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How Forces Affect Motion

NCERT Class 9 · Science Based on NCERT Class 9 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores the fundamental cause of motion—force—and builds on concepts of position, velocity, and acceleration from earlier chapters. It introduces Newton's three laws of motion, explaining how forces affect the motion of objects, including the role of balanced and unbalanced forces, friction, and the relationship between force, mass, and acceleration. The one big idea a student must walk away with is that a net force is required to change an object's state of motion, and that forces always occur in pairs acting on different objects.
Key Concepts (14)
Force
A push or pull that can make an object move from rest, change the speed or direction of a moving object, or change the shape of an object. It is a physical quantity that requires both magnitude and direction to be specified.
e.g. Kicking a football to make it move from rest, or squeezing a lemon to change its shape.
Newton (unit of force)
The SI unit of force, symbol N. One newton is the force that produces an acceleration of 1 m s⁻² on an object of mass 1 kg.
e.g. The upward force your palm applies on a 100 g mass is approximately 1 N.
Balanced forces
Two or more forces acting on an object that are equal in magnitude but opposite in direction, resulting in a net force of zero and no change in the object's state of motion.
e.g. In a tug of war, when both teams pull with equal force, the rope does not move.
Unbalanced forces
Forces acting on an object that are not equal in magnitude or not opposite in direction, resulting in a non-zero net force that changes the object's state of motion.
e.g. In a tug of war, when one team pulls harder, the rope moves in the direction of the larger force.
Net force
The overall force acting on an object when all individual forces are combined, considering both magnitude and direction. The motion of an object depends only on the net force.
e.g. If two people push a stalled car in the same direction with forces of 10 N and 6 N, the net force is 16 N in that direction.
Force of friction
A force that opposes the relative motion between two surfaces in contact, acting in the direction opposite to the direction of motion or attempted motion.
e.g. When pushing a box on a floor, friction acts opposite to the push, making it harder to move the box.
Gravitational force (weight)
The force with which the Earth pulls an object towards itself. It is equal to the product of the object's mass and the acceleration due to gravity (g).
e.g. A 30 kg barbell experiences a gravitational force of 294 N downward.
Normal force
The force exerted by a surface on an object placed on it, acting perpendicular to the surface.
e.g. A book resting on a table experiences an upward normal force from the table balancing its weight.
Inertia
The tendency of an object to resist any change in its state of rest or uniform motion. This concept is central to Newton's first law of motion.
e.g. A stack of coins at rest remains at rest until a net force acts on it.
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.
e.g. A box moving on a frictionless surface will continue moving forever at constant velocity if no net force acts on it.
Newton's second law of motion
When a net force acts on an object, the object accelerates in the direction of the net force. The magnitude of the acceleration is proportional to the magnitude of the net force and inversely proportional to the mass of the object.
e.g. Pushing a ball gently gives it a small acceleration; a strong push gives a larger acceleration.
Newton's third law of motion
Whenever one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first object. These forces act on two different objects.
e.g. When you kick a ball, the ball exerts an equal and opposite force on your foot.
Momentum
The product of an object's mass and its velocity. The direction of momentum is the same as that of velocity. Newton's second law can also be expressed as the rate of change of momentum being proportional to the net force.
e.g. A fast-moving cricket ball has high momentum, making it harder to stop.
Acceleration due to gravity (g)
