Physics Questions & Answers - Free Online Practice

701.

Which of the following representations is correct for an atom X with 28 electrons and 30 neutrons?

$^{58}_{28}X$

$^{30}_{28}X$

$^{28}_{30}X$

$^{58}_{30}X$

Correct answer is A

An atom can be represented thus,

$^{Z}_{A}X$ where Z = mass number

A = atomic number

A = number of proton = number of electrons in a free state = 28

Z = number of protons + number of neutrons = 28 + 30 = 58

$^{58}_{28}X$ is the representation

702.

The volume of a fixed mass of gas is 10cm³ when its pressure is 400cmHg. Determine the volume of the gas when its pressure is 200cmHg.

30cm³

40cm³

50cm³

20cm³

View answer & explanation

Correct answer is D

Using boyle's law

P₁V₁ = P₂V₂

V₂ = $\frac{P_1 V_1}{200}$ = $\frac{400 \times 10}{200}$

= 20cm³

703.

The velocity ratio of an inclined plane where angle of inclination in θ is

Sinθ

Cosθ

Tanθ

$\frac{1}{\sin \theta}$

View answer & explanation

Correct answer is D

Consider an inclined plane shown below, as the effort moves along OB, the load moves is lifted up through a vertical height AB

V.R = $\frac{\text{distance moved by effort}}{\text{distance moved by load}}$ = $\frac{OB}{AB}$

But Sinθ = $\frac{opp}{hyp}$ = $\frac{AB}{OB}$

therefore; = $\frac{OB}{AB}$ = $\frac{1}{\sin \theta}$

704.

Which of the following is not a mechanical wave?

wave in closed pipe

radio waves

water waves

sound waves

View answer & explanation

Correct answer is B

Mechanical waves require material medium for their propagation only radio waves among the options docs require a material medium

705.

Calculate the time taken for a mango fruit that fall from the tree 20 m to the ground [g = 10 ms^-2]

10s

View answer & explanation

Correct answer is D

u = , H = 20, g = 10

From equation of motion under gravity

H = ut ± $\frac{1}{2}$ gt², It is falling so g is +ve

H = 0 + $\frac{1}{2}$ gt²

H = $\frac{1}{2}$ gt²

2H = gt²

$\frac{2H}{g}$ = t²

t = $\frac{\sqrt{2H}}{g}$

= $\frac{\sqrt {2 \times 20}}{10}$

= √4

= 2s