- A force is an interaction that can change motion or shape and is measured in newtons.
- Friction and normal force require contact; gravitational and magnetic forces can act at a distance.
- Balanced forces sum to zero; unbalanced forces cause acceleration.
- Without net external force, an object remains at rest or moves at constant velocity.
- Inertia is resistance to changes in velocity; mass measures inertia.
- For constant mass, net force F=ma, with acceleration in the direction of net force.
- A free-body diagram represents all external forces on a chosen object with directions.
- Interaction forces between two bodies are equal in magnitude and opposite in direction, acting on different bodies.
- Linear momentum p=mv is a vector, measured in kg·m/s.
- Impulse J=FΔt for constant force and equals change in momentum.
- Total momentum remains constant in an isolated system with negligible net external impulse.
- Friction opposes relative motion or its tendency at contact surfaces.
Use these notes alongside your current textbook. Check important formulas, worked answers and syllabus coverage with your teacher or official learning material.
How Forces Affect Motion — Class 9 Science, Exploration (2026–27)
Chapter 6. This original study guide explains core concepts with examples and worked practice. It is not a page-by-page verified transcription of NCERT activities, diagrams or numbered exercises. Refer to the prescribed textbook for exact official question wording.
Learning outcomes
Explain the concepts accurately, solve numerical or reasoning problems, choose suitable methods, and verify answers with correct units and assumptions.
Force and interactions
A force is an interaction that can change motion or shape and is measured in newtons.
Example and interpretation
Pushing a trolley changes its motion if the net force is nonzero.
Contact and noncontact forces
Friction and normal force require contact; gravitational and magnetic forces can act at a distance.
Example and interpretation
A magnet attracts a paper clip without direct contact.
Balanced and unbalanced forces
Balanced forces sum to zero; unbalanced forces cause acceleration.
Example and interpretation
A book resting on a table has weight balanced by normal force.
Newton's first law
Without net external force, an object remains at rest or moves at constant velocity.
Example and interpretation
A puck continues moving nearly uniformly on low-friction ice.
Inertia
Inertia is resistance to changes in velocity; mass measures inertia.
Example and interpretation
A loaded trolley is harder to accelerate than an empty trolley under the same force.
Newton's second law
For constant mass, net force F=ma, with acceleration in the direction of net force.
Example and interpretation
A net 12 N on 3 kg produces 4 m/s².
Force diagrams
A free-body diagram represents all external forces on a chosen object with directions.
Example and interpretation
A hanging lamp has downward weight and upward tension.
Newton's third law
Interaction forces between two bodies are equal in magnitude and opposite in direction, acting on different bodies.
Example and interpretation
The foot pushes the ground backward; ground pushes foot forward.
Momentum
Linear momentum p=mv is a vector, measured in kg·m/s.
Example and interpretation
A 2 kg object moving 3 m/s east has momentum 6 kg·m/s east.
Impulse
Impulse J=FΔt for constant force and equals change in momentum.
Example and interpretation
A 10 N force applied for 0.2 s gives impulse 2 N·s.
Conservation of momentum
Total momentum remains constant in an isolated system with negligible net external impulse.
Example and interpretation
Two skaters push apart and gain equal and opposite momenta if initially at rest.
Friction
Friction opposes relative motion or its tendency at contact surfaces.
Example and interpretation
Friction helps shoes grip the ground.
Mass versus weight
Mass measures inertia in kg; weight is gravitational force W=mg in N.
Example and interpretation
For 5 kg at g≈9.8 m/s², weight≈49 N.
Circular motion
Changing direction requires acceleration even at constant speed; net force points inward for uniform circular motion.
Example and interpretation
A stone on a string needs inward tension.
Acceleration and direction
Acceleration need not point along velocity; negative acceleration relative to an axis does not always imply slowing.
Example and interpretation
An object moving left and accelerating left increases speed.
Multiple forces
Resolve forces by direction and calculate vector net force before applying F=ma.
Example and interpretation
10 N right and 6 N left give net 4 N right.
Common errors
Action and reaction forces do not cancel on one body because they act on different bodies.
Example and interpretation
The book's weight and table normal force are not a Newton-third-law pair.
Safe demonstrations
Use low masses, stable surfaces and careful measurement when testing acceleration.
Example and interpretation
Compare cart acceleration under different applied forces.
