give a net force of zero, so the body does not move.

3.5 MASS AND INERTIA

Inertia is the tendency to resist a change in state. That tendency comes from mass.

The greater the mass, the greater the inertia. A loaded trolley is harder to start moving than an empty one. Once moving, it is also harder to stop.

So mass is a measure of inertia. You feel this when pushing a loaded shopping cart. The same push produces a smaller acceleration than it would on an empty cart.

3.6 KINDS OF INERTIA AND ITS EXAMPLES AS ILLUSTRATION OF FIRST LAW

Inertia comes in two kinds.

Inertia of rest keeps a stationary body still. When a bus starts suddenly, passengers lean backward because their upper body resists the motion. Dust shakes out of a beaten carpet for the same reason — the dust stays put while the cloth moves.

Inertia of motion keeps a moving body moving. When a bus brakes suddenly, passengers lean forward. An athlete runs before a long jump so that inertia of motion carries him further through the air.

Both kinds illustrate the first law directly. No force is needed to keep a moving body moving. Only a force can change its state.

3.7 LINEAR MOMENTUM (p = mv)

Why does a heavy truck need more force to stop than a light car at the same speed? The force needed to stop a moving body in a given time depends on its mass and its velocity together.

Linear momentum is the product of mass and velocity:
p = mv

It is a vector, pointing along the velocity. Its SI unit is kg m s⁻¹, and its CGS unit is g cm s⁻¹.

Worked pattern: a 3 kg body moving at 4 m s⁻¹ has momentum p = 3 \times 4 = 12 kg m s⁻¹. If the same body doubles its speed, its momentum doubles too.

3.8 CHANGE IN MOMENTUM (Δp = mΔv)

The symbol Δ means a small change. The change in momentum is written:
\Delta p = \Delta(mv)

This form is always true. The product mv can change because mass changes, because velocity changes, or because both change.

When mass stays constant, the relation simplifies:
\Delta p = m\Delta v

That simplification has a condition. It holds only when the velocity is much smaller than the speed of light, v \ll c. Near light speed, mass increases with velocity, so \Delta p = m\Delta v fails. The general form \Delta p = \Delta(mv) never fails.

Worked pattern: a 4 kg body speeds up from 3 m s⁻¹ to 7 m s⁻¹. The change is \Delta p = 4(7 - 3) = 16 kg m s⁻¹.

3.9 RATE OF CHANGE OF MOMENTUM

Apply a force F to a body of mass m for time t, and its velocity changes from u to v.

Initial momentum is mu and final momentum is mv. The change is m(v - u). Dividing by time t gives the rate of change of momentum \frac{m(v-u)}{t}.

But acceleration is a = \frac{v-u}{t}. So the rate of change of momentum equals ma, mass times acceleration. This holds only while mass stays constant.

Worked pattern: a 2 kg body changes velocity from 1 m s⁻¹ to 5 m s⁻¹ in 2 s. The rate is 2 \times \frac{5-1}{2} = 4 N, which is the force needed to cause that change.

3.10 NEWTON’S SECOND LAW OF MOTION (DERIVATION OF F = ma)

Newton’s first law defines force only in words. The second law gives its value.

Experiment shows two facts. For a fixed mass, acceleration is directly proportional to force: a \propto F. For a fixed acceleration, force is proportional to mass: F \propto m.

Combining gives F = Kma. The unit of force is chosen so that K equals 1. Then F = ma.

The graphs matter. Acceleration against force is a straight line. Force against mass is a straight line. Acceleration against mass, for a constant force, is a hyperbola.

Worked pattern: the force needed to give a 4 kg body an acceleration of 3 m s⁻² is F = 4 \times 3 = 12 N.

3.11 C.G.S. AND S.I. UNITS OF FORCE

Force is measured in newtons in the SI system and dynes in the CGS system.

