JEE Physics Notes: Laws of Motion

Introduction to Laws of Motion

The laws of motion describe the relationship between the motion of an object and the forces acting upon it. Newton formulated three fundamental laws that govern classical mechanics.

Newton’s First Law of Motion (Law of Inertia)

Newton’s First Law states that an object at rest remains at rest, and an object in motion continues to move with uniform velocity unless acted upon by an external force.

Concept of Inertia

• Inertia of Rest: A body at rest remains at rest unless a force is applied.
• Inertia of Motion: A body in motion remains in motion with the same speed unless a force acts upon it.
• Inertia of Direction: A body continues to move in the same direction unless a force changes its path.

Examples of Inertia

• Passengers experience a jerk when a bus starts suddenly (Inertia of Rest).
• A moving bus stops suddenly, and passengers lean forward (Inertia of Motion).
• A car takes a turn, and passengers experience a side push (Inertia of Direction).

Newton’s Second Law of Motion

Newton’s Second Law states that the rate of change of momentum of a body is directly proportional to the applied force and occurs in the direction of the force.

F = ma

where:

• F = Force
• m = Mass of the object
• a = Acceleration

Momentum

Momentum is the product of mass and velocity, given by:

p = mv

where:

• p = Momentum
• m = Mass
• v = Velocity

Impulse

Impulse is the change in momentum due to an applied force over a short time interval.

Impulse = Force × Time = Δp

Newton’s Third Law of Motion

Newton’s Third Law states that for every action, there is an equal and opposite reaction.

Examples of Newton’s Third Law

• A rocket moves upward due to the downward thrust of expelled gases.
• A swimmer pushes water backward to move forward.
• A gun recoils when a bullet is fired.

Applications of Newton’s Laws

• Vehicle seat belts and airbags reduce injuries by minimizing the effects of inertia.
• Athletes use starting blocks to exert force and gain momentum.
• Walking and running depend on action-reaction forces.

Friction

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

Types of Friction

• Static Friction: Prevents motion between surfaces at rest.
• Kinetic Friction: Acts on moving objects.
• Rolling Friction: Acts when an object rolls over a surface.

Frictional Force Equation

Ff = μN

where:

• F_f = Frictional force
• μ = Coefficient of friction
• N = Normal reaction force

Free Body Diagrams (FBD)

A Free Body Diagram is a visual representation showing all forces acting on an object.

Steps to Draw FBD

• Identify the object of interest.
• Mark all external forces, including normal force, friction, tension, and applied forces.
• Choose a coordinate system to resolve forces.

Applications of Laws of Motion

• Designing transportation safety systems.
• Understanding planetary motion.
• Predicting motion in engineering and physics problems.

The Laws of Motion are fundamental in solving JEE Physics problems, especially in mechanics, dynamics, and force-related applications.