Chapter summary table
| Topic | Essential explanation | Example |
|---|---|---|
| Force and interactions | A force is an interaction that can change motion or shape and is measured in newtons. | Pushing a trolley changes its motion if the net force is nonzero. |
| Contact and noncontact forces | Friction and normal force require contact; gravitational and magnetic forces can act at a distance. | A magnet attracts a paper clip without direct contact. |
| Balanced and unbalanced forces | Balanced forces sum to zero; unbalanced forces cause acceleration. | A book resting on a table has weight balanced by normal force. |
| Newton's first law | Without net external force, an object remains at rest or moves at constant velocity. | A puck continues moving nearly uniformly on low-friction ice. |
| Inertia | Inertia is resistance to changes in velocity; mass measures inertia. | A loaded trolley is harder to accelerate than an empty trolley under the same force. |
| Newton's second law | For constant mass, net force F=ma, with acceleration in the direction of net force. | A net 12 N on 3 kg produces 4 m/s². |
| Force diagrams | A free-body diagram represents all external forces on a chosen object with directions. | A hanging lamp has downward weight and upward tension. |
| Newton's third law | Interaction forces between two bodies are equal in magnitude and opposite in direction, acting on different bodies. | The foot pushes the ground backward; ground pushes foot forward. |
| Momentum | Linear momentum p=mv is a vector, measured in kg·m/s. | A 2 kg object moving 3 m/s east has momentum 6 kg·m/s east. |
| Impulse | Impulse J=FΔt for constant force and equals change in momentum. | A 10 N force applied for 0.2 s gives impulse 2 N·s. |
| Conservation of momentum | Total momentum remains constant in an isolated system with negligible net external impulse. | Two skaters push apart and gain equal and opposite momenta if initially at rest. |
| Friction | Friction opposes relative motion or its tendency at contact surfaces. | Friction helps shoes grip the ground. |
| Mass versus weight | Mass measures inertia in kg; weight is gravitational force W=mg in N. | For 5 kg at g≈9.8 m/s², weight≈49 N. |
| Circular motion | Changing direction requires acceleration even at constant speed; net force points inward for uniform circular motion. | A stone on a string needs inward tension. |
| Acceleration and direction | Acceleration need not point along velocity; negative acceleration relative to an axis does not always imply slowing. | An object moving left and accelerating left increases speed. |
| Multiple forces | Resolve forces by direction and calculate vector net force before applying F=ma. | 10 N right and 6 N left give net 4 N right. |
| Common errors | Action and reaction forces do not cancel on one body because they act on different bodies. | The book's weight and table normal force are not a Newton-third-law pair. |
| Safe demonstrations | Use low masses, stable surfaces and careful measurement when testing acceleration. | Compare cart acceleration under different applied forces. |
Original practice exercise 1 — Solved applications
- Question: A 4 kg cart experiences a net 20 N force. Find acceleration.
Solution: a=F/m=20/4=5 m/s² in the force direction.
- Question: A 3 kg object accelerates at 2 m/s². Find net force.
Solution: F=ma=3×2=6 N.
- Question: Two horizontal forces 18 N east and 7 N west act on a body. Find resultant.
Solution: 18−7=11 N east.
- Question: A 2 kg ball moves at 6 m/s. Find momentum.
Solution: p=mv=2×6=12 kg·m/s in the direction of motion.
- Question: A constant force of 15 N acts for 0.4 s. Find impulse.
Solution: J=FΔt=15×0.4=6 N·s.
- Question: A 6 kg mass rests on a table. Estimate weight for g=10 m/s².
Solution: W=mg=6×10=60 N downward; table normal is 60 N upward if no other vertical forces.
- Question: Why do passengers lean forward when a bus stops suddenly?
Solution: Their bodies tend to continue moving forward because of inertia while the bus decelerates.
- Question: Are weight and normal force a Newton's third-law pair?
Solution: No. Both act on the book; a third-law pair acts on different objects.
- Question: Why can an object move at constant speed while accelerating?
Solution: In circular motion its velocity direction changes, producing acceleration.
- Question: Two skaters initially at rest push apart. What happens to total momentum?
Solution: If external horizontal forces are negligible, total momentum remains zero: their momenta are equal and opposite.
Original practice exercise 2 — Concept questions with answers
- Question: Explain force and interactions.
Answer: A force is an interaction that can change motion or shape and is measured in newtons.
- Question: Explain contact and noncontact forces.
Answer: Friction and normal force require contact; gravitational and magnetic forces can act at a distance.
- Question: Explain balanced and unbalanced forces.
Answer: Balanced forces sum to zero; unbalanced forces cause acceleration.
- Question: Explain newton's first law.
Answer: Without net external force, an object remains at rest or moves at constant velocity.
- Question: Explain inertia.
Answer: Inertia is resistance to changes in velocity; mass measures inertia.
- Question: Explain newton's second law.
Answer: For constant mass, net force F=ma, with acceleration in the direction of net force.
- Question: Explain force diagrams.
Answer: A free-body diagram represents all external forces on a chosen object with directions.
- Question: Explain newton's third law.