One newton is the force that gives a 1 kg body an acceleration of 1 m s⁻²:
1\ \text{N} = 1\ \text{kg} \times 1\ \text{m s}^{-2}

One dyne gives a 1 g body an acceleration of 1 cm s⁻²:
1\ \text{dyne} = 1\ \text{g} \times 1\ \text{cm s}^{-2}

The units connect by conversion. Since 1 kg = 1000 g and 1 m = 100 cm:
1\ \text{N} = 1000 \times 100\ \text{g cm s}^{-2} = 10^5\ \text{dyne}

Worked pattern: 5 N equals 5 \times 10^5 dyne.

3.12 NEWTON’S SECOND LAW OF MOTION IN TERMS OF RATE OF CHANGE OF MOMENTUM

The second law has a general form in terms of momentum:
F = \frac{\Delta p}{\Delta t} = \frac{\Delta(mv)}{\Delta t}

When mass stays constant, this reduces to F = ma.

The first law follows from the second. If F = 0, then a = 0. A body at rest stays at rest, and a moving body keeps moving uniformly. So the first law is a special case of the second.

The form \Delta p = F\Delta t explains impulse. A given change in momentum comes from a large force acting briefly, or a small force acting long. That is why a cricketer withdraws his hands while catching, and an athlete lands on sand. The longer time reduces the force.

Worked pattern: stopping a 0.2 kg ball moving at 10 m s⁻¹ in 0.1 s needs F = \frac{0.2 \times 10}{0.1} = 20 N.

3.13 NEWTON’S THIRD LAW OF MOTION

Newton’s third law states: to every action there is an equal and opposite reaction.

The key point is that action and reaction act on two different bodies. They never act on the same body, so they never cancel each other. A book on a table pushes down with its weight, and the table pushes up with an equal normal reaction.

Everyday examples follow the same pattern. A swimmer pushes water backward, and water pushes the swimmer forward. A gun recoils when it fires a bullet. A rocket moves forward because expelled gases push it forward with an equal reaction.

The spring balance demonstration shows this directly. Two balances joined and pulled apart show the same reading, proving the forces are equal and opposite.

A person pushing a wall can fall back because the wall pushes back with equal force.

3.14 UNIVERSAL LAW OF GRAVITATION

Every particle of mass attracts every other particle. Newton gave the law for this force.

The force is directly proportional to the product of the masses, and inversely proportional to the square of the separation, acting along the line joining them:
F = G\frac{m_1 m_2}{r^2}

G is the universal gravitational constant. Its value is 6.67 \times 10^{-11}\ \text{N m}^2\ \text{kg}^{-2}. It is the same everywhere, independent of the bodies, temperature, or medium.

The force is always attractive. It obeys the inverse square law. It is significant between heavenly bodies but tiny between ordinary objects, because G is so small.

Worked pattern: two masses of 2 kg and 3 kg separated by 0.5 m attract with F = 6.67 \times 10^{-11} \times \frac{6}{0.25} = 1.6 \times 10^{-9} N.

3.15 FORCE DUE TO GRAVITY

The earth attracts every body toward its centre. That pull is the force due to gravity, and it acts vertically downward at the centre of gravity of the body.

For a body of mass m on the earth’s surface, of mass M and radius R:
F = \frac{GMm}{R^2}

Using the earth’s mass and radius, the earth pulls a 1 kg body with a force of about 9.8 N. That number becomes the acceleration due to gravity in the next section.

The body also attracts the earth with an equal force. The earth does not move toward the body because its inertia is enormous.

Worked pattern: the force of gravity on a 2 kg body is 2 \times 9.8 = 19.6 N downward.

3.16 ACCELERATION DUE TO GRAVITY

Galileo found that all bodies fall with the same acceleration in vacuum, whatever their mass, size, or shape. This is the acceleration due to gravity, g.

It is the rate at which the velocity of a freely falling body increases. It is a vector directed downward, with SI unit m s⁻². Its mean value on the earth’s surface is 9.8 m s⁻². It is slightly less at the equator than at the poles, and it falls with altitude and depth, reaching zero at the earth’s centre.

The relation between g and G comes from equating two expressions for the force on a falling body:
mg = \frac{GMm}{R^2}, so g = \frac{GM}{R^2}

On the moon, g is nearly one-sixth of its value on the earth.

