Physics Basics - Motion, Force and Energy

Master the fundamental concepts of physics for RRB exam preparation with clear explanations and practical examples.

Motion and Kinematics

Basic Concepts

Types of Motion

• Translational Motion: Linear movement of objects
• Rotational Motion: Circular movement around an axis
• Oscillatory Motion: Back and forth movement
• Random Motion: Unpredictable movement pattern

Quantities of Motion

• Distance: Total path length traveled
• Displacement: Shortest distance between start and end point
• Speed: Distance covered per unit time
• Velocity: Displacement per unit time with direction
• Acceleration: Change in velocity per unit time

Key Formulas

Kinematics Equations

• First Equation: v = u + at
• Second Equation: s = ut + �at�
• Third Equation: v� = u� + 2as

Where:

• v = final velocity
• u = initial velocity
• a = acceleration
• t = time
• s = displacement

Practice Problems

Question 1

A car starts from rest and accelerates at 2 m/s� for 5 seconds. What is its final velocity?

Question 2

A train traveling at 60 km/h applies brakes and stops in 10 seconds. Find the retardation.

Question 3

An object travels 100 meters with uniform acceleration. If its final velocity is 20 m/s and initial velocity is 5 m/s, find the time taken.

Force and Newton’s Laws

Newton’s Laws of Motion

First Law (Law of Inertia)

• Object at rest stays at rest, object in motion stays in motion
• Unless acted upon by an external force
• Application: Seat belts in cars

Second Law (F = ma)

• Force equals mass times acceleration
• Formula: F = ma
• Unit: Newton (N)

Third Law (Action-Reaction)

• Every action has equal and opposite reaction
• Examples: Rocket propulsion, walking

Types of Forces

• Applied Force: Direct push or pull
• Friction Force: Opposing motion
• Normal Force: Perpendicular support force
• Tension Force: Force through ropes/strings
• Gravitational Force: Earth’s attraction
• Spring Force: Force exerted by springs

Friction

Static Friction

• Prevents motion between surfaces
• Maximum value: fs d �s � N

Kinetic Friction

• Opposes motion between moving surfaces
• Formula: fk = �k � N

Coefficients of Friction

• �s (static): 0.1-0.6 (depends on surfaces)
• �k (kinetic): 0.05-0.5 (depends on surfaces)

Practice Problems

Question 4

A 5 kg block is pushed with a force of 20 N on a horizontal surface with friction coefficient 0.2. Find the acceleration.

Question 5

A person weighing 70 kg is in an elevator accelerating upward at 2 m/s�. What is the apparent weight?

Work, Energy and Power

Work

• Definition: Force � displacement � cos(angle)
• Formula: W = F � d � cos(�)
• Unit: Joule (J)
• Condition: Force must cause displacement

Energy

Kinetic Energy

• Energy of motion
• Formula: KE = �mv�
• Depends on mass and velocity

Potential Energy

• Energy due to position
• Gravitational PE: PE = mgh
• Elastic PE: PE = �kx�

Conservation of Energy

• Total energy remains constant
• Energy transforms from one form to another

Power

• Definition: Work done per unit time
• Formula: P = W/t
• Unit: Watt (W)
• 1 Horsepower = 746 Watts

Practice Problems

Question 6

A 2 kg object is lifted to a height of 5 meters. What is its potential energy? (g = 9.8 m/s�)

Question 7

A car of mass 1000 kg is moving at 20 m/s. What is its kinetic energy?

Question 8

A machine lifts 500 kg of bricks to a height of 10 meters in 20 seconds. What is the power required?

Electricity and Magnetism

Electric Current

• Definition: Flow of electric charge
• Formula: I = Q/t
• Unit: Ampere (A)

Voltage

• Definition: Electric potential difference
• Unit: Volt (V)

Resistance

• Formula: R = V/I (Ohm’s Law)
• Unit: Ohm (�)

Electric Power

• Formula: P = V � I
• Unit: Watt (W)

Series and Parallel Circuits

Series Circuit

• Same current through all components
• Total resistance: Rtotal = R1 + R2 + R3

Parallel Circuit

• Same voltage across all components
• Total resistance: 1/Rtotal = 1/R1 + 1/R2 + 1/R3

Magnetism

Magnetic Field

• Region around magnet with magnetic influence
• Measured in Tesla (T)

Electromagnetism

• Current-carrying conductor produces magnetic field
• Used in motors, generators, transformers

Practice Problems

Question 9

A 12V battery is connected to a 4� resistor. What is the current flowing through the circuit?

Question 10

Three resistors of 2�, 3�, and 6� are connected in parallel to a 6V battery. Find the total current.

Heat and Thermodynamics

Temperature

• Celsius Scale: Water freezes at 0�C, boils at 100�C
• Fahrenheit Scale: Water freezes at 32�F, boils at 212�F
• Kelvin Scale: Absolute scale, 0K = -273�C

Heat Transfer

Conduction

• Direct transfer through material
• Good conductors: metals
• Poor conductors: wood, plastic

Convection

• Transfer through fluid movement
• Hot fluids rise, cold fluids sink

Radiation

• Transfer through electromagnetic waves
• No medium required

Specific Heat Capacity

• Definition: Heat required to raise temperature by 1�C
• Formula: Q = mc�T
• Water: 4186 J/kg�C (high specific heat)

Practice Problems

Question 11

How much heat is required to raise the temperature of 2 kg of water from 20�C to 80�C?

Question 12

A 500g piece of aluminum (specific heat = 900 J/kg�C) is heated from 25�C to 125�C. Find the heat absorbed.

Light and Optics

Properties of Light

• Speed: 3 � 10x m/s in vacuum
• Nature: Electromagnetic wave
• Colors: VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red)

Reflection

• Angle of incidence = Angle of reflection
• Used in mirrors, periscopes

Refraction

• Bending of light when changing mediums
• Snell’s Law: n�sin(��) = n�sin(��)

Lenses

Convex Lens

• Converging lens
• Forms real and virtual images
• Used in magnifying glasses

Concave Lens

• Diverging lens
• Forms virtual images only
• Used in correcting myopia

Practice Problems

Question 13

Light travels from air (n=1) to glass (n=1.5) at an angle of 30�. What is the angle of refraction?

Question 14

An object is placed 20 cm from a convex lens of focal length 10 cm. Where is the image formed?

Quick Reference Formulas

Motion

• v = u + at
• s = ut + �at�
• v� = u� + 2as

Force

• F = ma
• Weight = mg
• Friction = �N

Energy

• KE = �mv�
• PE = mgh
• Power = Work/Time

Electricity

• Ohm’s Law: V = IR
• Power: P = VI = I�R = V�/R
• Energy: E = P � t

Heat

• Q = mc�T
• Latent Heat: Q = mL

Back to Science Topics

All Topic Practice