Objective Physics Vol 1 for NEET

1. Physics Quantifies the World Through Measurement and Analysis

All the quantities which are used to describe the laws of physics are called physical quantities, e.g. length, mass, volume, etc.

Measurement is fundamental. Physics is built upon the precise measurement of physical quantities. These quantities, like length, mass, and time, are the building blocks used to describe the universe and its laws. Standard systems of units, such as the SI system, ensure consistency and comparability in measurements worldwide.

Units provide context. Every measurement requires a unit to give it meaning. The SI system defines base units for fundamental quantities (metre for length, kilogram for mass, second for time, etc.) and derived units for quantities like speed (m/s) or force (Newton). Understanding units is crucial for interpreting physical values.

Dimensions reveal relationships. Dimensional analysis uses the fundamental dimensions (like [M], [L], [T]) to check the consistency of equations and deduce relationships between physical quantities. Errors in measurement are inevitable but can be classified (systematic, random) and quantified (absolute, relative, percentage) to understand the reliability of results.

2. Motion is Relative and Precisely Described by Vectors

The angle is positive (or negative), if the initial line revolves in anti-clockwise (or clockwise) direction to get the terminal line.

Motion is relative. Whether an object is at rest or in motion depends entirely on the observer's frame of reference. An object can be at rest relative to one observer but in motion relative to another. This relativity is a core concept in describing movement.

Vectors describe motion. Quantities like displacement, velocity, and acceleration have both magnitude and direction, making them vector quantities. Vectors are represented graphically by arrows and mathematically using components (e.g., i, j, k unit vectors). Vector algebra (addition, subtraction, dot product, cross product) provides the tools to manipulate these quantities precisely.

Kinematics quantifies motion. Kinematics describes motion without considering its causes. Key concepts include:

• Distance (scalar path length) vs. Displacement (vector change in position)
• Speed (scalar rate of distance) vs. Velocity (vector rate of displacement)
• Acceleration (vector rate of change of velocity)

For uniformly accelerated motion, simple kinematic equations relate these quantities over time and displacement.

3. Forces Dictate Changes in Motion, Not Just Motion Itself

The factor which is necessary for causing motion or change in motion is termed as force.

Forces cause acceleration. Newton's laws of motion explain the relationship between force and motion. The first law describes inertia – a body at rest stays at rest, and a body in motion stays in motion unless acted upon by an external force. The second law quantifies this: the net force on a body is proportional to its mass and acceleration (F=ma).

Forces come in pairs. Newton's third law states that for every action, there is an equal and opposite reaction. Forces always occur in pairs acting on different bodies. Common forces include:

• Weight (gravity's pull)
• Normal reaction (surface support)
• Tension (pull in strings/ropes)
• Friction (opposing relative motion)
• Spring force (restoring force in springs)

Equilibrium means balanced forces. A body is in equilibrium if the net force acting on it is zero. This means it is either at rest or moving with constant velocity. Free body diagrams help visualize all forces acting on an object to analyze its motion or equilibrium state.

4. Energy is a Fundamental Quantity That Transforms But Is Conserved

The energy of a body is defined as its capacity or ability for doing work.

Work transfers energy. Work is done when a force causes displacement in the direction of the force. It is a scalar quantity representing energy transfer. Work done by a constant force is the dot product of force and displacement (W = F · s). For variable forces, work is calculated by integrating force over displacement.

Energy takes many forms. Energy exists in various forms, including mechanical energy (kinetic and potential). Kinetic energy is the energy of motion (KE = 1/2 mv²), while potential energy is stored energy due to position or configuration (e.g., gravitational PE = mgh, elastic PE = 1/2 kx²).

Energy is conserved. The work-energy theorem states that the net work done on a body equals the change in its kinetic energy. More broadly, the law of conservation of energy states that the total energy of an isolated system remains constant, although it can transform between different forms (e.g., PE to KE). Power is the rate at which work is done or energy is transferred.

5. Momentum Remains Constant in Isolated Systems During Interactions

The total momentum of an isolated system (a system having no external force acting on it) of constant mass remains constant or conserved and does not change with time.

Momentum quantifies motion. Linear momentum is a vector quantity defined as the product of mass and velocity (p = mv). It represents the "quantity of motion" a body possesses. The total linear momentum of a system is the vector sum of the momenta of its individual particles.

Momentum is conserved. The law of conservation of linear momentum is a direct consequence of Newton's second and third laws. It states that the total linear momentum of a system remains constant if no net external force acts on it. This principle is particularly useful for analyzing collisions and explosions.

Collisions conserve momentum. Collisions are interactions where bodies exert strong forces on each other for a short time. Momentum is conserved in all types of collisions (elastic, inelastic, perfectly inelastic). The center of mass of a system moves as if all the external forces were applied to a single particle with the total mass; its velocity remains constant if the net external force is zero.

6. Rotational Motion Mirrors Translational Motion with Analogous Concepts

Moment of inertia plays the same role in rotational motion as mass plays in translational motion.

Rotation about an axis. Rotational motion describes a body's movement around a fixed axis. Every particle in a rigid body undergoing pure rotation moves in a circle around this axis. Rotational kinematics uses angular displacement, velocity, and acceleration, analogous to their linear counterparts.

