Essential Physics Concepts, Laws, Units, and Formulas

Key Physics Concepts

KEY CONCEPTS

• 1. Law: An object stays in uniform motion unless a net force acts.
• 2. Law: Acceleration is proportional to net force and inversely proportional to mass.
• 3. Law: Forces come in equal and opposite pairs.
• Conservation of Energy: Total mechanical energy remains constant if no non-conservative forces act.
• Conservation of Momentum: Total momentum stays constant in isolated systems.
• Archimedes: Buoyant force equals the weight of displaced fluid.
• Bernoulli: Faster fluid → lower pressure; slower fluid → higher pressure.
• Pascal: Pressure applied to a fluid transmits equally everywhere.
• Electric Field: Region where a charge experiences force.
• Potential Difference (Voltage): Energy per charge.
• Current: Rate of charge flow.
• Resistance: Opposition to current.
• Magnetic Field: Region where magnetic forces act.
• Faraday: Changing magnetic flux induces EMF.
• Waves: Frequency ↑ → wavelength ↓ (wave speed constant in a given medium).
• Simple harmonic motion: Restoring force proportional to displacement.
• m (mass): kilogram (kg); grams (g) = 0.001 kg.
• W (weight): newton (N); depends on gravity.
• F (force): newton (N).
• d (distance/displacement): meter (m).
• t (time): second (s).
• v (velocity): m/s.
• a (acceleration): m/s².
• ρ (density): kg/m³; water = 1000 kg/m³.
• P (pressure): pascal (Pa) = N/m².
• V (volume): m³; 1 cm³ = 1 mL.
• Q (charge): coulomb (C).
• I (current): ampere (A).
• V (voltage): volt (V).
• R (resistance): ohm (Ω).
• C (capacitance): farad (F).
• B (magnetic field): tesla (T).
• Φ (magnetic flux): weber (Wb).
• f (frequency): hertz (Hz).
• T (period): second (s).
• λ (wavelength): meter (m).
• Inertia: Resistance to changing motion.
• Equilibrium: Net force and net torque are zero.
• Impulse: Effect of a force acting over time.
• Elastic Collision: Kinetic energy and momentum conserved.
• Inelastic Collision: Momentum conserved; kinetic energy lost.
• Buoyancy: Upward force due to displaced fluid.
• Conductors: Allow charge flow.
• Insulators: Resist charge flow.
• Capacitor: Stores electric energy.
• Magnetic Flux: Measure of magnetic field passing through an area.
• Induced EMF: Voltage created by changing magnetic flux.
• Node (waves): Point of zero displacement.
• Antinode: Point of maximum displacement.

Forces, Energy, Momentum

Forces: An object stays in motion unless a net force acts. Acceleration increases with net force and decreases with mass. Forces come in equal and opposite pairs but act on different bodies. Constant velocity means net force = 0. Equilibrium means net force = 0 and net torque = 0.

Energy: Energy is conserved when no non-conservative forces act. An object speeding up gains kinetic energy (KE); slowing down loses KE.

Momentum: Momentum is conserved in isolated systems. Elastic collisions conserve KE; inelastic collisions do not. “Stick together” indicates a perfectly inelastic collision.

Fluids and Buoyancy

Fluids: Pressure increases with depth. For the same force, larger area produces lower pressure. Buoyant force depends only on the displaced fluid. If an object's density is less than the fluid's, it floats; if greater, it sinks. If partially submerged, only the submerged volume is displaced. Underwater objects appear closer due to refraction.

Gravity and Motion

Gravity: Heavier objects do not fall faster in a vacuum. Weight changes with gravity; mass does not.

Simple Harmonic Motion (SHM)

SHM: The restoring force points toward equilibrium. Maximum speed occurs at equilibrium; maximum acceleration occurs at the endpoints.

Waves and Sound

Waves: As frequency increases, wavelength decreases (wave speed is constant in a given medium). Nodes are points of zero displacement; antinodes are points of maximum displacement. Sound travels faster in water than in air.

Electricity

Electricity: The electric field gives the direction a positive charge moves. Voltage is energy per charge. Current flows from higher to lower potential. In series circuits, current is the same everywhere. In parallel circuits, voltage is the same across branches. Resistance increases with longer or thinner wires. Parallel resistors decrease the total resistance. Capacitors store energy; closer plates increase capacitance.

Magnetism

Magnetism: Magnetic force is perpendicular to velocity and the magnetic field. There is no magnetic force if velocity is parallel to the field. Use the right-hand rule for positive charges; reverse for negative charges. Changing magnetic flux induces an EMF; no change produces no EMF. A faster change in flux produces a larger EMF.

Circular Motion and Orbits

Circular Motion: Constant speed in a circle still implies acceleration (centripetal). Satellites orbit by continuously falling while moving forward.

Torque and Rotation

Torque: A longer lever arm produces more torque. Rotational equilibrium requires net torque = 0.

Optics

Optics: The angle of incidence equals the angle of reflection. Exiting a denser medium causes light to bend away from the normal.

Units and Conventions

Units: kg = mass; N = weight/force; density in kg/m³; pressure in Pa; charge in C; current in A; voltage in V; magnetic field in T.

Object in water appears closer than it is: Light bends when entering air → objects look shallower.

Floating vs sinking intuition: If object density < fluid density → floats; if > fluid density → sinks.

Satellite in orbit is constantly falling: Forward speed makes it miss Earth → continuous free fall.

Weight vs mass: Mass never changes; weight changes with gravity.

Heavier objects don’t fall faster in vacuum: All objects accelerate equally without air resistance.

High pressure: Pushes in all directions.

Current is the same everywhere in series; voltage is the same everywhere in parallel.

Buoyant force doesn't depend on object weight—only on displaced fluid.

Electric field lines go from positive to negative; they never cross.

Magnetic force acts perpendicular to both velocity and magnetic field.

Important Reminders

• Velocity zero does NOT mean acceleration zero (example: top of projectile motion).
• Something can move in a circle at constant speed but still accelerate (centripetal acceleration).

Problem-Solving Steps (SOLVE)

• Forces: Draw a free-body diagram, identify all forces, determine the net direction.
• Momentum: Identify an isolated system; compare before and after velocities.
• Buoyancy: Compare object density vs fluid density to decide float or sink.
• Pressure: Remember depth increases pressure; area distributes force.
• Electricity: Identify series vs parallel; current is same in series, voltage is same in parallel.
• Magnetism: Use the right-hand rule to determine force direction.
• Waves: Identify given variables and relate frequency, wavelength, and medium properties (f, λ, and wave motion).