⚡ CLASS 9 SCIENCE · CHAPTER 6 · PART 1 of 2

How Forces Affect Motion

Force, Balanced & Unbalanced Forces, Friction, and Newton’s First Law of Motion — explained with activities, real-life examples, and exam-ready notes.

📖 Exploration · Grade 9
🧪 3 Activities
📝 2 Solved Examples
Quick Navigation
6.1 Concept of Force
6.2 Balanced & Unbalanced Forces
6.3 Force of Friction
6.4 Newton’s First Law
Final Quiz
At a Glance

6.1

The Concept of Force

A force can make an object move from rest, change the speed and direction of motion of a moving object, and can even change the shape of an object.

• Kicking a ball — a force applied by your foot makes a stationary ball move.
• Striking a ball with a bat — the force changes the direction of motion of a moving cricket ball.
• Squeezing a lemon — force applied by your fingers changes its shape.

🤔 Think It Over

Why does a canoe move forward when the canoeist pushes water backwards with their paddle, and why does it move faster when they push harder? Also — if the same canoeist uses the same paddle force in an empty canoe vs. one carrying a passenger, which canoe moves faster?

Force is a Vector Quantity

Whenever a force is described, its direction is also specified — e.g., friction acts opposite to motion; like poles of a magnet repel; unlike charges attract; gravity pulls objects towards Earth; buoyant force acts upward on an object in liquid.

Force is a physical quantity for which we need to specify direction along with magnitude and unit — just like position, displacement, velocity, and acceleration.

📘 Definition

The SI unit of force is the newton (small ‘n’), symbol N. The magnitude of the force expresses its strength.

📌 Note

If either the magnitude or direction, or both, of a force applied on an object changes, the effect of the force also changes.

6.1.1 Measuring the Magnitude of a Force

A spring balance can measure the magnitude of a force in general (not just weight). If you pull on the free end of the spring balance, it measures the force with which you pull on the spring inside.

Remember: the weight of an object is the gravitational force with which the Earth pulls the object.

🧵 Threads of Curiosity

In everyday life, the smallest forces we can directly feel are of the order of millinewtons (10⁻³ N), such as a light touch. Scientists can measure forces far smaller — down to yoctonewtons (10⁻²⁴ N) in specialised experiments (as of 2026).

Q1. Which of the following is NOT an effect a force can produce on an object?
Change its speed
Change its mass
Change its direction of motion
Change its shape
Q2. What instrument is commonly used to measure the magnitude of a force?
Spring balance
Thermometer
Stopwatch
Measuring tape
Q3. Force is correctly described as a quantity that requires:
Only a numerical value
Only a unit, no direction
Both magnitude and direction
Neither magnitude nor direction

6.2

Balanced and Unbalanced Forces

In real life, situations seldom exist where only one force acts on an object. Usually more than one force acts at the same time:

• Pushing a box on a surface — your push force + friction force (opposite direction).
• A ball floating on water — gravitational force (downward) + buoyant force (upward).

Tug of War — Understanding Balanced Forces

If both teams pull the rope with equal force, the rope does not move — these are balanced forces (equal in magnitude, opposite in direction).

If one team pulls harder (larger magnitude), the forces become unbalanced and the rope moves in the direction of the larger force.

📘 Definition — Net Force

If the forces applied on an object are not balanced, a non-zero net force acts on the object.

• Opposite, unequal forces: Net force = difference of magnitudes; direction = along the larger force.
• Same direction forces: Net force = sum of magnitudes; direction = same as both forces.

📝 Example 6.1 — Net Force on a Block

Two forces of 10 N and 6 N act on a block lying on a table in three different ways. Find the net force in each case.

(a) Both forces same direction (right): Net force = 10 N + 6 N = 16 N, towards right
(b) 10 N right, 6 N left: Net force = 10 N − 6 N = 4 N, towards right
(c) 6 N right, 10 N left: Net force = 10 N − 6 N = 4 N, towards left
🚀 Ready to Go Beyond

When forces act at an angle to each other (not parallel or opposite), you’ll learn to calculate net force in higher grades. Also, equal and opposite forces applied to two ends of an extended object can make it rotate — e.g., turning a handlebar or a tap.

Q1. Two forces of 8 N and 8 N act on a block in opposite directions. What is the net force?
16 N
0 N (balanced forces)
4 N
Cannot be determined
Q2. Three forces of 5 N, 3 N, and 2 N act on a block, all in the same direction. What is the net force?
5 N
3 N
10 N
0 N
Q3. If the net force acting on an object is zero, the forces acting on it are called:
Balanced forces
Unbalanced forces
Frictional forces
Gravitational forces

6.3

The Force of Friction: Often Overlooked but Always Present

Suppose an object is at rest on the floor and you apply a forward force on it. Many times the object does not move until you apply a larger force. Why?

This is because of the force of friction arising between the bottom surface of the box and the floor, acting opposite to the direction of your applied force. The box starts moving only when your applied force is larger than friction, so a net force acts in the direction of motion.

