β‘ Physics Study Guide
Common mistake: Without force, objects stop moving.
Reality: Galileo and Newton showed that without force, objects maintain their current state. A stationary object stays still; a moving object keeps moving at constant speed in a straight line (inertia). Objects stop on Earth because friction is a force acting on them.
- Basic algebra β solving equations, unit conversion
- Vectors basics β quantities with magnitude and direction (optional)
1. Newton's Three Laws of Motion
First Law β Law of Inertia
- Inertia is an object's resistance to changes in motion.
- Greater mass β greater inertia.
- Example: A passenger lurches forward when a car brakes β inertia keeps the passenger moving forward.
Second Law β F = ma AP Exam
F: net force (N), m: mass (kg), a: acceleration (m/sΒ²)
\(F = 10 \times 3 = 30 \text{ N}\)
Third Law β Action-Reaction
- The forces act on different objects β they do NOT cancel.
- Example: A rocket expels gas downward (action) β the rocket is pushed upward (reaction).
2. Kinematics β Equations of Motion
These equations apply to uniform acceleration (constant acceleration).
vβ = initial velocity, v = final velocity, a = acceleration, t = time, x = displacement
Free Fall AP Exam
Near Earth's surface, all objects fall with the same acceleration (ignoring air resistance).
\(45 = \dfrac{1}{2}(9.8)t^2 \Rightarrow t^2 = \dfrac{90}{9.8} \approx 9.18 \Rightarrow t \approx 3.03 \text{ s}\)
3. Work, Energy, and Power
Work
W: work (J), F: force (N), d: displacement (m), ΞΈ: angle between force and displacement
Kinetic and Potential Energy
Conservation of Energy
Power
P: power (W = J/s), W: work (J), t: time (s)
4. Waves
Waves transfer energy through a medium (or through space) without permanently displacing the medium.
- Wavelength (Ξ»): distance between two adjacent crests (m)
- Frequency (f): number of cycles per second (Hz)
- Period (T): time for one full cycle; \(T = \dfrac{1}{f}\)
- Amplitude (A): maximum displacement from equilibrium; determines energy
- Transverse: vibration β₯ to wave direction. Examples: light, water waves, guitar strings.
- Longitudinal: vibration β₯ to wave direction. Example: sound waves (compressions and rarefactions).
Speed of Light: \(c = 3 \times 10^8\) m/s in vacuum
5. Electricity
Ohm's Law
V: voltage (volts, V), I: current (amperes, A), R: resistance (ohms, Ξ©)
Series and Parallel Circuits
Parallel Circuit \[ \frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \cdots \] Same voltage across all resistors; current divides.
Power in Circuits
6. Practice Problems
- A 5 kg box is pushed with 30 N on a frictionless surface. Find its acceleration.
- A car starts from rest and reaches 60 m/s in 10 s. Find the distance traveled.
- A 2 kg book falls from 5 m. Find its speed just before hitting the ground (g = 9.8 m/sΒ²).
- A wave has frequency 440 Hz and wavelength 0.78 m. Find its speed.
- A 9V battery powers a bulb with resistance 18 Ξ©. Find the current and power.
- \(a = F/m = 30/5 = \mathbf{6 \text{ m/s}^2}\)
- \(x = \frac{1}{2}(0+60) \times 10 = \mathbf{300 \text{ m}}\)
- \(v = \sqrt{2gh} = \sqrt{2 \times 9.8 \times 5} = \sqrt{98} \approx \mathbf{9.9 \text{ m/s}}\)
- \(v = 440 \times 0.78 = \mathbf{343.2 \text{ m/s}}\)
- \(I = 9/18 = 0.5 \text{ A}\), \(P = 9 \times 0.5 = \mathbf{4.5 \text{ W}}\)
- Newton's 2nd Law: F=ma β units: N = kgΒ·m/sΒ²
- Kinematics: identify which variable is missing, then pick the right equation
- Conservation of energy: KE+PE = constant (no friction)
- Ohm's Law V=IR; series R adds up; parallel: 1/R total = sum of 1/R
Review this material at increasing intervals to commit it to long-term memory.