The acceleration experienced by an object falling freely under the influence of Earth's gravitational force. Its value is approximately 9.8 m s⁻² near the Earth's surface and does not depend on the mass of the object.
e.g. A feather and a hammer dropped in vacuum fall with the same acceleration g.
Formulas & Equations (5)
F = ma
Reads as: Force equals mass times acceleration
Variables: F = net force (newton, N), m = mass (kg), a = acceleration (m s⁻²)
Use when: Calculating the net force acting on an object when its mass and acceleration are known, or finding acceleration from force and mass.
⚠ Students often forget that F in this formula is the net force, not just any applied force. They also sometimes confuse mass with weight.
F = mg
Reads as: Gravitational force equals mass times acceleration due to gravity
Variables: F = gravitational force or weight (N), m = mass (kg), g = acceleration due to gravity (9.8 m s⁻², often taken as 10 m s⁻² for quick estimation)
Use when: Calculating the weight of an object or the gravitational force acting on it near the Earth's surface.
⚠ Students often use g = 9.8 m s⁻² without checking if the problem specifies a different value, or they forget that weight is a force and must be in newtons.
a = F/m
Reads as: Acceleration equals net force divided by mass
Variables: a = acceleration (m s⁻²), F = net force (N), m = mass (kg)
Use when: Finding the acceleration produced when a known net force acts on an object of known mass.
⚠ Students sometimes invert the formula and write a = m/F, leading to incorrect results.
s = ut + (1/2)at²
Reads as: Displacement equals initial velocity times time plus half acceleration times time squared
Variables: s = displacement (m), u = initial velocity (m s⁻¹), t = time (s), a = acceleration (m s⁻²)
Use when: Calculating displacement when initial velocity, acceleration, and time are known, especially in problems involving Newton's second law.
⚠ Students often forget to include the initial velocity term or misplace the factor of 1/2.
v = u + at
Reads as: Final velocity equals initial velocity plus acceleration times time
Variables: v = final velocity (m s⁻¹), u = initial velocity (m s⁻¹), a = acceleration (m s⁻²), t = time (s)
Use when: Finding final velocity after a period of constant acceleration, or finding acceleration from velocity change over time.
⚠ Students sometimes forget that acceleration can be negative (deceleration) and still use the formula correctly.
How It Works
Effect of balanced vs unbalanced forces on motion
  1. Identify all forces acting on the object.
  2. Determine if the forces are equal in magnitude and opposite in direction.
  3. If yes, the forces are balanced: net force is zero, object remains at rest or continues with constant velocity.
  4. If no, the forces are unbalanced: a non-zero net force acts, causing acceleration in the direction of the net force.
  5. For forces in the same direction, net force is the sum of magnitudes; for opposite directions, net force is the difference, acting in the direction of the larger force.
💡 Balanced forces do not cause a change in motion; only unbalanced forces produce acceleration.
Measuring force of friction using a spring balance
  1. Place a spring balance horizontally on a surface and ensure its reading is zero.
  2. Attach a wooden block to the hook of the spring balance.
  3. Pull the spring balance with gradually increasing force.
  4. Note the reading on the spring balance when the block just starts moving.
  5. This reading gives an approximate measure of the force of friction between the block and the surface.
💡 The reading indicates the force needed to overcome static friction; a smaller reading means less friction.
Demonstrating Newton's first law through thought experiment
  1. Imagine an object on a perfectly smooth horizontal surface with zero friction.
  2. Apply a force to set the object in motion from rest.
  3. Once moving, remove the applied force.
  4. Since no net force acts (friction is zero), the object continues moving with constant velocity forever.
  5. This shows that a force is needed only to change velocity, not to maintain constant velocity.
💡 In the absence of net force, an object in motion stays in motion with constant velocity.
