Physics Essentials: quick revision, 19 bites

Physics: The Study of Nature

Physics explains everything around you.

Physics is the branch of science that studies **matter**, **energy**, and their interactions. It seeks to understand the fundamental laws that govern the universe, from the smallest subatomic particles to the largest galaxies.

Takeaway. Physics is the fundamental science of matter and energy.

SI Units: The Language of Physics

Measure once, measure right.

Physics uses the **International System of Units (SI)** for consistency. The seven base units are the **meter** (length), **kilogram** (mass), **second** (time), **ampere** (electric current), **kelvin** (temperature), **mole** (amount of substance), and **candela** (luminous intensity).

Takeaway. SI units provide a universal standard for measurement.

Kinematics: Describing Motion

Motion in numbers.

Kinematics describes motion without considering forces. Key quantities are **displacement** ($\Delta x$), **velocity** ($v$), and **acceleration** ($a$).

Takeaway. Velocity is displacement per time, acceleration is velocity change per time.

Newton's First Law: Inertia

Objects resist change.

Newton's First Law states that an object at rest stays at rest, and an object in motion stays in motion with constant velocity, unless acted upon by a **net external force**. This property is called **inertia**.

Takeaway. Inertia is the tendency to resist changes in motion.

Newton's Second Law: F=ma

Force equals mass times acceleration.

This law quantifies how force affects motion. The net force on an object equals the product of its mass and acceleration.

Takeaway. The net force on an object equals mass times acceleration.

Newton's Third Law: Action-Reaction

Every action has an equal reaction.

Newton's Third Law states that for every action, there is an equal and opposite reaction. Forces always come in pairs acting on different objects.

Takeaway. Forces always occur in equal and opposite pairs.

Work and Energy

Work transfers energy.

Work is done when a force causes displacement. Energy is the ability to do work. The SI unit for both is the **joule (J)**.

Takeaway. Work is force times displacement times the cosine of the angle.

Kinetic and Potential Energy

Energy in motion and position.

**Kinetic energy** ($KE$) is energy of motion, while **potential energy** ($PE$) is stored energy due to position or configuration.

Takeaway. Kinetic energy depends on speed; potential energy depends on height.

Conservation of Energy

Energy is never lost, only transformed.

In a closed system, the total energy remains constant. Energy can change forms, such as from potential to kinetic, but the total amount stays the same.

Takeaway. Total energy in a closed system is conserved.

Power: Rate of Work

How fast work is done.

**Power** is the rate at which work is done or energy is transferred. Its SI unit is the **watt (W)**, equal to one joule per second.

Takeaway. Power measures how quickly work is done.

Momentum and Impulse

Mass in motion.

**Momentum** ($p$) is the product of mass and velocity. **Impulse** is the change in momentum, equal to force times time.

Takeaway. Momentum is mass times velocity; impulse changes momentum.

Conservation of Momentum

Momentum is conserved in collisions.

In an isolated system, the total momentum before a collision equals the total momentum after. This principle applies to all collisions, elastic and inelastic.

Takeaway. Total momentum remains constant in isolated systems.

Gravity and Gravitational Force

The force that keeps you grounded.

**Gravity** is an attractive force between any two masses. On Earth, it gives objects weight. The acceleration due to gravity is approximately $9.8 \, \text{m/s}^2$.

Takeaway. Gravity is a universal attractive force between masses.

Confusing Mass and Weight

Mass is not weight.

**Mass** is the amount of matter in an object (constant everywhere). **Weight** is the gravitational force on that mass ($W = mg$). Weight changes with gravity, mass does not.

Takeaway. Mass is intrinsic; weight depends on gravity.

Friction: Friend and Foe

Friction is everywhere.

**Friction** is a force that opposes relative motion between surfaces. It is essential for walking, driving, and holding objects, but also causes wear and energy loss.

Takeaway. Friction is a necessary force with both benefits and drawbacks.

Pressure in Fluids

Force spread over area.

**Pressure** is force per unit area. In fluids, pressure increases with depth. Atmospheric pressure is the weight of air above us.

Takeaway. Pressure is force per area and increases with depth in fluids.

Archimedes' Principle

Why ships float.

Archimedes' Principle states that a body immersed in a fluid experiences an upward **buoyant force** equal to the weight of the displaced fluid.

Takeaway. Buoyant force equals the weight of displaced fluid.

Simple Harmonic Motion

Oscillations everywhere.

**Simple harmonic motion (SHM)** is a periodic motion where the restoring force is proportional to displacement. Examples include a mass on a spring and a pendulum.

Takeaway. SHM is characterized by a restoring force proportional to displacement.

Problem-Solving Strategy

Tackle physics problems step-by-step.

Takeaway. A systematic approach prevents errors in physics problems.