Physics Basics
Understanding the fundamental rules that govern how everything moves and interacts.
Why Physics Matters
Physics explains everyday experiences:
- Why you feel pushed back when a car accelerates
- Why a heavy truck is harder to stop than a bicycle
- Why hot air rises
- Why the sky is blue
- How your phone screen works
- Why bridges can hold so much weight
Forces and Motion
What Is a Force?
A force is a push or pull on an object. Forces cause objects to:
| Effect | Example |
|---|---|
| Start moving | Kicking a ball |
| Stop moving | Brakes on a car |
| Speed up | Pressing gas pedal |
| Slow down | Friction on a sliding box |
| Change direction | Steering a car |
| Change shape | Squeezing clay |
Types of Forces
| Force | Description | Examples |
|---|---|---|
| Gravity | Attraction between masses | Falling objects, orbiting planets |
| Friction | Resistance to sliding motion | Walking, brakes, rubbing hands |
| Normal | Surface pushing back | Floor supporting you |
| Tension | Force through rope/string | Tug of war, hanging lamp |
| Applied | Direct push or pull | Pushing a cart |
| Air resistance | Drag from air | Parachute, wind on your face |
Mass vs. Weight
| Property | Mass | Weight |
|---|---|---|
| What it is | Amount of matter | Force of gravity on mass |
| Units | Kilograms (kg) | Newtons (N) |
| Location dependent? | No (same everywhere) | Yes (less on Moon) |
| How measured | Balance scale | Spring scale |
Example: An astronaut has the same mass on Earth and Moon, but weighs less on the Moon because gravity is weaker.
Newton's Laws of Motion
First Law: Inertia
Objects at rest stay at rest. Objects in motion stay in motion (at constant speed in a straight line). Unless acted on by a force.
| Situation | What Happens | Why |
|---|---|---|
| Car stops suddenly | You lurch forward | Body wants to keep moving |
| Tablecloth pulled quickly | Dishes stay put | Inertia keeps them in place |
| Spacecraft in space | Travels forever without fuel | No friction to slow it |
| Book on table | Stays still | No force to move it |
Second Law: F = ma
Force equals mass times acceleration.
| More... | Means... |
|---|---|
| Force | More acceleration |
| Mass | Less acceleration (harder to move) |
Practical examples:
| Scenario | Explanation |
|---|---|
| Pushing empty vs. loaded shopping cart | Same force, loaded cart accelerates less |
| Throwing baseball vs. bowling ball | Same force, bowling ball accelerates less |
| Small engine vs. large engine car | More force, faster acceleration |
Third Law: Action and Reaction
Every action has an equal and opposite reaction.
| Action | Reaction |
|---|---|
| You push on wall | Wall pushes back on you |
| Rocket expels gas downward | Gas pushes rocket upward |
| You push boat from dock | Boat pushes you toward dock |
| Bird pushes air down with wings | Air pushes bird up |
Energy
What Is Energy?
Energy is the ability to do work or cause change. It comes in many forms.
Types of Energy
| Type | Description | Examples |
|---|---|---|
| Kinetic | Energy of motion | Moving car, running water |
| Potential (gravitational) | Stored due to position | Raised hammer, water behind dam |
| Potential (elastic) | Stored in stretched/compressed objects | Stretched rubber band, compressed spring |
| Chemical | Stored in molecular bonds | Gasoline, food, batteries |
| Electrical | Moving electrons | Lightning, current in wires |
| Thermal (heat) | Vibration of atoms | Hot stove, body heat |
| Nuclear | Stored in atomic nuclei | Sun, nuclear power plants |
| Light (radiant) | Electromagnetic waves | Sunlight, lasers |
| Sound | Vibration through medium | Music, speech |
Conservation of Energy
Energy cannot be created or destroyed, only converted from one form to another.
| Energy Conversion | What Happens |
|---|---|
| Car engine | Chemical (fuel) → Thermal → Kinetic |
| Hydroelectric dam | Potential (water height) → Kinetic → Electrical |
| Solar panel | Light → Electrical |
| Your body | Chemical (food) → Kinetic (movement) + Thermal (body heat) |
| Friction | Kinetic → Thermal |
Power
Power is how fast energy is transferred or converted.
| Concept | Definition | Units |
|---|---|---|
| Energy | Ability to do work | Joules (J) |
| Power | Rate of energy transfer | Watts (W) = Joules/second |
Example: A 60W light bulb converts 60 joules of electrical energy to light and heat every second.
Heat and Temperature
Temperature vs. Heat
| Concept | What It Is | Units |
|---|---|---|
| Temperature | Average kinetic energy of molecules | Celsius, Fahrenheit, Kelvin |
| Heat | Transfer of thermal energy | Joules |
Key insight: A cup of hot coffee has higher temperature than a bathtub of warm water, but the bathtub contains more total thermal energy.
