# Laws of Motion Notes, Class 11 Physics | SwaVid

Laws of Motion, NCERT Class 11 Physics: notes with key ideas, formulas, common mistakes and 3 quick checks with worked answers.

Canonical: https://www.swavid.com/cbse/class-11/physics/chapter/laws-of-motion

Source: https://www.swavid.com/cbse/class-11/physics/chapter/laws-of-motion

# Laws of Motion

## Formulas and definitions

## Key ideas

## Common mistakes

## Quick checks

## Keep going

## Questions parents ask

## Class 11 Physics on SwaVid

### Aristotle's fallacy and Galileo's insight

### Newton's first and second laws

### Impulse and the third law

### Conservation of linear momentum

### Equilibrium and free-body diagrams

### Static and kinetic friction

### Dynamics of circular motion

### Next chapter

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SwaVid › CBSE › Class 11 › Physics › Laws of Motion

Notes written by SwaVid from the NCERT textbook.

Class 11 Physics, Chapter 4. Key ideas, formulas and common mistakes.

CBSE prints 23 marks for Units I-III together, not for this chapter alone.

Short answer. This chapter explains the basic principles that govern the motion of bodies under the influence of forces. It builds on Galileo's ideas of inertia to establish Newton's three laws of motion, momentum conservation, friction, and circular dynamics.

Source: NCERT Physics Part I (Reprint 2026-27) , Chapter 4, Laws of Motion

Aristotle held that an external force is required to keep a body in uniform motion. Galileo showed that an external force is needed only to counter resistive forces like friction, and a body on a frictionless surface moves with constant velocity on its own.

The first law defines inertia: a body remains at rest or in uniform motion unless acted upon by a net external force. The second law defines force quantitatively as the rate of change of momentum, giving F = dp/dt = m a for constant mass.

Impulse is the product of force and time duration, equal to the change in momentum. The third law states that to every action, there is always an equal and opposite reaction acting simultaneously on two different bodies.

In an isolated system where the net external force is zero, the total vector momentum remains constant. This principle governs phenomena such as the recoil of a gun and rocket propulsion.

A particle is in equilibrium when the vector sum of all external forces acting on it is zero. Free-body diagrams isolate the body and show all contact and non-contact forces acting directly on it.

Static friction opposes impending motion and adjusts itself up to a maximum value fs(max) = μs N. Once sliding begins, kinetic friction acts with magnitude fk = μk N, where μk is typically less than μs.

Circular motion requires a centripetal force directed toward the centre. On curved roads, friction provides this force on level ground, while banking reduces dependence on friction and allows safe turning up to an optimum speed.

Try each question first. Then tap it to see the answer.

Initial velocity u = 90 m s⁻¹, final velocity v = 0 m s⁻¹, distance s = 0.60 m

Using v² = u² + 2 a s: 0 = 90² + 2 × a × 0.60, so a = -8100 / 1.2 = -6750 m s⁻²

Resistive force F = m |a| = 0.04 × 6750 = 270 N

270 N

Weight of the mass W = m g = 6 × 10 = 60 N

Let T be tension in upper rope, making angle θ with vertical: T cos θ = W = 60 N

Horizontal equilibrium gives T sin θ = 50 N

Dividing gives tan θ = 50 / 60 = 5 / 6, so θ = tan⁻¹(5/6)

tan⁻¹(5/6) (or approximately 40°)

Speed v = 18 × (5 / 18) = 5 m s⁻¹

Maximum safe speed without slipping v max = √(μs R g) = √(0.1 × 3 × 9.8) = √2.94 ≈ 1.71 m s⁻¹

Since actual speed (5 m s⁻¹) is greater than v max (1.71 m s⁻¹), the cyclist will slip

Yes, the cyclist will slip

Work, Energy and Power .

Motion in a Straight Line .

Motion in a Plane .

The 14 chapters of Class 11 Physics .

CBSE does not give marks to one chapter. It prints 23 marks for Units I-III together (Physical World and Measurement; Kinematics; Laws of Motion). Laws of Motion is part of that group.

No. They are written by SwaVid from the NCERT textbook. The CBSE curriculum is the reference for what is assessed.

Work, Energy and Power.

SwaVid is an AI tutor for Indian students in Classes 6 to 12. It finds the earlier ideas a child missed, then teaches today’s chapter. Start free to see how it works.

- Ch 4 NCERT book
- 23 marks, Units I-III together
- 3 quick checks
- Treating action and reaction as forces that cancel each other on the same body . Action and reaction act on two entirely different bodies simultaneously. Therefore, they never act on the same body and never cancel each other out.
- Assuming static friction is always equal to μs N . The term μs N gives only the maximum limiting value of static friction. For any applied force smaller than this limit, static friction equals the applied force itself.
- Treating centripetal force as an extra real force in a free-body diagram . Centripetal force is not a new physical force. It is the net inward resultant of real forces such as tension, friction, or normal reaction.
- Including internal forces when applying Newton's second law to a system . Only net external forces cause the acceleration of a system. Internal forces occur in action-reaction pairs between parts of the system and sum to zero.

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