➡️

Applied Force

Pushed by hand, forward
⬅️

Friction

Opposes motion
⬇️

Gravitational Force

Weight, downward
⬆️

Normal Force

Surface pushes up
🚀 Ready to Go Beyond

For a pushed object, the weight (downward) and normal force (upward, perpendicular to surface) are balanced with each other. Air around the object also exerts friction as it moves through air, but its magnitude is often small enough to neglect.

📌 Note

Multiple forces may act on an object, but its motion depends only on the net force.

⏸️ Pause and Ponder

1. A weightlifter lifts a barbell. List two forces acting on the barbell. Are these forces balanced if the weightlifter keeps the barbell steady?
2. Two players are arm-wrestling. At the instant their arms tilt forward, are the forces exerted balanced? If not, which player exerted the larger force?

Once a moving box loses your applied force, it slows down and eventually stops — because friction continues to act opposite to motion. On a moving object, you must continuously apply a force to counter friction; otherwise friction brings it to rest.

🤔 What If…

…the force of friction disappears in the world? How will the motion of objects be impacted?

Activity 6.1: Let Us Investigate

🔬

Rubber Band & Coin Stack Experiment

1. Stack 4 coins of ₹10 secured with tape. Locate surfaces of different materials (wooden table, cemented floor, laminated table, polished marble/tiled floor).
2. Stretch a rubber band between forefinger and thumb. Mark points A, B (ends) and C (stretch point).
3. Place the coin stack near the middle of A-B. Push it back till the band reaches mark C. Release and observe its motion.
4. Measure the distance travelled from C. Repeat on each surface, keeping A, B, C at the same distances.
5. Compare: does the coin stack travel farther and decelerate more slowly on smoother surfaces (laminated, polished marble) than on rougher ones (wooden table)?

Conclusion: Before release, the stack is stationary — forces are balanced. Upon release, the rubber band’s force (forward) exceeds friction, giving a net forward force and acceleration. Once contact is lost, only friction acts — opposite to motion — gradually decreasing velocity to zero. Distance travelled differs across surfaces because the force of friction is different for different surfaces.

Activity 6.2: Let Us Measure

⚖️

Spring Balance & Wooden Block

1. Place a spring balance horizontally on a surface; check the reading is zero. Attach a wooden block to its hook.
2. Pull the spring balance with gradually increasing force; note the reading when the block just starts moving.
3. Repeat on the remaining three surfaces from Activity 6.1.
4. Compare readings — is the smallest reading for the surface where the coin stack travelled farthest?

Conclusion: The spring balance reading gives an approximate measure of the force of friction. Smaller reading → smaller friction → coin travels farther before stopping. Larger reading → larger friction → coin stops sooner.
🤔 Think as a Scientist — Thought Experiment

A thought experiment is conducted when real-world conditions are too hard to recreate. Suppose an object and floor have surfaces so smooth that friction between them is zero. If you repeat Activity 6.1’s steps with such a frictionless object and floor — will its velocity decrease? Will it ever come to rest, or continue moving forever?

Q1. In Activity 6.1, why does the coin stack travel a larger distance on a polished marble floor than on a wooden table top?
The rubber band is stretched more on marble
Marble floor has more gravity
Friction is smaller on the polished marble floor
The coins are heavier on marble
Q2. The force of friction always acts in a direction:
Opposite to the direction of motion (or attempted motion)
Same as the direction of motion
Perpendicular to motion always
Upward only
Q3. In Activity 6.2, a smaller spring balance reading when the block just starts moving indicates:
A heavier block
A larger normal force
A rougher surface
A smaller force of friction

6.4

Newton’s First Law of Motion

GG

Galileo Galilei

In ancient times, it was believed a force was needed even to keep an object moving at constant velocity. In the 17th century, Galileo argued through thought experiments that if a body moves on a horizontal plane and all impediments to motion are removed, it will continue to move indefinitely.

IN

Isaac Newton

Used the word ‘inertia’ to describe an object’s tendency to resist change in its state of rest or uniform motion. Presented three laws of motion in 1687 — a defining moment in the history of science. The unit of force (newton) is named after him.

📌 Note — Naming Convention

When a unit is named after a person, its full form begins with small case (newton, not Newton), while its symbol is capitalised (N, not n).

NEWTON’S FIRST LAW OF MOTION

An object at rest remains at rest, and an object in motion continues to move with a constant velocity, unless a net force acts upon the object.

In other words, if the net force on an object is zero, the object cannot begin to move or change its velocity — its acceleration is zero.

📌 Note

An object at rest has zero velocity. Constant velocity means no change in magnitude or direction. If this constant velocity is non-zero, motion is in a straight line, same direction, with constant magnitude.

📝 Example 6.2 — Box Moving at Constant Velocity

A person pushes a moving box forward with a force exactly equal to the friction force. Will the box continue moving or come to rest?

