Science Class 9 — Exploration
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Force, Types of Forces and Balanced & Unbalanced Forces

🎓 Class 9 Science CBSE Theory Ch 6 — How Forces Affect Motion ⏱ ~13 min
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Introduction: What Sets Things Moving?

Imagine you are seated on a stationary bicycle. To start moving, you press the pedals — your muscles apply something on the pedal that makes it turn. To stop, you press the brakes — friction grips the wheel and slows it down. To turn left, you tug the handle. In every case, something invisible is acting on the bicycle to change the way it moves. That something is what physicists call a force.

In daily life, you apply forces all the time without thinking about it. Kicking a football, striking a cricket ball with a bat, squeezing a lemon, pulling open a drawer, pushing a shopping trolley — each of these is a force at work. In this chapter we will learn what forces are, how they are classified, and how they decide whether an object will move, stop, speed up, slow down, or change direction.

Key Idea: A force cannot be seen directly. We recognise a force only through the changes it produces — in motion, in speed, in direction, or in shape.

6.1 The Concept of Force

The simplest way to describe a force is as a push or a pull. When you open a door by pulling the handle towards you, you are applying a pulling force. When you close it by pressing it away, you are applying a pushing force. Forces always act between two objects — one applies the force, the other receives it.

Effects produced by a force

When a force acts on an object, it can produce one or more of the following observable effects:

  • Change in state of motion: A stationary object can be set in motion, or a moving object can be brought to rest. Kicking a stationary ball makes it roll; applying brakes brings a moving cycle to a stop.
  • Change in speed: A moving object can be made to move faster (accelerate) or slower (decelerate). Pressing the accelerator of a car increases speed.
  • Change in direction: A force applied sideways on a moving body changes its direction. A batsman striking a moving cricket ball deflects it to the boundary.
  • Change in shape or size: Pressing a sponge, squeezing a lemon, or stretching a rubber band changes the shape of the object even if the object as a whole does not move.
Sets in motion Stops motion Changes direction Changes shape Effects of a Force
Fig 6.1: A force can set objects in motion, stop them, change their direction, or change their shape.

6.2 Activity — Feeling Force in Action

Activity 6.1 — Pushes, Pulls and SqueezesL3 Apply
Predict first: Try squeezing a sponge with both hands. Does the sponge move from its place? What changes about it?
  1. Place a small ball on a smooth tabletop. Give it a gentle push with your finger. Note what happens.
  2. Now stop the moving ball with your other hand. Note the effect of your hand on the ball.
  3. Take a fresh sponge or a soft rubber ball and squeeze it tightly. Observe the change in shape.
  4. Stretch a rubber band lightly. Look at how its length changes.
Observations: In step 1, the push made the stationary ball move (state of motion changed). In step 2, your hand stopped the moving ball (motion changed to rest). In step 3, the sponge did not shift but its shape clearly changed under the squeezing force. In step 4, the rubber band became longer because of a stretching force.

Conclusion: A force does not always cause motion — sometimes it only changes the shape or size of an object. The key signature of a force is that it produces some kind of change.

6.3 Types of Forces

Based on whether the two objects involved must touch each other, forces are divided into two broad classes — contact forces and non-contact forces.

A. Contact Forces

A contact force requires the two objects to be physically touching. The four common contact forces in our syllabus are:

  • Muscular force: The force exerted by the muscles of humans and animals — lifting a bag, kicking a ball, an ox pulling a cart.
  • Frictional force: The opposing force between two surfaces in contact when one slides or tries to slide over the other. Friction makes a rolling ball slow down on the ground.
  • Normal force: The perpendicular push that a surface exerts on an object resting on it. A book on a table experiences a normal force from the table pushing upward.
  • Tension: The pulling force transmitted through a rope, string or cable when it is stretched tight. The rope of a swing experiences tension when a child sits on it.

B. Non-Contact Forces

A non-contact force can act across a distance without the two bodies touching. The three important non-contact forces are:

  • Gravitational force: The attractive pull that any two objects with mass exert on each other. The Earth pulls a falling apple downward; the Sun holds the planets in their orbits.
  • Electrostatic force: The push or pull between charged objects. A plastic comb rubbed on dry hair attracts small bits of paper without touching them.
  • Magnetic force: The attractive or repulsive force between magnets, or between a magnet and certain metals like iron. A magnet picks up an iron pin from a small distance away.
FORCES Contact Forces Non-Contact Forces Muscular Friction Normal Tension Gravitational Electrostatic Magnetic Need touch ←——— vs ———→ Act at a distance
Fig 6.2: Classification of forces into contact and non-contact categories.

