Complete Physics Notes on Laws of Motion, Force, Work, Energy and Power  for BPSC and Other Competitive Exams in 2025

Complete Physics Notes on Laws of Motion, Force, Work, Energy and Power  for BPSC and Other Competitive Exams in 2025

General Science-1 (Physics)

Basic Concepts of Laws of Motion, Force, Work, Energy and Power for BPSC (CCE)

1. Laws of Motion

Newton’s Laws of Motion are fundamental principles describing the relationship between a body and the forces acting upon it, and its motion in response to those forces.

• Newton’s First Law (Law of Inertia):
  • Concept: An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced external force.
  • Key Idea: Inertia is the property of an object to resist changes in its state of motion. Mass is a measure of inertia.
  • Examples: A book on a table remaining at rest, a passenger lurching forward when a bus suddenly brakes.
• Newton’s Second Law:
  • Concept: The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. The direction of the acceleration is in the direction of the net force.
  • Formula: F=ma (Force = mass × acceleration)
  • Key Idea: This law quantifies the relationship between force, mass, and acceleration. A larger force produces a larger acceleration for a given mass, and a larger mass requires a larger force to achieve the same acceleration.
  • Examples: Pushing a lighter cart versus a heavier cart with the same force, kicking a football (applying force to change its momentum).
• Newton’s Third Law:
  • Concept: For every action, there is an equal and opposite reaction. When one object exerts a force on a second object, the second object simultaneously exerts a force equal in magnitude and opposite in direction on the first object.
  • Key Idea: Forces always occur in pairs. These forces act on different bodies.
  • Examples: A rocket propelling upwards (exhaust gases pushed downwards, rocket pushed upwards), a bird flying (wings push air down, air pushes bird up), swimming (pushing water backward, water pushes swimmer forward), walking (pushing the ground backward, the ground pushes you forward).

2. Force

• Concept: A force is a push or pull that can change the state of motion of an object (make it move, stop, speed up, slow down, or change direction) or deform it.
• Nature: It is a vector quantity, meaning it has both magnitude and direction.
• SI Unit: Newton (N). 1 N=1 kg⋅m/s2.
• Types of Forces (Commonly Encountered):
  • Contact Forces: Forces that result from direct physical contact between two objects (e.g., muscular force, frictional force, normal force, tension).
  • Non-Contact Forces (Field Forces): Forces that act on an object without direct physical contact (e.g., gravitational force, magnetic force, electrostatic force).
• Net Force/Resultant Force: The vector sum of all forces acting on an object. If the net force is zero, the object is in equilibrium (either at rest or moving with constant velocity).

3. Work

• Concept: In physics, work is done when a force causes a displacement of an object in the direction of the force.
• Formula: W=F⋅d⋅cos(θ)
  • W = Work done
  • F = Magnitude of the force
  • d = Magnitude of the displacement
  • θ = Angle between the force and the displacement
• Key Points:
  • Work is a scalar quantity.
  • Work is zero if:
    • The force applied is zero.
    • The displacement is zero (object doesn’t move).
    • The force and displacement are perpendicular to each other (θ=90∘, so cos(90∘)=0). For example, the work done by gravity on a satellite moving in a circular orbit around the Earth is zero because the gravitational force is towards the center (perpendicular to displacement).
• SI Unit: Joule (J). 1 J=1 N⋅m.

4. Energy

• Concept: Energy is the capacity to do work. It exists in various forms and can be converted from one form to another. The total energy in an isolated system remains constant (Law of Conservation of Energy).
• SI Unit: Joule (J), same as work.
• Common Forms of Mechanical Energy:
  • Kinetic Energy (KE): Energy possessed by an object due to its motion.
    • Formula: KE=21​mv2
      • m = mass of the object
      • v = velocity of the object
    • Key Idea: Kinetic energy is proportional to mass and the square of velocity.
  • Potential Energy (PE): Stored energy an object possesses due to its position or state.
    • Gravitational Potential Energy: Energy stored due to an object’s height in a gravitational field.
      • Formula: PE=mgh
        • m = mass
        • g = acceleration due to gravity
        • h = height above a reference point
    • Elastic Potential Energy: Energy stored in a stretched or compressed elastic object (like a spring).
• Work-Energy Theorem: The net work done on an object is equal to the change in its kinetic energy.
  • Wnet​=ΔKE=KEfinal​−KEinitial​

5. Power

• Concept: Power is the rate at which work is done or the rate at which energy is transferred or converted.
• Formula:
  • P=tW​ (Power = Work done / Time taken)
  • P=F⋅v (Power = Force × Velocity, when force and velocity are in the same direction)
• Key Idea: Power tells us how quickly work is done. A powerful machine can do a lot of work in a short amount of time.
• SI Unit: Watt (W). 1 W=1 J/s.
• Other Units: Horsepower (hp) is a common non-SI unit of power (1 hp≈746 W).