Chapter 4 - Laws of Motion

Kinematics tells us how an object moves. Laws of Motion Class 11 explains why its motion changes.

The chapter introduces force, inertia, momentum and Newton’s three laws of motion. These principles are then applied to connected bodies, friction, equilibrium and circular motion.

Newton’s first law and inertia

Newton’s first law states that an object remains at rest or continues moving with constant velocity unless acted upon by an unbalanced external force.

This law introduces inertia—the tendency of an object to resist a change in its state of motion. Mass is a measure of inertia.

Students use the first law to understand why passengers move forward when a bus stops suddenly and why seat belts are necessary.

Newton’s second law

Newton’s second law relates net external force to the rate of change of momentum.

For constant mass, it leads to the familiar relationship between force, mass and acceleration.

The acceleration of an object is determined by the net force, not by any single force acting on it. Students must therefore identify and add all forces vectorially.

Momentum and impulse

Linear momentum depends on mass and velocity.

Impulse measures the effect of a force acting over a time interval and equals the change in momentum. This explains why increasing the stopping time can reduce the average force during a collision.

Examples include airbags, padded surfaces and following through while catching a ball.

Newton’s third law

For every force exerted by one object on another, the second object exerts an equal and opposite force on the first.

The two forces:

• Are equal in magnitude

• Act in opposite directions

• Occur simultaneously

• Act on different objects

Because they act on different bodies, they do not cancel while analysing the motion of one body.

Conservation of linear momentum

When the net external force on a system is zero, its total linear momentum remains constant.

Students apply this principle to collisions, recoil and interactions between objects.

Free-body diagrams and equilibrium

A free-body diagram shows all external forces acting on one selected object.

Common forces include:

• Weight

• Normal reaction

• Tension

• Friction

• Applied force

• Spring force

For equilibrium, the vector sum of all forces must be zero.

Friction

The chapter distinguishes between static, limiting, kinetic and rolling friction.

Static friction is self-adjusting. It matches the applied force up to a maximum limiting value. Therefore, static friction is not always equal to its limiting value.

Students study the laws of friction, coefficient of friction and methods of reducing friction through lubrication and rolling.

Circular motion

An object moving in a circle requires a net inward force called centripetal force.

Centripetal force is not a separate new type of force. It may be provided by friction, tension, gravity, normal reaction or a combination of forces.

Students apply this idea to vehicles moving on level and banked circular roads.

Common student mistakes

Students often:

• Write equations without drawing a free-body diagram

• Include forces acting on another body

• Treat action and reaction as forces on the same body

• Assume friction always equals (\mu N)

• Add forces without considering direction

• Treat centripetal force as an additional force

• Confuse mass with weight

Teachoo’s solutions begin by selecting one body and identifying all forces acting on it.

How should you study Laws of Motion?

For every question:

1. Select the body or system

2. Draw its free-body diagram

3. Choose coordinate axes

4. Resolve inclined forces

5. Apply Newton’s second law along each axis

6. Check limiting conditions for friction

7. Interpret the signs in the final answer

Frequently Asked Questions

What is the main idea of Laws of Motion?

The chapter explains how forces affect motion using Newton’s three laws, momentum, impulse and conservation of momentum.

Do action and reaction forces cancel each other?

Not when analysing one object, because the forces act on different objects. They may cancel only when considering the two objects together as one system.

Is friction always equal to (\mu N)?

No. Static friction adjusts according to the applied force up to its limiting value. The expression (\mu N) gives the maximum static friction or the kinetic friction under the relevant model.

Is centripetal force a separate force?

No. It is the name given to the net inward force required for circular motion.

Why is this chapter important?

It provides the foundation for almost all mechanics problems, including work and energy, rotational motion, gravitation and oscillations.