Question

1. the maximum frequency of incidence radiation on a metal surface below which no electron is emitted from the surface of a metal is called

a. threshold frequency.
b. critical frequency.
c. maximum frequency.
d. resonance frequency.

1. convection takes place in

a. solid and liquid only.
b. solid and gas only.
c. liquid and gas only.
d. liquid only.

1. the s.i. unit for work is derived from the combination of the units

a. foot, pound and second.
b. kilogram, metre and second.
c. gram, metre and second.
d. kilogram, centimetre and second.

1. which of the following statements about a curved mirror is correct?

a. the pole of a curved mirror is at the center of its reflecting surface.
b. a converging mirror diverges parallel rays from a point call real focus.
c. a curved mirror converges parallel rays to a point called virtual focus.
d. the focal length of a spherical mirror is twice the radius of curvature.

1. a cargo truck of mass 8000 kg is moving at a constant speed of 15 ms⁻¹. the brakes are suddenly applied and the truck comes to rest in 3 s. determine the average braking force.

a. 40 kn
b. 80 kn
c. 120 kn
d. 24 kn

Explanation:

Question 21

The threshold frequency is defined as the minimum (not maximum, note the question might have a typo but the concept is threshold) frequency of incident radiation below which no photoelectrons are emitted from a metal surface. Option A matches this definition. Critical frequency is for ionosphere, maximum frequency is incorrect term here, resonance frequency is for oscillations.

Convection is heat transfer by movement of fluid (liquid or gas) particles. Solids don't have fluid-like movement for convection (heat in solids is mainly conduction). So convection occurs in liquid and gas only.

Work \( W = F \times d \), force \( F = ma \), so units of work: \( \text{kg} \times \text{m/s}^2 \times \text{m}=\text{kg·m}^2/\text{s}^2 \) (Joule). So derived from kilogram, metre, second. Foot - pound - second is imperial, gram is not SI base for mass (SI base is kg), centimetre is not SI base for length (SI base is m).

Answer:

A. threshold frequency.

Question 22