Applying Newton's second law to find acceleration from net force
  1. Identify the net force acting on the object by summing all forces vectorially.
  2. Determine the mass of the object.
  3. Use the formula a = F/m to calculate the acceleration.
  4. Ensure the direction of acceleration is the same as the direction of the net force.
  5. Use kinematic equations to find displacement, velocity, or time as needed.
💡 The net force is the vector sum of all forces; do not use individual forces directly in F = ma.
Demonstrating Newton's third law with spring balances
  1. Take two identical spring balances and connect them by their hooks.
  2. Fix the free end of one spring balance to an immovable object or hold it fixed.
  3. Pull the free end of the other spring balance with your hand.
  4. Observe the readings on both spring balances when they are stationary.
  5. Both readings are equal, showing that the forces exerted on each other are equal in magnitude and opposite in direction.
💡 The two forces act on different objects, so they do not cancel each other.
Rocket propulsion using Newton's third law
  1. The rocket engine burns fuel and produces hot gases.
  2. These gases are expelled downward at high speed.
  3. The expelled gases exert an equal and opposite force on the rocket upward.
  4. If this upward force exceeds the weight of the rocket, the net force is upward, causing lift-off.
  5. The rocket continues to accelerate upward as long as the thrust exceeds weight.
💡 The action and reaction forces act on different bodies (gases and rocket), so they do not cancel.
Catching a cricket ball safely using Newton's second law
  1. A fast-moving cricket ball has high velocity and needs to be stopped.
  2. The fielder pulls their hands backward while catching the ball.
  3. This increases the time over which the ball's velocity reduces to zero.
  4. Since acceleration = change in velocity / time, a longer time means smaller acceleration.
  5. From F = ma, smaller acceleration means a smaller force on the fielder's hands, reducing injury.
💡 Increasing the time of impact reduces the force experienced.
Exam Traps — Don't Make These Mistakes
⚠ Confusing net force with applied force in F = ma
✗ A student writes: 'The force applied is 10 N, so using F = ma, acceleration = 10 N / 5 kg = 2 m/s².'
→ The student used the applied force directly without considering other forces like friction. The net force is the vector sum of all forces, not just the applied force.
✓ First calculate net force: applied force minus friction. Then use F_net = ma to find acceleration.
⚠ Thinking action-reaction forces cancel each other
✗ A student writes: 'When a book rests on a table, the gravitational force and the normal force are action-reaction pairs, so they cancel and the book is at rest.'
→ Action-reaction forces act on different objects. The gravitational force on the book (by Earth) and the normal force on the book (by table) act on the same object (the book), so they are not an action-reaction pair. They are balanced forces, not action-reaction.
✓ The action-reaction pair is: Earth pulls book down (action), book pulls Earth up (reaction). The normal force and weight are balanced forces acting on the same object.
⚠ Assuming constant velocity requires a constant force
✗ A student writes: 'To keep a car moving at constant speed, you must keep applying a constant force.'
→ According to Newton's first law, an object in motion continues with constant velocity unless a net force acts. In real life, friction opposes motion, so a force is needed to balance friction, but if net force is zero, constant velocity is maintained without any net force.
✓ To maintain constant velocity, the net force must be zero. The applied force must equal the opposing friction force, so that they balance.
⚠ Confusing mass and weight in calculations
✗ A student writes: 'The weight of a 5 kg object is 5 N.'
→ Weight is the gravitational force, calculated as F = mg. For a 5 kg mass, weight = 5 kg × 9.8 m/s² = 49 N, not 5 N. Mass is a scalar quantity in kg, weight is a force in newtons.
✓ Weight = mass × acceleration due to gravity. For a 5 kg object, weight = 5 × 9.8 = 49 N.