Answer: Interaction forces between two bodies are equal in magnitude and opposite in direction, acting on different bodies.
- Question: Explain momentum.
Answer: Linear momentum p=mv is a vector, measured in kg·m/s.
- Question: Explain impulse.
Answer: Impulse J=FΔt for constant force and equals change in momentum.
- Question: Explain conservation of momentum.
Answer: Total momentum remains constant in an isolated system with negligible net external impulse.
- Question: Explain friction.
Answer: Friction opposes relative motion or its tendency at contact surfaces.
Original practice exercise 3 — Higher-order reasoning
- Identify an everyday situation that demonstrates two different chapter concepts. Explain how they apply.
- Compare two methods, models or quantities introduced in the chapter and identify their limitations.
- Design a safe observation or calculation that tests one claim, listing variables and assumptions.
- Construct a multistep numerical or classification problem and verify its answer.
- Identify a common misconception and correct it using the chapter principles.
Frequently asked questions
A 4 kg cart experiences a net 20 N force. Find acceleration.
a=F/m=20/4=5 m/s² in the force direction.
A 3 kg object accelerates at 2 m/s². Find net force.
F=ma=3×2=6 N.
Two horizontal forces 18 N east and 7 N west act on a body. Find resultant.
18−7=11 N east.
A 2 kg ball moves at 6 m/s. Find momentum.
p=mv=2×6=12 kg·m/s in the direction of motion.
A constant force of 15 N acts for 0.4 s. Find impulse.
J=FΔt=15×0.4=6 N·s.
A 6 kg mass rests on a table. Estimate weight for g=10 m/s².
W=mg=6×10=60 N downward; table normal is 60 N upward if no other vertical forces.
Why do passengers lean forward when a bus stops suddenly?
Their bodies tend to continue moving forward because of inertia while the bus decelerates.
Are weight and normal force a Newton's third-law pair?
No. Both act on the book; a third-law pair acts on different objects.
Why can an object move at constant speed while accelerating?
In circular motion its velocity direction changes, producing acceleration.
Two skaters initially at rest push apart. What happens to total momentum?
If external horizontal forces are negligible, total momentum remains zero: their momenta are equal and opposite.
Explain force and interactions.
A force is an interaction that can change motion or shape and is measured in newtons.
Explain contact and noncontact forces.
Friction and normal force require contact; gravitational and magnetic forces can act at a distance.
Explain balanced and unbalanced forces.
Balanced forces sum to zero; unbalanced forces cause acceleration.
Explain newton's first law.
Without net external force, an object remains at rest or moves at constant velocity.
Explain inertia.
Inertia is resistance to changes in velocity; mass measures inertia.
Revision checklist
- Explain and apply force and interactions.
- Explain and apply contact and noncontact forces.
- Explain and apply balanced and unbalanced forces.
- Explain and apply newton's first law.
- Explain and apply inertia.
- Explain and apply newton's second law.
- Explain and apply force diagrams.
- Explain and apply newton's third law.
- Explain and apply momentum.
- Explain and apply impulse.
- Explain and apply conservation of momentum.
- Explain and apply friction.
- Explain and apply mass versus weight.
- Explain and apply circular motion.
- Explain and apply acceleration and direction.
- Explain and apply multiple forces.
- Explain and apply common errors.
- Explain and apply safe demonstrations.
Official textbook
NCERT ePathshala: Exploration. The practice exercises here are original and are not claimed to be the complete official NCERT exercise answer key.
Frequently asked questions
A 4 kg cart experiences a net 20 N force. Find acceleration.
a=F/m=20/4=5 m/s² in the force direction.
A 3 kg object accelerates at 2 m/s². Find net force.
F=ma=3×2=6 N.
Two horizontal forces 18 N east and 7 N west act on a body. Find resultant.
18−7=11 N east.
A 2 kg ball moves at 6 m/s. Find momentum.
p=mv=2×6=12 kg·m/s in the direction of motion.
A constant force of 15 N acts for 0.4 s. Find impulse.
J=FΔt=15×0.4=6 N·s.
A 6 kg mass rests on a table. Estimate weight for g=10 m/s².
W=mg=6×10=60 N downward; table normal is 60 N upward if no other vertical forces.
Explain force and interactions.
A force is an interaction that can change motion or shape and is measured in newtons.
Explain contact and noncontact forces.
Friction and normal force require contact; gravitational and magnetic forces can act at a distance.
Explain balanced and unbalanced forces.
Balanced forces sum to zero; unbalanced forces cause acceleration.
Explain newton's first law.
Without net external force, an object remains at rest or moves at constant velocity.
Explain inertia.
Inertia is resistance to changes in velocity; mass measures inertia.
