Class 11 Physics Laws of Motion Notes

Class 11 Physics Laws of Motion Notes

Force

A force is a push or pull that can:

  • Change the shape of an object
  • Change its speed
  • Change the direction of motion

SI unit of force is newton (N).

🔹 Inertia

Inertia is the property of a body by which it resists any change in its state of rest or motion.

Types of Inertia:

  1. Inertia of rest
  2. Inertia of motion
  3. Inertia of direction

Greater the mass of an object, greater is its inertia.

🔹 Newton’s First Law of Motion

Statement:
A body remains at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force.

This law is also known as the Law of Inertia.

Examples:

  • Passengers fall backward when a bus starts suddenly
  • A book remains at rest on a table until pushed

🔹 Momentum

Momentum is the product of mass and velocity.Momentum=m×v\text{Momentum} = m \times vMomentum=m×v

  • Vector quantity
  • SI unit: kg m/s

🔹 Newton’s Second Law of Motion

Statement:
The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of force.

Mathematically:F=maF = maF=ma

Where:

  • FFF = force
  • mmm = mass
  • aaa = acceleration

This law gives the definition of force.

🔹 Newton’s Third Law of Motion

Statement:
For every action, there is an equal and opposite reaction.

Key Points:

  • Action and reaction act on different bodies
  • They do not cancel each other

Examples:

  • Recoil of a gun
  • Walking on the ground
  • Swimming in water

🔹 Conservation of Momentum

If no external force acts on a system, the total momentum remains constant.

Example:

  • Collision of two balls
  • Explosion of a bomb

🔹 Impulse

Impulse is the product of force and time.Impulse=F×t\text{Impulse} = F \times tImpulse=F×t

Impulse is equal to the change in momentum.

🔹 Friction

Friction is the force that opposes the relative motion between two surfaces in contact.

Types of Friction:

  1. Static friction
  2. Kinetic friction
  3. Rolling friction

Advantages of Friction:

  • Helps in walking
  • Allows writing on paper

Disadvantages of Friction:

  • Causes wear and tear
  • Produces heat

🔹 Circular Motion and Force

For an object moving in a circular path:

  • A force acts towards the center
  • This force is called centripetal force

F=mv2rF = \frac{mv^2}{r}F=rmv2​