Friction acts backward, equal and opposite to the applied force — they balance. Net force = 0. As per Newton’s first law, the box will continue moving with constant velocity.
📝 Example 6.3 — Graphs for Zero Net Force

Draw position-time and velocity-time graphs for an object on which no net force acts.

Two cases possible:
At rest: position-time graph = horizontal line at fixed position; velocity-time graph = horizontal line at zero.
Constant velocity: position-time graph = straight inclined line; velocity-time graph = horizontal line at a non-zero value.
📌 Note — Key Insight

If friction is zero, a net force is needed to set an object at rest into motion. But once moving, no further force is needed to keep it moving at constant velocity. However, to change velocity (magnitude or direction) or to stop a moving object, a force must be applied.

⏸️ Pause and Ponder

3. An object is moving with constant velocity. Is there a net force acting upon it?
4. Suppose no net force acts on an object. Which situations are possible: (i) remains at rest if at rest (ii) keeps moving at constant velocity if already moving (iii) moves with constant acceleration?
5. In the real world, it’s hard to find a situation with truly zero forces. But by applying additional forces, a condition of zero net force can be achieved. Explain with an example.

💡 Looking Ahead

Newton’s first law describes motion in the absence of net force. What happens when a net force does act on an object? That’s answered by Newton’s Second Law of Motion — covered in Part 2 of these notes.

Q3. According to Newton’s First Law, what happens to an object’s velocity if the net force acting on it is zero?
It always becomes zero
It remains unchanged (constant)
It keeps increasing
It keeps decreasing
Q4. Who first argued through thought experiments that a body in motion on a frictionless horizontal plane would move indefinitely?
Isaac Newton
Albert Einstein
Galileo Galilei
Robert Hooke
Q5. Newton’s First Law of Motion is also known as the law of:
Acceleration
Inertia
Momentum
Gravitation
Q6. If an object is at rest with zero net force, its position-time graph is:
A horizontal line
A straight inclined line
A curved line
A vertical line

🏆 Part 1 Final Quiz

15 questions covering Force, Balanced/Unbalanced Forces, Friction & Newton’s First Law

1. What is the SI unit of force?
Joule
Watt
Newton
Pascal
2. Force is a physical quantity that requires specifying:
Magnitude and direction
Only magnitude
Only direction
Neither magnitude nor direction
3. Which instrument is used to measure the magnitude of a force?
Thermometer
Spring balance
Barometer
Voltmeter
4. In a tug of war, if both teams pull with equal force, the rope:
Moves towards the stronger team
Moves randomly
Does not move (balanced forces)
Breaks
5. Two forces of 12 N and 5 N act in opposite directions on an object. What is the net force?
17 N
12 N
5 N
7 N
6. The force of friction acts on a moving object in a direction:
Same as the direction of motion
Opposite to the direction of motion
Perpendicular to the direction of motion
Friction has no specific direction
7. The normal force acting on a box resting on a table acts:
Upward, perpendicular to the surface
Downward
Sideways
In the direction of motion
8. In Activity 6.1, the coin stack travels the LARGEST distance on the surface with:
Highest friction
Highest gravity
Lowest friction
Highest temperature
9. Newton’s First Law of Motion is also known as the law of:
Acceleration
Inertia
Momentum
Gravitation
10. Who formulated the three laws of motion, and in which year?
Galileo, 1609
Einstein, 1905
Hooke, 1665
Newton, 1687
11. A box moves at constant velocity because a person pushes it with a force equal to friction. The net force on the box is:
Twice the applied force
Equal to friction
Zero
Equal to the applied force
12. If an object is at rest, its position-time graph is:
A horizontal line
A straight inclined line
A curved line
A vertical line
13. Once an object is moving at constant velocity with zero net force, to change its velocity you need:
Nothing — it changes on its own
A force to be applied
More time
Less mass
14. The smallest force humans can directly feel (like a light touch) is of the order of:
Yoctonewtons (10⁻²⁴ N)
Kilonewtons (10³ N)
Meganewtons (10⁶ N)
Millinewtons (10⁻³ N)
15. Two forces of 7 N act on an object in the same direction. What is the net force?
0 N
7 N
14 N
49 N

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📋 At a Glance — Part 1 Summary

• Force: Can start, stop, change speed/direction, or change the shape of an object. SI unit = newton (N).
• Balanced forces: Equal in magnitude, opposite in direction → no change in motion (net force = 0).
• Unbalanced forces: Net force ≠ 0 → object’s motion changes.
• Friction: Always opposes relative motion between two surfaces in contact; depends on the nature of the surfaces.
• Newton’s First Law: An object at rest remains at rest, and an object in motion continues with constant velocity, unless acted upon by a net force. Also called the Law of Inertia.

📘 Part 1: Force, Friction & Newton’s First Law
Part 2 → Newton’s 2nd Law, 3rd Law, Systems & Numericals