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, as the question's wording might have a typo, but the concept is the minimum frequency below which photoelectric emission doesn't occur) frequency of incident radiation below which no electrons are emitted from a metal surface. Critical frequency is used in other contexts (e.g., ionosphere), maximum frequency is incorrect as it's the minimum, and resonance frequency is for oscillatory systems. So the correct term is threshold frequency.

Convection is heat transfer by the movement of fluid (liquid or gas) particles. Solids transfer heat mainly by conduction as their particles are tightly packed and don't move freely. So convection occurs in liquids and gases only.

Work is calculated as \( W = F \times d \), and force \( F = ma \) (mass × acceleration). The SI units are mass (kilogram, kg), length (metre, m), and time (second, s). Foot - pound - second is imperial, gram is not SI base for mass (kg is), and centimetre is not SI base for length (m is). So the SI unit of work (joule) is derived from kg, m, s.

Answer:

A. threshold frequency.

Question 22