Heat Transfer Methods
| Method | How It Works | Example |
|---|---|---|
| Conduction | Direct contact | Touching a hot pan |
| Convection | Movement of fluid/gas | Hot air rising |
| Radiation | Electromagnetic waves | Sun warming your face |
Practical Applications
| Principle | Application |
|---|---|
| Hot air rises (convection) | Heating vents near floor |
| Dark colors absorb more radiation | Black car gets hotter |
| Metals conduct heat well | Metal pot handles get hot |
| Air is a poor conductor | Double-pane windows insulate |
Waves and Sound
Wave Properties
| Property | Definition | Determines |
|---|---|---|
| Wavelength | Distance between wave peaks | Color (light), pitch (sound) |
| Frequency | Waves per second (Hz) | Pitch for sound |
| Amplitude | Height of wave | Loudness (sound), brightness (light) |
| Speed | How fast wave travels | Depends on medium |
Sound
| Fact | Explanation |
|---|---|
| Sound needs a medium | Vibrations travel through air, water, or solids |
| No sound in space | No medium to carry vibrations |
| Speed of sound | About 343 m/s in air (faster in water and solids) |
| Thunder after lightning | Light travels faster than sound |
The Doppler Effect
When a source of waves moves relative to you, the frequency changes:
| Motion | Effect | Example |
|---|---|---|
| Approaching | Higher pitch | Ambulance siren approaching |
| Receding | Lower pitch | Ambulance siren moving away |
Light
Light Basics
| Property | Details |
|---|---|
| Speed | 300,000 km/s (fastest thing in universe) |
| Nature | Both wave and particle (photon) |
| Medium needed? | No (travels through vacuum) |
| What we see | Reflected light entering our eyes |
Color
| Phenomenon | Explanation |
|---|---|
| White light | All colors combined |
| Object color | Wavelengths reflected (absorbed colors not seen) |
| Blue sky | Short blue wavelengths scattered by atmosphere |
| Red sunset | Blue light scattered away, red remains |
| Rainbow | Light refracted and separated by water droplets |
Reflection and Refraction
| Phenomenon | What Happens | Example |
|---|---|---|
| Reflection | Light bounces off surface | Mirror, still water |
| Refraction | Light bends when entering new medium | Straw looks bent in water |
| Total internal reflection | Light trapped inside medium | Fiber optics |
Electricity and Magnetism
Basic Concepts
| Term | Definition | Analogy |
|---|---|---|
| Voltage | Electrical "pressure" | Water pressure in pipe |
| Current | Flow of electrons | Water flow rate |
| Resistance | Opposition to flow | Narrow pipe |
| Power | Voltage x Current | Work being done |
Static vs. Current Electricity
| Type | Description | Examples |
|---|---|---|
| Static | Charges built up on surface | Shock from doorknob, lightning |
| Current | Continuous flow of charges | Household electricity |
Magnetism
| Fact | Explanation |
|---|---|
| Magnets have poles | North and South (always together) |
| Like poles repel | N-N or S-S push apart |
| Opposite poles attract | N-S pull together |
| Electricity and magnetism linked | Moving charges create magnetic fields |
Electromagnets: Electric current creates magnetic field. This is how electric motors and generators work.
Practical Physics
Around the House
| Situation | Physics Principle |
|---|---|
| Microwave heats food | Electromagnetic waves vibrate water molecules |
| Insulation keeps house warm | Slows heat transfer (conduction) |
| Refrigerator | Removes heat using evaporation/compression |
| LED lights | Electrons emit photons in semiconductor |
| Mirrors reflect | Smooth surface reflects light uniformly |
In Transportation
| Feature | Physics |
|---|---|
| Seatbelts | Counter inertia in sudden stop |
| Airbags | Increase stopping time, reduce force |
| Aerodynamic shape | Reduce air resistance |
| Antilock brakes | Maintain friction while slowing |
| Crumple zones | Absorb energy, increase stopping time |
Understanding Everyday Phenomena
| Question | Physics Answer |
|---|---|
| Why is ice slippery? | Its surface has a thin quasi-liquid layer even below freezing |
| Why does hot water freeze faster sometimes? | Mpemba effect (complex, not fully understood) |
| Why do pipes burst in winter? | Water expands when freezing |
| Why can you hear around corners? | Sound waves diffract (bend) around obstacles |
| Why do helium balloons float? | Helium is less dense than air |
Key Takeaways
Forces change motion - Objects need a force to start, stop, speed up, slow down, or change direction
Inertia resists change - Objects "want" to keep doing what they're doing
F = ma governs motion - Force, mass, and acceleration are related
Energy is conserved - It changes form but is never created or destroyed
Heat moves from hot to cold - Never the reverse without work input
Waves transfer energy - Sound needs a medium; light doesn't
Electricity and magnetism are connected - Moving charges create magnetic fields and vice versa
Physics explains daily life - From why you slip on ice to how your phone works
Same rules everywhere - Physics works the same on Earth, in space, everywhere in the universe
Simple rules, complex outcomes - A few basic principles explain an enormous range of phenomena