MCAT Physics 3 — Questions and Answers
Question 1: A photon has a wavelength of 400 nm. What is its energy? (h = 6.63 × 10⁻³⁴ J·s, c = 3 × 10⁸ m/s)
- 4.97 × 10⁻¹⁹ J (Correct answer)
- 3.31 × 10⁻¹⁹ J
- 1.66 × 10⁻¹⁹ J
- 9.94 × 10⁻¹⁹ J
Correct answer: 4.97 × 10⁻¹⁹ J
E = hc/λ = (6.63 × 10⁻³⁴ × 3 × 10⁸) / (400 × 10⁻⁹) ≈ 4.97 × 10⁻¹⁹ J.
Question 2: An ideal gas undergoes an isothermal compression. Which of the following remains constant?
- Pressure
- Volume
- Temperature (Correct answer)
- Kinetic energy of molecules
Correct answer: Temperature
By definition, isothermal means constant temperature; kinetic energy of molecules (∝ T) also stays constant, but 'temperature' is the direct answer.
Question 3: A wave on a string has a frequency of 50 Hz and a wavelength of 0.4 m. What is the wave speed?
- 125 m/s
- 50 m/s
- 20 m/s (Correct answer)
- 0.008 m/s
Correct answer: 20 m/s
Wave speed v = fλ = 50 × 0.4 = 20 m/s.
Question 4: A 70 kg person stands on a scale in an elevator accelerating upward at 2 m/s². What does the scale read? (g = 10 m/s²)
- 560 N
- 700 N
- 840 N (Correct answer)
- 140 N
Correct answer: 840 N
Normal force N = m(g + a) = 70 × (10 + 2) = 70 × 12 = 840 N.
Question 5: Light travels from glass (n = 1.5) into air (n = 1.0). If the angle of incidence is 30°, what is the angle of refraction?
- 19.5°
- 30°
- 48.6° (Correct answer)
- 90°
Correct answer: 48.6°
By Snell's law, n₁sinθ₁ = n₂sinθ₂: 1.5 × sin30° = 1.0 × sinθ₂, so sinθ₂ = 0.75, θ₂ ≈ 48.6°.
Question 6: Which thermodynamic process occurs when a gas expands against zero external pressure (free expansion)?
- Isothermal
- Adiabatic
- Isobaric
- Neither heat nor work is exchanged with the surroundings (Correct answer)
Correct answer: Neither heat nor work is exchanged with the surroundings
In free expansion, no work is done (P_ext = 0) and for an ideal gas in an insulated container no heat is exchanged, so ΔU = 0.
Question 7: A 0.5 kg ball is thrown vertically upward with an initial velocity of 20 m/s. What maximum height does it reach? (g = 10 m/s²)
- 10 m
- 20 m (Correct answer)
- 40 m
- 5 m
Correct answer: 20 m
Using v² = v₀² − 2gh, at maximum height v = 0: h = v₀²/(2g) = 400/20 = 20 m.
MCAT Physics 3
A photon has a wavelength of 400 nm.
What is its energy? (h = 6.63 × 10⁻³⁴ J·s, c = 3 × 10⁸ m/s)