AP Physics – Wave Properties (Wavelength, Frequency, Speed, Amplitude)

AP Physics – Wave Properties (Wavelength, Frequency, Speed, Amplitude)

AP Physics: Wave Properties Study Guide

(Wavelength, Frequency, Speed, Amplitude)

What This Is

Waves are disturbances that transfer energy without transferring matter. On the AP Physics exam, you’ll need to understand how waves behave—how fast they move, how often they repeat, and how big they are. These concepts apply to sound, light, ocean waves, and even earthquakes. Example: When you strum a guitar string, it vibrates at a certain frequency (pitch) and amplitude (loudness), creating a wave that travels through the air as sound.

Key Terms & Concepts

• Wave: A disturbance that transfers energy from one place to another without transferring matter.
• Mechanical Wave: Requires a medium (e.g., sound waves in air, water waves). Electromagnetic waves (e.g., light) do not need a medium.
• Transverse Wave: Particles move perpendicular to the wave direction (e.g., light, ocean waves).
• Longitudinal Wave: Particles move parallel to the wave direction (e.g., sound waves).
• Wavelength (?): Distance between two identical points on a wave (e.g., crest to crest). Unit: meters (m).
• Frequency (f): Number of waves passing a point per second. Unit: Hertz (Hz = 1/s).
• Period (T): Time for one complete wave cycle. T = 1/f (inverse of frequency).
• Wave Speed (v): How fast the wave travels. v = f × ? (speed = frequency × wavelength).
• Amplitude (A): Maximum displacement from equilibrium (determines energy/intensity).
• Phase: Position of a point in a wave cycle (e.g., in-phase = crests align; out-of-phase = crests and troughs align).
• Superposition: When two waves overlap, their amplitudes add (constructive/destructive interference).
• Doppler Effect: Change in frequency due to relative motion (e.g., a siren’s pitch changes as it passes you).

Step-by-Step: Solving Wave Problems

1. Identify the wave type (transverse/longitudinal, mechanical/electromagnetic).
2. List given values (e.g., frequency, wavelength, speed, amplitude).
3. Choose the right formula (e.g., v = f × ? for speed, T = 1/f for period).
4. Convert units if needed (e.g., kHz-Hz, cm-m).
5. Solve for the unknown (plug in numbers and calculate).
6. Check for reasonableness (e.g., sound waves in air ~340 m/s; light ~3 × 10? m/s).

Example Problem: A sound wave has a frequency of 500 Hz and a wavelength of 0.68 m. What is its speed? - Step 1: Mechanical, longitudinal wave. - Step 2: f = 500 Hz, ? = 0.68 m. - Step 3: v = f × ?. - Step 4: Units are already in Hz and m. - Step 5: v = 500 × 0.68 = 340 m/s.

Common Mistakes

• Mistake: Confusing frequency and period. Correction: Frequency (f) is waves per second; period (T) is seconds per wave. T = 1/f.

• Mistake: Forgetting units (e.g., using cm instead of m in v = f × ?). Correction: Always convert to SI units (meters, seconds, Hertz).

• Mistake: Assuming all waves travel at the same speed. Correction: Speed depends on the medium (e.g., sound is faster in water than air; light is fastest in a vacuum).

• Mistake: Mixing up amplitude and wavelength. Correction: Amplitude = energy/intensity; wavelength = distance between crests.

• Mistake: Ignoring the Doppler effect in moving-source problems. Correction: Frequency increases if the source moves toward you, decreases if it moves away.

AP Exam Insights

• Multiple-Choice Traps:
• Questions may give period but ask for frequency (or vice versa).
• Watch for unit conversions (e.g., MHz-Hz).
• Doppler effect questions often involve relative motion (e.g., a moving ambulance).
• FRQ Trends:
• Calculate wave speed given f and ? (or vice versa).
• Explain how amplitude affects energy (e.g., louder sound = larger amplitude).
• Draw/interpret wave diagrams (label ?, A, crests, troughs).
• Tricky Distinction:
• Wave speed (v) depends on the medium (e.g., sound in air vs. water).
• Frequency (f) depends on the source (e.g., a tuning fork’s pitch).
• Wavelength (?) changes if v or f changes (? = v/f).

Quick Check Questions

1. A wave has a period of 0.02 s. What is its frequency?
2. (A) 0.02 Hz
3. (B) 20 Hz
4. (C) 50 Hz

5. (D) 500 Hz Answer: (C) 50 Hz. Frequency = 1/period = 1/0.02 s = 50 Hz.

6. A sound wave travels at 340 m/s with a frequency of 170 Hz. What is its wavelength?

7. (A) 0.5 m
8. (B) 2 m
9. (C) 57,800 m

10. (D) 0.0059 m Answer: (B) 2 m. ? = v/f = 340 m/s ÷ 170 Hz = 2 m.

11. Short FRQ: A wave has an amplitude of 0.5 m and a wavelength of 2 m. If the wave speed is 10 m/s, what is its frequency? Show your work. Answer: f = v/? = 10 m/s ÷ 2 m = 5 Hz.

Last-Minute Cram Sheet

1. v = f × ? (wave speed = frequency × wavelength).
2. T = 1/f (period = 1/frequency).
3. Amplitude (A) = energy/intensity (not speed!).
4. Sound waves are longitudinal; light waves are transverse.
5. Doppler effect: Frequency-if source moves toward you.
6. Speed depends on medium (e.g., sound faster in water than air).
7. Units: Always use meters (m), seconds (s), Hertz (Hz).
8. Superposition: Waves add (constructive) or cancel (destructive).
9. Phase: In-phase = crests align; out-of-phase = crests/troughs align.
10. Electromagnetic waves (e.g., light) don’t need a medium!