⚠ Forgetting that acceleration due to gravity (g) is independent of mass
✗ A student writes: 'A heavier object falls faster because it has more mass, so g is larger for heavier objects.'
→ The acceleration due to gravity (g) is constant near Earth's surface (≈9.8 m/s²) and does not depend on the mass of the falling object. All objects fall with the same acceleration in the absence of air resistance.
✓ The value of g is approximately 9.8 m/s² for all objects near Earth's surface, regardless of their mass.
Compare & Contrast
Effect on motion
Balanced forces
Unbalanced forces
Balanced forces produce zero net force, so no change in motion (object remains at rest or moves with constant velocity). Unbalanced forces produce a non-zero net force, causing acceleration (change in speed, direction, or both).
Nature and units
Mass
Weight
Mass is a scalar quantity measuring the amount of matter in an object, measured in kilograms (kg). Weight is a vector force due to gravity, calculated as mass × g, measured in newtons (N). Mass is constant everywhere; weight varies with gravitational field strength.
What they describe
Newton's first law of motion
Newton's second law of motion
Newton's first law describes the behaviour of an object when no net force acts (zero acceleration). Newton's second law describes what happens when a net force acts (acceleration proportional to force and inversely proportional to mass).
Objects on which forces act
Action-reaction forces (Newton's third law)
Balanced forces
Action-reaction forces act on two different objects (e.g., Earth pulls book, book pulls Earth). Balanced forces act on the same object (e.g., weight and normal force on a book). Action-reaction forces never cancel because they act on different bodies.
Exam Focus
Marks pattern: Typically 2-3 MCQs (1 mark each), 1-2 short answer questions (2-3 marks each), and 1 long answer numerical or conceptual question (5 marks). Total weightage around 8-12 marks in the board exam.
Most asked: Newton's three laws of motion with examples · Balanced and unbalanced forces and their effect on motion · Numerical problems using F = ma and kinematic equations · Force of friction and its role in motion · Action-reaction pairs in everyday situations
Don't skip: Numerical problems combining Newton's second law with kinematic equations (e.g., finding force from velocity-time graph); Conceptual questions on Newton's third law with real-life examples (e.g., rocket propulsion, walking, canoe rowing); Differentiating between balanced and unbalanced forces with diagrams
One-Line Revision (read aloud before the exam)
  1. Force is a push or pull that can change an object's state of motion or shape, and it has both magnitude and direction.
  2. The SI unit of force is the newton (N); 1 N = 1 kg m s⁻².
  3. Balanced forces produce zero net force and no change in motion; unbalanced forces produce a net force and cause acceleration.
  4. Friction is a force that opposes relative motion between surfaces in contact, acting opposite to the direction of motion.
  5. Newton's first law: An object at rest stays at rest, and an object in motion stays in motion with constant velocity, unless acted upon by a net force.
  6. Newton's second law: F = ma; acceleration is directly proportional to net force and inversely proportional to mass.
  7. Newton's third law: For every action, there is an equal and opposite reaction; the two forces act on different objects.
  8. Weight is the gravitational force on an object, calculated as W = mg, where g = 9.8 m s⁻².
  9. The acceleration due to gravity (g) is the same for all objects near Earth's surface, regardless of mass.
  10. In the absence of net force, an object in motion continues with constant velocity (Galileo's insight).
  11. Action-reaction forces do not cancel because they act on different bodies.
  12. Increasing the time of impact reduces the force experienced (e.g., pulling hands back while catching a ball).
  13. Rockets work by expelling gases downward; the gases exert an equal upward force on the rocket.
  14. Friction can be both helpful (walking, driving) and opposing (slowing down moving objects).
  15. The normal force is the perpendicular force exerted by a surface on an object placed on it.