Rotational inertia. Moment of inertia (I) is the rotational analogue of mass; it measures a body's resistance to changes in its rotational motion. It depends on mass distribution and the axis of rotation. Theorems like the parallel axis theorem (I = I_CM + MR²) and perpendicular axis theorem (I_z = I_x + I_y) help calculate moment of inertia for complex shapes.

Torque causes angular acceleration. Torque (τ), the rotational analogue of force, is the turning effect of a force. It is the product of force and the perpendicular distance from the axis (τ = r x F). Torque causes angular acceleration (τ = Iα). Angular momentum (L), the rotational analogue of linear momentum, is conserved if no external torque acts on the system (L = Iω).

7. Gravity Governs Universal Attraction

every particle in the universe attracts every other particle with a force whose magnitude is directly proportional to the product of their masses and inversely proportional to the square of distances between their centres.

Newton's law of gravitation. This fundamental law describes the attractive force between any two masses in the universe. The force is proportional to the product of the masses and inversely proportional to the square of the distance between them (F = G m1m2/r²). This force is always attractive, acts along the line joining the centers, and is conservative.

Gravity is Earth's pull. Gravity is the specific term for the gravitational force exerted by the Earth on other bodies. Acceleration due to gravity (g) is the acceleration experienced by a body due to Earth's gravitational pull. Its value varies with altitude, depth, the shape of the Earth, and its rotation.

Gravitational field and potential. A gravitational field exists around any mass, where other masses experience a force. Gravitational field intensity is the force per unit mass. Gravitational potential is the potential energy per unit mass, representing the work done to bring a unit mass from infinity to a point. Gravitational potential energy is the energy stored in a system of masses due to their gravitational interaction.

8. Heat is Energy Transferred Due to Temperature Differences and Phase Changes

The form of energy which is exchanged among various bodies or system on account of temperature difference is defined as heat.

Heat is energy transfer. Heat is energy that flows between systems solely due to a temperature difference. Temperature is a measure of the degree of hotness or coldness, quantified using scales like Celsius, Fahrenheit, and Kelvin. Heat capacity and specific heat quantify how much heat is needed to change a substance's temperature.

Phase changes require latent heat. Substances can change state (solid, liquid, gas) at constant temperature by absorbing or releasing latent heat. Latent heat of fusion is for melting/freezing, and latent heat of vaporization is for boiling/condensation. Heating curves show temperature changes and phase transitions as heat is added.

Heat transfers via mechanisms. Heat can transfer through:

• Conduction (molecular vibrations in solids)
• Convection (mass movement in fluids)
• Radiation (electromagnetic waves, requires no medium)

Thermal conductivity quantifies a material's ability to conduct heat. Radiation follows laws like Stefan's law (energy radiated ∝ T⁴) and Wien's displacement law (peak wavelength ∝ 1/T).

9. Gases Behave Based on the Collective Motion of Their Molecules

The average kinetic energy of the gas particles is proportional to the temperature of the sample (in kelvin).

Kinetic theory postulates. This theory describes gases as collections of randomly moving molecules. Key assumptions include negligible molecular volume, no intermolecular forces, elastic collisions, and average kinetic energy proportional to absolute temperature.

Pressure from collisions. Gas pressure arises from the molecules colliding with the container walls. The pressure is proportional to the number density and the average kinetic energy of the molecules. Ideal gases obey simple laws (Boyle's, Charles', Gay-Lussac's) relating pressure, volume, and temperature.

Molecular velocities and free path. Gas molecules have a distribution of speeds (average, rms, most probable). The rms speed is related to temperature and molar mass. Mean free path is the average distance a molecule travels between collisions, related to molecular size and number density. Real gases deviate from ideal behavior at high pressure and low temperature due to finite molecular volume and intermolecular forces, described by van der Waals' equation.

10. Energy Transformations Obey Fundamental Laws Setting Limits on Processes

if some quantity of heat is supplied to a system capable of doing external work, then the quantity of heat absorbed by the system is equal to the sum of the increase in the internal energy of the system and the external work done by the system.

Thermodynamic systems and states. Thermodynamics studies energy transformations in systems defined by boundaries and surroundings. The state of a system is described by variables like pressure, volume, and temperature, related by an equation of state. Processes describe changes between states.

First Law: Energy Conservation. The first law states that heat added to a system equals the change in its internal energy plus the work done by the system (∆Q = ∆U + ∆W). Internal energy is a state function, depending only on the current state, while heat and work are path-dependent.

Thermodynamic Processes. Specific processes occur under constant conditions:

• Isobaric (constant pressure)
• Isochoric (constant volume, W=0)
• Isothermal (constant temperature, ∆U=0 for ideal gas)
• Adiabatic (no heat transfer, ∆Q=0)
• Cyclic (returns to initial state, ∆U=0 for the cycle)

Second Law: Limits on Efficiency. The second law imposes limits on energy conversion. It states that heat cannot be completely converted to work (Kelvin-Planck) and heat cannot spontaneously flow from cold to hot (Clausius). This limits heat engine efficiency (Carnot efficiency depends on temperature difference) and refrigerator performance. Reversible processes are idealizations of infinitely slow, frictionless changes.

Last updated: May 19, 2025