MCAT Physics 4 — Questions and Answers
Question 1: A concave mirror has a focal length of 10 cm. An object is placed 30 cm from the mirror. Where is the image formed?
- 15 cm in front of the mirror (Correct answer)
- 10 cm behind the mirror
- 20 cm in front of the mirror
- 5 cm behind the mirror
Correct answer: 15 cm in front of the mirror
Using 1/f = 1/do + 1/di: 1/10 = 1/30 + 1/di → 1/di = 1/10 − 1/30 = 2/30, di = 15 cm in front.
Question 2: A capacitor with capacitance 4 μF is connected to a 12 V battery. What charge is stored on the capacitor?
- 3 μC
- 48 μC (Correct answer)
- 0.33 μC
- 16 μC
Correct answer: 48 μC
Q = CV = 4 × 10⁻⁶ × 12 = 48 × 10⁻⁶ C = 48 μC.
Question 3: An object of mass m traveling at velocity v collides with an identical stationary object and the two stick together. What is their combined velocity?
- v
- v/2 (Correct answer)
- 2v
- v/4
Correct answer: v/2
By conservation of momentum, mv = 2m·v_f, so v_f = v/2.
Question 4: Which of the following is true about the electric field inside a conductor in electrostatic equilibrium?
- It points radially outward
- It is equal to the surface charge density divided by ε₀
- It is zero (Correct answer)
- It equals kQ/r²
Correct answer: It is zero
In electrostatic equilibrium, charges redistribute to the surface, resulting in zero electric field inside the conductor.
Question 5: A sound wave in air has an intensity of 10⁻⁶ W/m². What is the sound level in decibels? (I₀ = 10⁻¹² W/m²)
- 40 dB
- 60 dB (Correct answer)
- 80 dB
- 100 dB
Correct answer: 60 dB
β = 10 log(I/I₀) = 10 log(10⁻⁶/10⁻¹²) = 10 log(10⁶) = 10 × 6 = 60 dB.
Question 6: A radioactive isotope has a half-life of 5 years. What fraction of the original sample remains after 20 years?
- 1/2
- 1/8
- 1/16 (Correct answer)
- 1/4
Correct answer: 1/16
20 years = 4 half-lives; remaining fraction = (1/2)⁴ = 1/16.
Question 7: A uniform rod of length L and mass m is pivoted at one end. What is its moment of inertia about the pivot?
- mL²
- mL²/2
- mL²/3 (Correct answer)
- mL²/4
Correct answer: mL²/3
For a uniform rod pivoted at one end, I = mL²/3.
MCAT Physics 4
A concave mirror has a focal length of 10 cm.
An object is placed 30 cm from the mirror.
Where is the image formed?