MCQs — 10 Questions with Answers

Q1. Which of the following is the SI unit of force?

  • A. Newton (N) ✓
  • B. Kilogram (kg)
  • C. Joule (J)
  • D. Meter per second (m/s)

Answer: A — Newton (N) is the standard SI unit for measuring force; kilogram measures mass, joule measures energy, and m/s measures velocity.

Q2. In a tug of war, if Team X pulls with 80 N to the right and Team Y pulls with 80 N to the left, what is the net force on the rope?

  • A. 160 N to the right
  • B. 160 N to the left
  • C. 0 N ✓
  • D. 80 N to the right

Answer: C — Equal magnitudes in opposite directions create balanced forces with net force = 0, so the rope remains stationary.

Q3. Which statement best describes balanced forces?

  • A. Two forces of the same magnitude acting in the same direction
  • B. Two forces of equal magnitude acting in opposite directions ✓
  • C. A single force acting on an object at rest
  • D. Two forces that always prevent any motion

Answer: B — Balanced forces are specifically equal in magnitude and opposite in direction; they cancel out to produce zero net force.

Q4. A box experiences an applied force of 50 N to the right and friction force of 30 N to the left. What is the net force and its direction?

  • A. 80 N to the right
  • B. 20 N to the right ✓
  • C. 30 N to the left
  • D. 50 N to the right

Answer: B — Forces in opposite directions: net force = 50 N − 30 N = 20 N in the direction of the larger force (right).

Q5. When you push a heavy box on the floor and it doesn't move initially, which of the following is NOT correct?

  • A. Friction force is acting on the box opposite to your push
  • B. Your applied force equals the friction force
  • C. The net force on the box is zero
  • D. Your applied force is greater than the maximum static friction ✓

Answer: D — If the box doesn't move, the applied force must be less than or equal to static friction, not greater; the forces are balanced or friction dominates.

Q6. Two people push a stalled car in the same direction with forces of 300 N and 200 N. What is the magnitude and direction of the net force?

  • A. 100 N in the direction of the larger force
  • B. 500 N in the direction of both forces ✓
  • C. 250 N in the average direction
  • D. 300 N only from the stronger person

Answer: B — When forces act in the same direction, net force = sum of magnitudes = 300 N + 200 N = 500 N in that direction.

Q7. A ball floats stationary on water. Which of the following best explains this situation?

  • A. Only gravitational force acts downward on the ball
  • B. Only buoyant force acts upward on the ball
  • C. Gravitational force (downward) and buoyant force (upward) are balanced ✓
  • D. The ball experiences a net force pushing it downward

Answer: C — A floating object is in equilibrium; the downward gravitational force and upward buoyant force are equal and opposite, creating balanced forces.

Q8. Ramesh observes that when he applies a 10 N force on a toy car, it doesn't move, but when he applies 20 N, it starts moving. Which concept best explains this observation?

  • A. Gravitational force is stronger than applied force
  • B. Static friction increases with applied force
  • C. The 10 N applied force is balanced by static friction, but 20 N overcomes static friction ✓
  • D. The toy car has no mass when the force is 10 N

Answer: C — Static friction acts to resist motion; when applied force equals or is less than maximum static friction, the car stays still; 20 N exceeds this friction limit.

Q9. A force can change an object's motion in which of the following ways?

  • A. Change its speed only
  • B. Change its direction only
  • C. Change its shape only
  • D. Change its speed, direction, or shape ✓

Answer: D — Force can produce all three effects: change speed (ball kicked harder goes faster), change direction (bat deflects ball), and change shape (squeezing lemon deforms it).

Q10. Which of the following scenarios involves unbalanced forces resulting in motion?

  • A. A book resting on a table with weight and normal force balanced
  • B. A canoeist pushing water backward with increasing force, causing faster forward motion ✓
  • C. Two equal teams in tug of war pulling a rope with equal strength
  • D. A ball floating motionless on water with gravity and buoyancy balanced

Answer: B — Increased paddle force creates unbalanced forces (net force forward), accelerating the canoe; other options show balanced forces or no net force.

Flashcards

What is a force in simple terms?

A push or pull on an object that can change its motion, speed, direction, or shape.

What is the SI unit of force and its symbol?

The SI unit is newton, with symbol N.

Define balanced forces with an example.

Balanced forces are two forces equal in magnitude but opposite in direction, like both teams pulling a tug-of-war rope with equal strength — the rope doesn't move.

What happens when forces acting on an object are unbalanced?

An unbalanced or net force acts on the object, causing it to accelerate in the direction of the larger force.

In a tug of war, Team A pulls with 50 N right, Team B pulls with 30 N left. What is the net force?

Net force is 20 N towards the right (50 N − 30 N = 20 N in the direction of the larger force).

How do you find net force when two forces act in the same direction?

Add the magnitudes of both forces; the net force direction is the same as both individual forces.

Why doesn't a box move immediately when you push it with small force?

The force of friction acting opposite to your push is greater than or equal to your applied force, resulting in balanced forces.

A ball floats on water. What two forces act on it and in which directions?

Gravitational force acts downward (Earth pulling the ball) and buoyant force acts upward (water pushing the ball).

What does 'net force' mean in physics?

