Physics Flashcards

A concave mirror has a focal length of 10 cm. An object is placed 30 cm from the mirror. Where is the image formed?

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.

A capacitor with capacitance 4 μF is connected to a 12 V battery. What charge is stored on the capacitor?

Answer: 48 μC

Q = CV = 4 × 10⁻⁶ × 12 = 48 × 10⁻⁶ C = 48 μC.

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?

Answer: v/2

By conservation of momentum, mv = 2m·v_f, so v_f = v/2.

Which of the following is true about the electric field inside a conductor in electrostatic equilibrium?

Answer: It is zero

In electrostatic equilibrium, charges redistribute to the surface, resulting in zero electric field inside the conductor.

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)

Answer: 60 dB

β = 10 log(I/I₀) = 10 log(10⁻⁶/10⁻¹²) = 10 log(10⁶) = 10 × 6 = 60 dB.

A radioactive isotope has a half-life of 5 years. What fraction of the original sample remains after 20 years?

Answer: 1/16

20 years = 4 half-lives; remaining fraction = (1/2)⁴ = 1/16.

A uniform rod of length L and mass m is pivoted at one end. What is its moment of inertia about the pivot?

Answer: mL²/3

For a uniform rod pivoted at one end, I = mL²/3.