Science & TechnologyNCERT Class 9 · Science (Class 9)

How Forces Affect Motion

Force is the cause behind any change in an object's motion or shape, and — contrary to old intuition — a body keeps moving on its own at constant velocity unless an unbalanced (net) force such as friction acts on it (Newton's First Law).

⏱ 7 min readGS-III7 sections5 memory tricks

Why this matters for UPSC

Basic physics fundamentals occasionally surface in Prelims general-science questions — SI units, measuring instruments, types of forces, and Newton's laws with their everyday applications like inertia and seat belts. For Mains GS-III (Science & Technology), a clear grasp of force, friction and Newton's laws underpins reasoning about transport safety, engineering and space technology. UPSC favours application and match-the-concept recall over numerical derivations.

Understand the chapter

The Concept of Force: What It Does

A force is a push or pull that can set a stationary object into motion, change the speed or direction of a moving object, or even change an object's shape. Earlier chapters described motion (position, velocity, acceleration) but not its cause; this chapter establishes force as that cause. Everyday examples are kicking a ball (starts motion), a bat striking a ball (changes direction) and squeezing a lemon (changes shape).

Four effects: start/stop motion, change speed, change direction, change shape.
Force answers the 'why' of motion that kinematics leaves out.
A canoe moves forward because the paddle pushes water backward; harder push, faster motion.

Force is a Vector: Its Unit and Measurement

Force has both magnitude and direction, so it is a vector quantity — every force is described along with the direction in which it acts (e.g., friction opposite to motion, gravity toward Earth). The SI unit of force is the newton (N); when a unit is named after a person, the name is written in lower case (newton) but the symbol is capitalised (N). A spring balance measures the magnitude of a force, including weight, which is the gravitational force with which Earth pulls an object.

Vector = magnitude + direction, like displacement, velocity, acceleration.
SI unit: newton (N), named after Isaac Newton.
Smallest force felt ~ millinewton (10^-3 N); scientists measure down to yoctonewton (10^-24 N) as of 2026.
Spring balance = instrument for measuring force and weight.

Balanced and Unbalanced Forces and Net Force

Usually several forces act on an object at once; what determines its motion is the net (resultant) force, not the individual forces. Two forces equal in magnitude and opposite in direction are balanced — they give zero net force and no change in motion, like a tug-of-war that does not move. If the forces are unbalanced, a non-zero net force acts and the object's motion changes.

Balanced forces: equal + opposite, net zero, state of motion unchanged.
Opposite and unequal: net = difference, in the direction of the larger force.
Same direction: net = sum of magnitudes, same direction.
Motion depends only on the net force.

The Force of Friction

Friction is a force that arises between surfaces in contact and always acts opposite to the direction of (attempted) motion. An object starts moving only when the applied force exceeds friction; once you stop pushing, friction decelerates the object until it stops — which is why a coasting bicycle or a rolling ball eventually halts. Activities show friction depends on the nature of the surfaces in contact: smoother surfaces give smaller friction, so the object travels farther.

Friction opposes relative motion; depends on the nature of the surfaces in contact.
Smaller friction means slower deceleration and a larger distance travelled.
Air also exerts friction, usually negligible for slow everyday motion.
Spring-balance reading when a block just starts to move approximates the force of friction.

Other Forces: Weight and Normal Force

For an object resting on or pushed along a surface, besides the applied force and friction, two more forces act: its weight (gravitational force) acting straight down, and the normal force exerted by the surface acting upward, perpendicular to it. These two are balanced, so the object neither sinks into nor lifts off the surface. Identifying every force — applied, friction, weight, normal — is the key to finding the net force.

Weight = gravitational force, acts vertically downward.
Normal force = surface's push, upward and perpendicular to the surface.
Weight and normal force are balanced (cancel) on a flat surface.
Other named forces: buoyant (upward), magnetic, electrostatic, gravitational.

From Galileo to Newton: The Idea of Inertia

In ancient times people wrongly believed a force was needed to keep an object moving at constant velocity. In the 17th century Galileo Galilei argued through thought experiments that, with all impediments such as friction removed, a moving body would continue to move indefinitely. Isaac Newton named this tendency to resist a change in the state of rest or uniform motion 'inertia', and built it into his three laws of motion, published in 1687.

Old (Aristotelian) view: a force is needed to sustain motion — wrong.
Galileo (17th century): frictionless motion continues forever — established by thought experiments.
Newton coined the term 'inertia'; the three laws were published in 1687 (Principia).

Newton's First Law of Motion

Newton's First Law states: an object at rest stays at rest, and an object in motion continues to move with constant velocity, unless a net force acts upon it. Equivalently, if the net force is zero the object cannot begin to move or change its velocity — its acceleration is zero. Constant velocity means unchanged speed and direction; if it is non-zero, the motion is in a straight line.

Also called the law of inertia.
Net force = 0 implies acceleration = 0 (no change in velocity).
Explains why no force is needed to keep moving — force is needed only to overcome friction.

