Physics — Sample Question Bank

acceleration should be zero

Reason — A body is said to be in dynamic equilibrium when it is moving with uniform velocity. It means that the body is not at rest, its velocity is constant and hence its acceleration is zero.

Therefore, for dynamic equilibrium, acceleration should be zero.

(a) At the first surface, from air to glass:

$\frac{\text {sin i}_1}{\text {sin r}_1} = \text μ$

where μ is the refractive index of glass with respect to air.

At the second surface, from glass to air:

$\frac{\text {sin i}_2}{\text {sin r}_2} = \frac{1}{\text μ}$ .

Therefore, the product of the two ratios is given by,

$\frac{\text {sin i}_1}{\text {sin r}_1}\times \frac{\text {sin i}_2}{\text {sin r}_2} = \text μ \times \frac{1}{\text μ} = 1$

Hence, the product of the ratio is 1.

(b) The opposite surfaces are not parallel.

(c) If the opposite surfaces were parallel, the angle of refraction at the first surface would be equal to the angle of incidence at the second surface and since these angles are not equal, the surfaces cannot be parallel.

have only rotational motion

Reason — Forces are equal and opposite, but not along same straight line.

So, the forces will form a couple and the body will have rotational motion only.

(a)

From the above diagram, it is visible that inside the prism, the angles of incidence of rays I, J and H are respectively 30˚, 60˚ and 30˚. Hence only the ray J will suffer total internal reflection.

(b)

kinetic energy to sound energy

Reason — When you ring a cycle bell, you apply a force (mechanical/kinetic energy) by pressing or rotating the lever. This mechanical movement causes a metal striker to hit the bell, producing sound energy.

Given,

Object distance (u) = - 36 cm

As, image is formed on a screen then,

Image distance (v) = + 72 cm

Let, focal length be f.

From lens formula,

$$\begin{aligned}\frac{1}{\text f} = \frac{1}{\text v} - \frac{1}{\text u} \\[1 em] \frac{1}{\text f} = \frac{1}{72}-\frac{1}{(-36)}=\frac{1}{72}+\frac{1}{36}= \frac{1+2}{72} \\[1em] \frac{1}{\text f} = \frac{3}{72} \\[1 em] \text f = \frac{72}{3} = + 24 \text { cm}\end{aligned}$$

Now,

$\text {Power} = \frac{1}{\text {f (in metre)}} = \frac{1}{24\times 10^{-2}}=\frac{100}{24} ≈ 4.17\ \text D$

Negative

Reason — If the effect on the body is to turn it clockwise, moment of force is called the clockwise moment and it is taken as negative, while if the effect on the body is to turn it anticlockwise, moment of force is called the anticlockwise moment and it is taken as positive.

(a) Given,

angle of prism (A) = 60° for an equilateral prism.

angle of incidence (i₁) = 65°

Angle of deviation (δ) = 43°

𝑒 = i₂ = ?

From formula,

Angle of deviation (δ) = 𝑖 + 𝑒 − 𝐴

Substituting the values we get:

43 = 65 + i₂ - 60°

i₂ = 43 - 5 = 38°

Hence, i₂ = 38°

(b) Emergent ray

(i) The moment of force is equal to the product of the magnitude of the force and the perpendicular distance of the line of action of force from the axis of rotation.

(ii) The S.I. and C.G.S. unit of moment of force are related as follows —

1 N m = 10⁵ dyne x 10² cm = 10⁷dyne cm

One kilowatt hour is the electrical energy consumed by an electrical appliance of power 1 kilowatt when it is used for 1 hour.

1 kilowatt hour = 1 kilowatt x 1 hour
= 1000 J s^-1 x 3600 s = 3.6 x 10⁶ J