6.4 Balanced and Unbalanced Forces

It is rare for only one force to act on an object. Usually several forces act together, and the overall effect depends on their net outcome. The single force that has the same effect as all the individual forces taken together is called the net force (or resultant force).

Balanced forces

When two or more forces acting on a body add up to zero, they are called balanced forces. A book lying on a table is pulled down by gravity and pushed up by the normal force from the table. These two forces are equal in size but opposite in direction, so the net force is zero. Result: the book stays at rest. Balanced forces do not change the state of motion; they may, however, deform the object slightly.

Unbalanced forces

When the forces on a body do not cancel out, the body experiences an unbalanced force — a non-zero net force. An unbalanced force causes a change in the body's state of motion: it speeds up, slows down, starts moving, stops, or changes direction.

Definition. Balanced forces produce no change in motion (net force = 0). Unbalanced forces produce a change in motion (net force ≠ 0).

Tug of War — A Classic Example

In a tug of war, two teams pull a rope from opposite ends. Each team applies a muscular force. As long as both teams pull with equal strength, the forces are balanced and the rope (with its central knot) does not move. As soon as one team pulls harder than the other, the forces become unbalanced — the net force points toward the stronger team, and the rope begins to move in that direction.

🪢 Tug of War — Compare balanced vs unbalanced L4 Analyse

Same rope, same children — only the size of the pulls differs. Click each row to analyse the net force and the resulting effect on motion.

Balanced (no motion) 100 N 100 N Net force = 0 — knot stays put Unbalanced (motion to right) 80 N 120 N Net force = 40 N to the right — knot moves right
Fig 6.3: Tug of war — equal pulls give zero net force; unequal pulls create motion toward the stronger side.
Click either tug-of-war row above to compare net force and motion outcome.

Worked example — net force

Q: Two children push a box from opposite sides — one with 30 N to the right, the other with 18 N to the left. What is the net force?
A: Take right as positive. Net force \(= +30 - 18 = +12\) N to the right. The box accelerates to the right.

Quick Recap

ConceptQuick definition
ForceA push or pull producing change in motion or shape. SI unit: newton (N).
Contact forceActs only on touching — muscular, friction, normal, tension.
Non-contact forceActs at a distance — gravitational, electrostatic, magnetic.
Balanced forceNet force = 0; no change in motion.
Unbalanced forceNet force ≠ 0; motion changes.

Competency-Based Questions

Riya places a heavy carton on a wooden floor. She pushes it with 25 N but the carton does not move. Her elder brother joins and together they push with 60 N — the carton finally slides. The friction between the carton and the floor is 50 N.
Q1. When Riya alone pushes with 25 N, why does the carton not move? L2
  • (a) Riya's push is greater than friction
  • (b) Friction (50 N) exceeds her push, so net force is zero on a stationary body
  • (c) Gravity is too strong
  • (d) The floor is frictionless
(b) Static friction adjusts to match the applied push up to a maximum value. Since 25 N is less than the limiting friction of 50 N, the forces remain balanced and the carton stays at rest.
Q2. Calculate the net force on the carton when both children push together with 60 N. L3
Applied force = 60 N (forward). Friction = 50 N (backward). Net force \(= 60 - 50 = 10\) N forward — unbalanced, so carton accelerates.
Q3. Classify the forces acting on the carton as contact or non-contact. L2
Contact: applied push (muscular), friction, normal force from floor. Non-contact: gravitational pull from Earth (weight).
Q4. State whether the following is True or False: "A balanced force can change the shape of a body but cannot change its state of motion." L1
True. Squeezing a rubber ball — equal forces from both hands cancel out (no motion), yet the ball deforms.
Q5. A magnet attracts an iron nail kept 2 cm away. What kind of force is this and which property of the nail makes the force possible? L4
It is a non-contact magnetic force. Iron is a ferromagnetic material — its atoms can align with an external magnetic field, which causes the magnet to pull it.

Assertion–Reason Questions

Options: (A) Both A and R are true and R is the correct explanation of A. (B) Both true but R is not the correct explanation. (C) A true, R false. (D) A false, R true.

A: A book lying on a table is in equilibrium.
R: The weight of the book and the normal force from the table are equal in magnitude and opposite in direction.
(A) Both statements are true and R correctly explains A. The two balanced forces give zero net force, so the book stays at rest.
A: Friction is a non-contact force.
R: Friction acts between two surfaces in physical contact.
(D) Assertion is false — friction is a contact force. Reason is true and is the correct definition of friction.
A: When equal and opposite forces act on a balloon by squeezing, it changes shape but does not move.
R: Balanced forces never have any effect on a body.
(C) Assertion is true. Reason is false — balanced forces can still deform a body even if they cannot move it.
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