New NCERT Exploration · Class 9 Science · Chapter 6: How Forces Affect Motion
This chapter explains what a force is and how it changes the state of rest or motion of an object. It covers balanced and unbalanced forces, net force, friction and Galileo's experiment with inclined planes. It then explains inertia and Newton's three laws of motion with everyday applications.
From the Class 9 Science Short Notes, made as per the new NCERT book Exploration · Pages 39–45 of the book.
Key points
- A force is a push or pull that can change an object's state of rest or motion, direction, shape or size; its SI unit is newton (N).
- Force is a vector quantity and can be measured with a spring balance.
- Balanced forces are equal and opposite, so the resultant force is zero; they do not change rest or motion but may change shape.
- Unbalanced forces give a non-zero resultant force and can start, stop, speed up or turn a moving object.
- Forces in the same direction are added; forces in opposite directions are subtracted, and the net force acts in the direction of the larger force.
- Friction opposes motion between two surfaces in contact; rough surfaces and greater weight produce more friction.
- Galileo showed that objects stop mainly because of friction; without friction a moving object would keep moving with constant speed.
- Newton's first law (law of inertia): an object stays at rest or in uniform motion unless an unbalanced external force acts on it.
- Inertia is of three types: inertia of rest, inertia of motion and inertia of direction.
- Newton's second law: rate of change of momentum is proportional to net external force; for constant mass F = m × a.
- Newton's third law: for every action there is an equal and opposite reaction; these forces act on different objects and do not cancel.
- In a system of objects, internal forces cancel each other and only the net external force changes the motion.
Formulas
| Balanced forces | F(net) = 0 |
| Net force, same direction | F(net) = F₁ + F₂ |
| Net force, opposite direction | F(net) = F₁ − F₂ |
| Newton's second law | F = m × a For a body of constant mass |
| One newton | 1 N = 1 kg × 1 m/s² |
| Total mass of a system | M = m₁ + m₂ |
| Net force on a system | F(net) = (m₁ + m₂) × a |
Key terms
- Force: A push or pull that can change an object's state of rest or motion, direction, shape or size.
- Balanced forces: Forces equal in size and opposite in direction, giving zero resultant force.
- Unbalanced forces: Forces that are not equal, so the resultant force is not zero.
- Net force: The overall force on an object when all forces acting on it are combined.
- Friction: The force that opposes motion or the tendency of motion between two surfaces in contact.
- Inertia: The property of an object by which it resists any change in its state of rest or motion.
- One newton: The force needed to give a 1 kg mass an acceleration of 1 m/s².
Diagrams to learn
- Examples of force: pushing a door, pulling a cart, kicking a football, stretching a rubber band
- Spring balance
- Tug of war showing balanced and unbalanced forces
- Friction on a pushed box
- Passengers in a bus that suddenly starts (inertia)
- Airbag in a car (application of the second law)
- Walking and swimming as action and reaction
- Action and reaction forces act on different objects, so they never cancel each other.
- When the net force is zero, a stationary object stays at rest, a moving object keeps the same speed in a straight line, and there is no acceleration.
- Acceleration is always in the direction of the applied force.
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This page is a summary. The Class 9 Science Short Notes has the complete chapter in colour, with every diagram and worked example.
Frequently asked questions
What is Newton's first law of motion?
An object remains at rest or continues to move with uniform velocity in a straight line unless an unbalanced external force acts on it. It is also called the law of inertia.
Why don't action and reaction forces cancel each other?
Action and reaction forces are equal and opposite, but they act on two different objects. So they cannot cancel each other.
What are the types of inertia?
Inertia of rest (a book stays on a table until pushed), inertia of motion (passengers move forward when a bus stops suddenly) and inertia of direction (passengers are pushed sideways when a car takes a sharp turn).
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