Chapter 3: Dynamics – Physics Class 9 Notes

Kinematics describes motion without considering its cause. Dynamics is concerned with the forces that produce and change the motion of bodies — whenever there is acceleration, a force is present to cause it.

This chapter covers the concept and types of force (contact and non-contact), the four fundamental forces of nature, free-body diagrams, Newton's three laws of motion, mass and weight, instruments used to measure mass and force, friction and terminal velocity, and momentum, impulse, and the principle of conservation of momentum.

Learning Objectives

• Differentiate between contact and non-contact forces, and identify different types of forces
• State the four fundamental forces of nature and compare their relative strengths
• Represent forces acting on a body using free-body diagrams
• State and apply Newton's first, second and third laws of motion
• Differentiate between mass and weight, and calculate weight using w = mg
• Justify the use of mechanical/electronic balances and a force meter to measure mass and weight
• Analyse the dissipative effect of friction and the concept of terminal velocity
• Define and calculate momentum and impulse, and apply the principle of conservation of momentum

Key Concepts

3.1 Concept and Types of Force

A force is a push or pull that starts, stops, or changes the magnitude or direction of the velocity of a body. Forces are of two major types. Contact forces act at the point of contact between two objects — examples include friction (resists relative motion between surfaces), drag (resistance from a fluid), thrust (upward force from a liquid on an immersed object), normal force (reaction from a surface, perpendicular to it), air resistance, tension force, and elastic force (restores deformed materials to their original shape). Non-contact (field) forces act between objects that are not touching, also called action-at-a-distance forces — examples include gravitational force, electrostatic force, and magnetic force.

3.2 The Four Fundamental Forces

All forces in nature fall under four fundamental forces. Gravitational force is the weakest of the four, but is long-range, extending to infinite distance while weakening with distance; it is described by Newton's law of gravitation, F = Gm₁m₂/r², where G = 6.67 × 10⁻¹¹ N m² kg⁻². Electromagnetic force, which includes electrostatic and magnetic forces, is stronger than gravity, is also long-range, and is responsible for chemical reactions and (at the macroscopic level) friction. Strong nuclear force binds protons and neutrons together in the nucleus, overcoming electromagnetic repulsion between protons; it is short-range, acting only over about 10⁻¹⁴ m. Weak nuclear force is responsible for radioactive decay (such as beta decay), is stronger than gravity but weaker than electromagnetic force, and is short-range, acting over about 10⁻¹⁷ m. Pakistani scientist Dr. Abdus Salam, along with Sheldon Glashow and Steven Weinberg, won the 1979 Nobel Prize in Physics for showing that the weak nuclear force and electromagnetic force are two aspects of a single unified electroweak force.

3.3 Free-Body Diagrams

A free-body diagram shows the relative magnitudes and directions of all forces acting on an object in a given situation. The object is represented as a box, and force arrows are drawn outward from its centre in the direction of each force, with arrow length representing magnitude and each force labelled by type (e.g. weight, friction, normal force).

3.4 Newton's Laws of Motion

Newton's First Law: a body continues its state of rest or of uniform motion in a straight line unless acted upon by some external force. This is also called the law of inertia — inertia is the property of a body to resist a change in its state of rest or motion, and mass is a measure of an object's inertia (greater mass means greater inertia).

Newton's Second Law: if a net external force acts on a body, it produces acceleration in the direction of the force; the acceleration is directly proportional to the force and inversely proportional to the mass, giving F = ma. One newton is defined as the force that produces an acceleration of 1 m s⁻² in a body of mass 1 kg (1 N = 1 kg m s⁻²). Since velocity is a vector, a force can change a body's speed, direction, or both.

Newton's Third Law: for every action, there is an equal and opposite reaction — if one body exerts a force on a second body, the second body exerts an equal and opposite force on the first. Action and reaction act on two different bodies, so they never cancel each other. Examples: a block resting on a table exerts its weight downward on the table, and the table exerts an equal normal reaction force upward on the block; when a bullet is fired, it moves forward with force F (action) while the gun recoils backward with reaction force R.

Newton's laws apply with very high accuracy to everyday motion but are not exact for particles moving at speeds close to the speed of light, where Einstein's relativistic mechanics is needed instead.

