⚛️ What is Physics?
Physics is the fundamental science that studies matter, energy, and the interactions between them. From the smallest particles to the largest galaxies, physics explains how the universe works. Understanding physics helps us design technology, predict natural phenomena, and comprehend our reality.
Why Study Physics?
- Technology: Electronics, computers, smartphones, satellites
- Engineering: Buildings, bridges, vehicles, machines
- Medicine: X-rays, MRI, radiation therapy
- Understanding: Weather, earthquakes, stars, the universe
Essential Units & Conversions
| Quantity | SI Unit | Symbol |
|---|---|---|
| Distance | meter | m |
| Time | second | s |
| Mass | kilogram | kg |
| Force | Newton | N (kg·m/s²) |
| Energy | Joule | J (N·m) |
| Power | Watt | W (J/s) |
🚀 Motion (Kinematics)
Key Definitions
Scalar vs Vector Quantities
- Scalar: Magnitude only (speed, distance, time, mass)
- Vector: Magnitude + direction (velocity, displacement, acceleration, force)
Distance vs Displacement
- Distance: Total path length traveled (scalar)
- Displacement: Change in position from start to end (vector)
Speed vs Velocity
- Speed: How fast something moves (scalar)
- Velocity: Speed with direction (vector)
Kinematic Equations
For constant acceleration (a = constant):
Δx = v₀t + ½at²
v² = v₀² + 2aΔx
Δx = ½(v + v₀)t
| Variable | Meaning | Unit |
|---|---|---|
| v | Final velocity | m/s |
| v₀ | Initial velocity | m/s |
| a | Acceleration | m/s² |
| t | Time | s |
| Δx | Displacement | m |
Free Fall
Choose a positive direction (usually up or right). Acceleration due to gravity is negative if up is positive: a = -9.8 m/s²
💪 Forces (Dynamics)
Newton's Three Laws of Motion
Newton's First Law (Inertia)
An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by a net external force.
In simple terms: Objects resist changes in motion.
Newton's Second Law (F = ma)
The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
Newton's Third Law (Action-Reaction)
For every action, there is an equal and opposite reaction.
Important: The action and reaction forces act on DIFFERENT objects!
Common Forces
| Force | Symbol | Formula | Direction |
|---|---|---|---|
| Weight | W or Fg | W = mg | Toward Earth's center |
| Normal | N or Fn | ⊥ to surface | Perpendicular to surface |
| Friction | f | f = μN | Opposite to motion |
| Tension | T | Along rope/string | Away from object |
| Applied | Fa | Varies | Direction of push/pull |
Friction
Kinetic friction: f_k = μ_k N
Static friction keeps objects from moving (it's usually stronger). Kinetic friction acts on moving objects. μ_s > μ_k typically.
⚡ Energy & Work
Work
When is Work Done?
- Force parallel to motion → Positive work
- Force opposite to motion → Negative work
- Force perpendicular to motion → Zero work
Types of Energy
Conservation of Energy
The Law of Conservation of Energy
Energy cannot be created or destroyed, only transformed from one form to another.
Total Energy (initial) = Total Energy (final)
Power
- Identify initial and final states
- List all forms of energy at each state
- Apply conservation: Total E_initial = Total E_final
- Solve for the unknown
🌊 Waves & Sound
Wave Properties
| Property | Symbol | Definition | Unit |
|---|---|---|---|
| Wavelength | λ (lambda) | Distance of one complete wave | m |
| Frequency | f | Waves per second | Hz (1/s) |
| Period | T | Time for one wave | s |
| Amplitude | A | Maximum displacement | m |
| Wave Speed | v | How fast wave travels | m/s |
Types of Waves
Transverse vs Longitudinal
- Transverse: Particles move perpendicular to wave direction (light, water waves)
- Longitudinal: Particles move parallel to wave direction (sound waves)
Sound
Sound Wave Properties
- Sound is a longitudinal mechanical wave
- Requires a medium (cannot travel in vacuum)
- Speed in air ≈ 343 m/s at 20°C
- Pitch ∝ frequency (higher f = higher pitch)
- Loudness ∝ amplitude (larger A = louder)
Electromagnetic Spectrum
Radio, Microwave, Infrared, Visible, UV, X-ray, Gamma
✏️ Practice Problems
Answer: v = 15 m/s
Answer: a = 4 m/s²
Answer: v = 20 m/s downward
Answer: v = 340 m/s, traveling through air
💡 Tips & Tricks
- Read the problem carefully
- Draw a diagram
- List knowns and unknowns
- Choose the right equation
- Solve algebraically first, then plug in numbers
- Check units and reasonableness
ALWAYS draw free body diagrams for force problems. Show all forces acting on the object as arrows starting from the center.
Convert everything to SI units (m, kg, s, N, J) before calculating. Check that your final answer has the right units.
Use SUVAT to track which variables you know!
⚠️ Common Mistakes to Avoid
Velocity, acceleration, and force are vectors! Always consider direction. A negative sign usually means opposite to your chosen positive direction.
Kinematic equations only work for CONSTANT acceleration. Free fall? Yes. Car speeding up then slowing down? Split into parts!
Mass (kg) is the amount of matter. Weight (N) is the force of gravity: W = mg. They're related but not the same!
Speed is always positive. Velocity can be negative (opposite direction). Be careful which one the problem asks for!
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