Worked pattern: a planet of mass 1.2 \times 10^{23} kg and radius 2.4 \times 10^6 m gives g = \frac{6.67 \times 10^{-11} \times 1.2 \times 10^{23}}{(2.4 \times 10^6)^2} \approx 1.4\ \text{m s}^{-2}.

3.17 FREE FALL

Free fall is one-dimensional motion under gravity. It uses the equations of motion you studied in Chapter 2, Motion in One Dimension. For a falling body, acceleration is +g. For a body thrown upward, it is −g.

A body falling from rest, with u = 0, obeys:
v = gt, h = \frac{1}{2}gt^2, v^2 = 2gh

If it starts with velocity u:
v = u + gt, h = ut + \frac{1}{2}gt^2, v^2 = u^2 + 2gh

For a body thrown upward, replace g by −g in those equations. At the highest point, v = 0, so:
h_{\max} = \frac{u^2}{2g}, time to rise t = \frac{u}{g}, total time \frac{2u}{g}

Worked pattern: a stone dropped from rest from a 45 m bridge, with g = 10 m s⁻², takes t = \sqrt{\frac{2 \times 45}{10}} = 3 s to fall. It strikes with velocity v = 10 \times 3 = 30 m s⁻¹.

3.18 MASS AND WEIGHT

Mass is the quantity of matter in a body. It is a scalar, measured in kilograms by a physical balance. It stays constant everywhere.

Weight is the force with which the earth attracts the body. It is a vector, directed downward, measured in newtons. Weight changes from place to place because g changes.

The two connect by:
W = mg

The gravitational units are useful here. One kilogram force equals 9.8 N, and one gram force equals 980 dyne.

Worked pattern: a 4 kg body weighs 4 \times 9.8 = 39.2 N on earth. On the moon, where g = 1.6 m s⁻², it weighs 4 \times 1.6 = 6.4 N, while its mass stays 4 kg.

What to practise from Laws of Motion Class 9 ICSE

The exercise blocks below follow the chapter’s own structure within the CISCE syllabus. Each block tests a defined part of this chapter.

Laws of Motion Class 9 ICSE frequently asked questions

Why is the acceleration due to gravity the same for all bodies in free fall in the laws of motion class 9 ICSE chapter?

In vacuum, no air acts on the falling body, so only gravity pulls it. The gravitational force on a body is proportional to its mass. In g = F/m, the mass cancels, so every body falls with the same acceleration.

What is the difference between mass and weight in this chapter?

Mass is the quantity of matter in a body, a scalar measured in kilograms, and it stays constant everywhere. Weight is the force of gravity on the body, a vector in newtons given by W = mg, and it changes where g changes.

When does the relation force equals mass times acceleration stop being valid?

F = ma holds when mass stays constant and velocity is far below the speed of light. Near light speed, mass grows with velocity, so you must use the general form F = \Delta p / \Delta t.

How do I pick the right equation of motion for a body thrown vertically upwards?

For upward motion the acceleration is −g. Use v = u - gt, h = ut - \frac{1}{2}gt^2, and v^2 = u^2 - 2gh. At the highest point v = 0, so h_{\max} = u^2 / 2g. Choose the equation that contains the quantity you want and the quantities you know.

What mistakes do students make when applying the three laws of motion in this ICSE chapter?

Three errors are common. Students say action and reaction cancel, but they act on different bodies. They use F = ma when mass changes, such as in a rocket. They also forget the minus sign on g for a body moving upward, and they confuse mass with weight.

Reference: ICSE Class 9 Physics textbook material, chapter 3: Laws of Motion — CISCE-aligned.

More from this book

• Chapter 1: Measurements and Experimentation
• Chapter 2: Motion in One Dimension
• Chapter 4: Pressure in Fluids and Atmospheric Pressure
• Chapter 5: Upthrust in Fluids Archimedes Principle and Floatation
• Chapter 6: Heat and Energy
• Chapter 7: Reflection of Light
• Chapter 8: Propagation of Sound Waves
• Chapter 9: Current Electricity
• Chapter 10: Magnetism