3.5 Mass and Weight

Mass is the quantity of matter in a body and is a measure of the body's inertia — it is a scalar quantity, remains constant everywhere, has SI unit kilogram (kg), and is measured with an ordinary balance. Weight is the gravitational force with which the Earth (or another planet) attracts a body toward its centre — it is a vector quantity directed downward, with SI unit newton (N), and is measured using a spring balance (force meter), not an ordinary balance.

A gravitational field is the region around a mass in which another mass experiences a gravitational force. Gravitational field strength g is the gravitational force per unit mass: g = w/m, so weight is given by w = mg (equivalent to the acceleration of free fall). Since g varies with location and altitude, an object's weight varies from place to place even though its mass stays the same; at any given location, weight remains proportional to mass, so masses can be compared by comparing weights there.

3.6 Measuring Mass and Weight

A mechanical balance uses a rigid beam pivoting on a central knife-edge, with pans on each end; the object is balanced against standard weights. A mechanical platform balance (for heavy items like flour or cement bags) places the fulcrum near one end so small calibrated weights at the other end balance a much heavier load. An electronic balance requires no standard weights, is more precise, and displays mass directly (some models also compute price from a per-kg rate). A force meter (spring balance or newton meter) measures force/weight directly in newtons using the compression or stretching of an internal spring against a calibrated scale; digital force meters are also available.

3.7 Friction and Terminal Velocity

Friction is a dissipative force — energy used to overcome it is converted to heat (e.g. warming of rubbed hands, hot machine parts, worn tyres, and burning meteors from air friction). Sliding friction between two solid surfaces has two forms: static friction (opposes the start of motion, up to a maximum called limiting friction) and kinetic friction (acts once the object is sliding). Rolling friction occurs when an object rolls over a surface (e.g. wheels, ball bearings) and is roughly one hundred times smaller than sliding friction, because the rolling surface touches only at a point with no relative sliding motion.

Terminal velocity is the constant velocity reached by a falling object when the upward force of air resistance exactly balances the downward force of gravity, so the object stops accelerating (e.g. a parachutist descending safely). Methods used to reduce friction include polishing sliding surfaces, applying oil or grease (liquid friction is less than solid friction), converting sliding friction to rolling friction using ball bearings, and streamlining vehicles/vessels to allow smooth (streamline) airflow or water flow around them.

3.8 Momentum, Impulse and Conservation of Momentum

Momentum is the quantity of motion of a moving body, defined as the product of its mass and velocity: p = mv. It is a vector quantity with SI unit kg m s⁻¹ (equivalent to N s). Impulse is the product of a large force acting for a short time interval and equals the total change in momentum it produces: Impulse = F × Δt = m × Δv = m(v_f − v_i). Rearranged, this gives Newton's second law in terms of momentum: F = Δp/Δt — the rate of change of momentum of a body equals the net force acting on it.

The principle of conservation of momentum states that if no external force acts on an isolated system, the total momentum of the system after an interaction (such as a collision) equals its total momentum before the interaction: m₁v₁ + m₂v₂ (before) = m₁v₁′ + m₂v₂′ (after). This principle applies to both macroscopic objects (e.g. colliding balls, a recoiling gun) and microscopic particles such as atoms and molecules. Everyday applications of impulse and momentum concepts include drawing hands back while catching a fast ball (increasing contact time to reduce force), soft packing materials for fragile goods, seatbelts, airbags, and vehicle crumple zones — all reduce the force of impact by increasing the time over which momentum changes.

Important Definitions

Key Formulas

Topic	Formula
Newton's law of gravitation	F = G m₁m₂ / r² (G = 6.67 × 10⁻¹¹ N m² kg⁻²)
Weight	w = mg (g ≈ 10 N kg⁻¹ or 10 m s⁻²)
Gravitational field strength	g = w/m
Newton's second law	F = ma (1 N = 1 kg m s⁻²)
Momentum	p = mv
Impulse	Impulse = F × Δt = m × Δv = m(v_f − v_i)
Force as rate of change of momentum	F = Δp/Δt
Conservation of momentum	m₁v₁ + m₂v₂ (before) = m₁v₁′ + m₂v₂′ (after)

Diagrams

Types of Forces: Classification of contact and non-contact forces with examples of each

Newton's Three Laws of Motion: Summary of the first, second and third laws with their key statements

Terminal Velocity: Velocity-time graph showing a falling object's speed levelling off at terminal velocity

Short Questions & Answers

Long Questions & Answers

State and explain Newton's three laws of motion with examples.