Key terms

Force: A push or pull that can change an object's state of rest or motion, its speed, direction or shape; a vector quantity.
Newton (N): The SI unit of force, named after Isaac Newton; name written in lower case, symbol as capital N.
Vector quantity: A quantity needing both magnitude and direction, such as force, velocity or acceleration.
Spring balance: An instrument that measures the magnitude of a force, including the weight of an object.
Weight: The gravitational force with which Earth pulls an object, directed downward.
Balanced forces: Forces equal in magnitude and opposite in direction, producing zero net force and no change in motion.
Net force: The single resultant of all forces acting on an object; it alone determines the object's motion.
Friction: A contact force opposing motion, depending on the nature of the surfaces in contact.
Normal force: The upward force a surface exerts perpendicular to itself, balancing the object's weight.
Inertia: The tendency of an object to resist a change in its state of rest or uniform motion.

Must-know facts exam-ready

SI unit of force is the newton (N), named after Isaac Newton.
Convention: unit name in lower case (newton), symbol capitalised (N).
Force is a vector — it needs both magnitude and direction.
Smallest force we can feel is ~ millinewton (10^-3 N); scientists measure down to yoctonewton (10^-24 N) as of 2026.
Weight is the gravitational force of Earth on an object and is measured with a spring balance.
Balanced forces are equal in magnitude, opposite in direction, give zero net force, and leave motion unchanged.
Opposite unequal forces: net = difference toward the larger; same-direction forces: net = sum.
Friction always opposes motion and depends on the nature of the surfaces in contact.
Normal force acts upward, perpendicular to the surface, and balances the weight.
Galileo (17th century) argued via thought experiments that motion continues indefinitely once impediments are removed.
Newton coined the term 'inertia' and published his three laws of motion in 1687 (Principia).
First Law (law of inertia): zero net force means no change in motion and zero acceleration.

Timeline

17th centuryGalileo Galilei argues through thought experiments that a body keeps moving indefinitely once all impediments to its motion are removed.
1687Isaac Newton publishes his three laws of motion (Principia), formalising the concept of inertia.

Memory tricks remember it for good

4-S of Force

Start/Stop, Speed, Steer (direction), Shape — the four things a force can change.

💡 Recall every effect a force can produce on an object.

WANF

Weight, Applied force, Normal force, Friction — the forces on a box pushed along a surface.

💡 List all acting forces before computing the net force.

Same = Sum, Opposite = Subtract

Forces in the same direction add; opposite forces give the difference, directed toward the larger.

💡 Quickly find the net force in straight-line (collinear) cases.

G-then-N, 1687

Galileo first (thought experiments, motion continues), then Newton names inertia and publishes 3 laws in 1687.

💡 Order the history of motion and pin the date of Newton's laws.

Zero Net, Zero Accel

If the net force is zero, acceleration is zero and velocity stays unchanged.

💡 Recall the core of Newton's First Law.

Traps to avoid

Mass vs weight: a spring balance measures weight (a force in newtons), not mass; weight equals the gravitational force.
newton vs Newton: the unit's name is lower case, its symbol is capital N — UPSC tests this convention.
'Balanced forces' does not mean 'no force' — it means zero net force; the object can still move at constant velocity.
A moving object does NOT need a continuous force to keep moving; force is needed only to overcome friction (rejecting the old Aristotelian view).
Opposite forces are 'balanced' only if equal; if unequal, the net force is the difference, not zero.
Inertia was named by Newton, but the insight that motion persists without impediments is Galileo's — do not swap the two.

Exam focus

🧠 Prelims angles

SI unit of force (newton) and order-of-magnitude prefixes (milli-, yocto-newton).
Newton's laws — year 1687, the term 'inertia', and the statement of the law of inertia.
Distinguishing balanced vs unbalanced forces and computing net force (sum or difference).
Types of forces and their directions: gravitational, friction, normal, buoyant, magnetic, electrostatic.
Instruments: the spring balance measures force and weight.
Match-the-scientist: Galileo's contribution vs Newton's contribution.

✍️ Mains angles GS-III

Friction is a 'necessary evil' in technology and everyday life — discuss.Show the dual role: friction is essential for walking, braking and gripping, yet wasteful as it causes energy loss and wear — hence lubricants, ball-bearings and streamlining to manage it.
How do Newton's laws of motion underpin modern transport and space technology?Link inertia to seat belts, airbags and headrests for safety; use net-force and first-law reasoning to explain vehicle braking distances and the principles behind rocket launches.

Practice Science & Technology questions from this syllabus →

Last-minute revision tick as you recall

Force = push/pull; a vector (magnitude + direction); SI unit newton (N).
Force can start/stop motion, change speed, change direction, or change shape.
Spring balance measures force; weight = Earth's gravitational pull on the object.
Balanced forces (equal + opposite) give zero net force and no change in motion.
Unbalanced forces give a net force and acceleration; same direction add, opposite subtract.
Friction opposes motion and depends on the surfaces in contact; air friction is usually negligible.
Weight (downward) and normal force (upward) balance on a flat surface.
Galileo: motion persists once impediments are removed; Newton: inertia plus 3 laws (1687).
First Law: zero net force means zero acceleration and unchanged velocity.

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Distilled from NCERT Class 9 · Science (Class 9) for UPSC. Always cross-check facts with the original NCERT.