Lessons — Visiphysics

Mechanics

Projectiles, forces, energy, momentum, circular motion

Everything here comes back to Newton's second law: a net force causes acceleration, and acceleration changes velocity over time. Projectile motion is just this applied separately to the horizontal (constant velocity, no force) and vertical (constant acceleration, gravity) directions at once. That's why a thrown ball's horizontal speed never changes while its vertical speed does.

A ball is launched at 20 m/s at 45°, how far does it land?

Block sliding down a 30° incline with friction

Elastic collision between two balls

Car rounding a circular curve

Block pushed across a table with friction

Electricity & magnetism

Point charges, circuits, fields

Coulomb's law is the electric version of gravity's inverse-square law. Like charges push apart, opposite charges pull together, and the force fades fast with distance. In circuits, Ohm's law relates voltage, current, and resistance, and resistors in series just add up directly.

Force between two opposite point charges

Series circuit with two resistors

Three collinear point charges

Oscillations & waves

Springs, pendulums, resonance

Simple harmonic motion shows up anywhere a restoring force grows with displacement: a spring pulling back harder the more you stretch it, or gravity pulling a pendulum back toward vertical. The period only depends on the system's own properties (mass and stiffness, or length and gravity), never on how far you pull it back.

Mass-spring oscillator period

Simple pendulum period

Orbital mechanics

Satellites, planetary motion

A stable orbit is really just free-fall that keeps missing the ground. Gravity supplies exactly the centripetal force needed to keep curving the orbiting body's path into a circle. Faster orbital speed only makes sense at a smaller radius; the two are locked together by the central body's mass.

Satellite orbital velocity

Momentum & energy

Explosions, conservation laws

Momentum is conserved in every collision or explosion, no exceptions: total momentum before always equals total momentum after. Kinetic energy is only conserved in elastic collisions; in an explosion or a "stick together" collision, it's momentum you should reach for first.

Explosion fragment velocities

Three cars, different speeds and masses

Rotation & torque

Angular acceleration, equilibrium, rolling motion

Torque is force's rotational twin. It's what actually causes angular acceleration, the same way force causes linear acceleration, and rotational inertia plays the role mass does. A rolling object splits its kinetic energy between moving forward and spinning, which is exactly why a hoop and a solid disk don't roll down a ramp at the same rate.

Torque and angular acceleration of a wheel

Beam in static equilibrium

Rolling race: hoop vs. disk

Fluids

Pressure, buoyancy, hydraulics

Pascal's principle is the whole idea behind hydraulic lifts: pressure applied anywhere in an enclosed fluid transmits equally everywhere, so a small force on a small piston becomes a large force on a large piston. You trade distance for force, never get something for nothing.

Hydraulic lift force ratio

Thermodynamics

Heat engines, calorimetry, gas laws

No real engine converts heat to work perfectly. A Carnot engine is the theoretical best case, and its efficiency depends only on the temperatures it runs between. Calorimetry problems are just energy conservation: heat lost by the hotter object equals heat gained by the cooler one.

Carnot engine efficiency

Calorimetry: mixing hot and cold

Optics

Mirrors, refraction, interference

Mirrors and lenses both obey the same relationship between object distance, image distance, and focal length. Refraction bends light because it changes speed crossing between materials. Pushed far enough, that bending becomes total internal reflection instead of transmission, which is the critical angle.

Image distance from a concave mirror

Critical angle for total internal reflection

Modern physics

Photons, atoms, radioactivity

Light comes in discrete packets of energy. A photon's energy depends only on its frequency (or equivalently, its wavelength), which is why blue light packs more punch per photon than red. Radioactive decay is random for any single atom, but predictable in bulk: a fixed fraction of whatever's left decays every half-life, so the amount remaining shrinks geometrically, not linearly.

Photon energy from wavelength

Radioactive decay over multiple half-lives

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