Newton's first law states that a body continues its state of rest or uniform motion unless acted on by an external force — e.g. passengers lurch forward when a bus stops suddenly because their bodies tend to continue moving. Newton's second law states that a net force produces acceleration in its own direction, proportional to the force and inversely proportional to mass, giving F = ma — e.g. a truck accelerates less than a bicycle under the same applied force because of its greater mass. Newton's third law states that for every action there is an equal and opposite reaction acting on a different body — e.g. a bullet fired forward from a gun (action) causes the gun to recoil backward (reaction).

Differentiate between mass and weight.

Mass is the quantity of matter in a body, a scalar quantity measured in kilograms, that stays constant everywhere and is measured with an ordinary balance. Weight is the gravitational force pulling a body toward the centre of a planet, a vector quantity measured in newtons (w = mg), that varies with location because gravitational field strength g varies with location and altitude, and is measured with a spring balance (force meter) rather than an ordinary balance.

Describe static and kinetic friction using the example of a block on a table, and explain terminal velocity.

When a horizontal pulling force is gradually applied to a block resting on a table (e.g. via weights added to a pan connected by a string), an opposing force called static friction initially prevents the block from moving; this increases up to a maximum called limiting friction. Once the applied force exceeds limiting friction, the block begins to slide, and the opposing friction during sliding is called kinetic friction. Terminal velocity occurs when a body falling through air (or another fluid) accelerates until the upward air resistance force grows large enough to balance the downward weight; beyond this point, the net force is zero, so the body falls at a constant (terminal) velocity — as seen in a parachutist's steady descent.

State the principle of conservation of momentum and explain it using a collision example.

The principle states that if no external force acts on an isolated system, the total momentum of the system after an interaction equals its total momentum before the interaction. For two balls of masses m₁ and m₂ moving with velocities v₁ and v₂ that collide and move off with velocities v₁′ and v₂′, m₁v₁ + m₂v₂ = m₁v₁′ + m₂v₂′. For example, when a moving ball strikes an identical stationary ball, momentum transfers from the first ball to the second: the striking ball can come to rest while the second ball moves off with the same momentum the first ball had before the collision, showing total momentum is conserved.

Multiple Choice Questions (MCQs)

Quick Revision Summary

• Force = push/pull that changes speed, direction, or state of motion
• Contact forces: friction, drag, thrust, normal, tension, elastic, air resistance
• Non-contact forces: gravitational, electrostatic, magnetic (also strong/weak nuclear)
• 4 fundamental forces (weakest to strongest): gravitational, weak nuclear, electromagnetic, strong nuclear
• Newton's 1st law: inertia; 2nd law: F=ma; 3rd law: action-reaction pairs on different bodies
• Mass = quantity of matter (scalar, kg, constant); Weight = mg (vector, N, varies with location)
• Static friction ≤ limiting friction; kinetic friction acts during sliding; rolling friction ≪ sliding friction
• Terminal velocity: air resistance balances weight, so acceleration becomes zero
• Momentum p = mv; Impulse = FΔt = Δp
• Conservation of momentum: total momentum before collision = total momentum after (isolated system)

Exam Tips

• Memorise the three statements of Newton's laws precisely — exam questions often ask to 'state' them exactly
• Practice distinguishing contact vs non-contact forces and classifying examples correctly
• Be clear on the mass vs weight distinction: mass is constant and scalar; weight = mg, is a vector, and varies with location
• Work through F=ma, p=mv, and impulse numericals step by step, listing given values before selecting the formula
• Remember real-life applications of momentum conservation and impulse (airbags, seatbelts, crumple zones, catching a ball) — these are common short-answer topics
• Know the order-of-magnitude ranges for strong (10⁻¹⁴ m) and weak (10⁻¹⁷ m) nuclear forces, and that Dr. Abdus Salam shared the 1979 Nobel Prize for the electroweak unification