Net force is the single resultant force that has the same effect as all individual forces combined acting on an object.

How does force direction affect the outcome when applied to an object?

Changing the direction of force changes the effect; only forces with proper direction specification give correct results of motion.

Important Board Questions

Define a balanced force with an example. How is it different from an unbalanced force? [2 marks]

State that balanced forces are equal in magnitude, opposite in direction, and produce zero net force (example: tug of war with equal teams). Unbalanced forces are unequal and produce non-zero net force causing motion.

Two forces of 15 N and 10 N act on a block. Calculate the net force when: (a) both forces act in the same direction, (b) forces act in opposite directions. Explain how the net force differs in both cases. [3 marks]

Case (a): Add magnitudes = 15 + 10 = 25 N in the same direction. Case (b): Subtract magnitudes = 15 − 10 = 5 N in the direction of larger force. Explain that same direction results in larger net force, opposite direction results in smaller net force.

A box lying on the floor doesn't move when a 30 N force is applied, but moves when a 50 N force is applied. Explain this using the concept of balanced and unbalanced forces. What role does friction play? How would the motion change if the floor were frictionless? [5 marks]

At 30 N: applied force = static friction, balanced forces, net force = 0, box remains stationary. At 50 N: applied force > friction, unbalanced forces, net force ≠ 0, box accelerates. Friction opposes motion and increases with applied force up to a limit. On frictionless surface, any force > 0 would cause motion immediately since friction = 0 and net force would always be unbalanced.

Assertion-Reason Questions for How Forces Affect Motion

Q1. Assertion (A):

Force is a vector quantity that affects the motion of objects.

Reason (R):

Force requires both magnitude and direction to be completely described, similar to position, displacement, velocity, and acceleration.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why force is a vector quantity. Force, like other vector quantities, needs magnitude and direction for complete specification.

Q2. Assertion (A):

When balanced forces act on an object, the object accelerates in the direction of the larger force.

Reason (R):

Balanced forces are equal in magnitude but opposite in direction, resulting in a net force of zero.

Answer: D — A is false but R is true — The assertion is false because balanced forces produce zero net force, causing no acceleration. The reason correctly defines balanced forces, but since the assertion is false, the overall answer is D (both false). The assertion should describe unbalanced forces.

Q3. Assertion (A):

A spring balance can measure force because the spring inside stretches proportionally to the applied force.

Reason (R):

All measuring instruments operate on the principle of proportional response to the quantity being measured.

Answer: C — A is true but R is false — The assertion is true—spring balances work via proportional stretching. However, the reason is too broad and not specific to why spring balances measure force. Not all measuring instruments rely on proportional response in the same way.

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Case-Based Questions (CBQ) for How Forces Affect Motion

Case Study:

A physics teacher conducted an experiment with a wooden block on a wooden table. Initially, when the teacher applied 5 N force horizontally, the block did not move. When 10 N force was applied, it still didn't move. At 12 N force, the block just began to slide. Once sliding started, the teacher found that only 9 N force was needed to keep the block moving at constant velocity. The teacher measured the mass of the block as 2 kg.

Q1. What is the maximum static friction force between the block and the table?

  • A. 5 N
  • B. 10 N
  • C. 12 N ✓
  • D. 9 N

Answer: C

Q2. When 12 N force is applied and the block just begins to move, what is the net force acting on the block?

  • A. 0 N (approximately zero at the threshold) ✓
  • B. 3 N
  • C. 12 N
  • D. 2.4 N

Answer: A

Q3. What is the kinetic friction force when the block is sliding at constant velocity with 9 N applied force?

  • A. 0 N
  • B. 9 N ✓
  • C. 12 N
  • D. 6 N

Answer: B

Q4. Which statement correctly compares static and kinetic friction in this experiment?

  • A. Static friction is greater than kinetic friction ✓
  • B. Kinetic friction is greater than static friction
  • C. Both are equal at 12 N
  • D. Static friction is zero when the block is not moving

Answer: A

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