Physics Test

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Physics Test - 13

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Weekly Quiz Competition

Question 1
4 / -1

During projectile motion, the horizontal velocity

A
first increases then decreases

B
first decreases then increases

C
always increases

D
always constant

Solution

Projectile is the name given to a body thrown with some initial velocity with the horizontal direction, and then allowed to move in two dimensions under the action of gravity alone, without being propelled by any engine or fuel. After projection, the object will move under the combined effect of two independent perpendicular velocities.
(i) Horizontal velocity, which remains constant throughout the motion (neglecting the air friction).
(ii) Vertical velocity which increases due to gravity. Initial value of this velocity at origin is zero.
Question 2
4 / -1

The depletion layer in a p-n junction diode is 10-6m wide and its knee potential is 0.5 V. What is the inner electric field in the depletion region?

A
5 x (10)6 $\frac{V}{M}$

B
5 x (10)-7 $\frac{V}{M}$

C
5 x (10)5 $\frac{V}{M}$

D
None of these

Solution

In both forward biasing and reverse biasing, applied potential establishes an internal electric field, which acts against or towards the potential barrier. This internal electric field is weakened or strengthened at the junction. In forward voltage at which the current through the junction starts to increase rapidly, once the applied forward voltage exceeds the knee voltage, the current starts increasing rapidly. In forward biasing condition, the inner electric field is given by:

$E=\frac{\Delta V}{\Delta r}$
or $\quad|E|=\frac{\Delta V}{\Delta r}=\frac{5 \times 10^{-1}}{10^{-6}}$
$=5 \times 10^{5} \vee / m$
Question 3
4 / -1

A train of length 80 m is moving with a uniform velocity of 25 m/sec. The time taken by the train to cross a bridge 120 m long is

A
3.2 sec

B
4.8 sec

C
8 sec

D
None of these

Solution

Time taken by train to cross the bridge = (length of train + length of bridge)/ speed of the train.

Or 200/ 25 or 8 sec is the answer.
Question 4
4 / -1

An incompressible fluid flows steadily through a cylindrical pipe which has radius '2R' at a point 'A' and radius 'R' at a point 'B'. If the velocity at point 'A' is 'v' along the flow of direction, what will be its velocity at point 'B'?

A
v/4

B
2v

C
4v

D
$-\frac{v}{2}$

Solution

since, the volume of liquid flowing per second at $A$
$=$ the volume of liquid flowing per second at $B$
$\therefore \quad v \times \pi \times(2 R)^{2}=V \times \pi R^{2}$
$\Rightarrow$
$V=4 v$
Question 5
4 / -1

A sound wave reflected at the boundary will undergo a phase change on reflection at

A
all reflections

B
air water boundary

C
metal - air boundary

D
none of these

Solution

metal - air boundary.When light is reflected off a more dense medium with higher index of refraction, crests get reflected as troughs and troughs get reflected as crests. The wave is said undergo a180^o change of phase on reflection.
Question 6
4 / -1

A source is moving towards an observer with a speed of 20 m/s and having frequency 240 Hz, and the observer is moving towards the source with a velocity of 20 m/s. What is the apparent frequency heard by the observer, if velocity of sound is 340 m/s?

A
270 Hz

B
240 Hz

C
268 Hz

D
360 Hz

Solution

Key Idea The perceived frequency heard by the observer depends upon the relative motion between observer and source. If the source and observer are approaching each other, then velocity of the source $v_{s}$ is positive and velocity of the listener $v_{l}$ is negative, the perceived frequency will be higher than the original. Perceived frequency is given by
\[
n^{\prime}=n\left(\frac{v+v_{0}}{v-v_{s}}\right) \quad\left[\because n^{\prime}=n \frac{v-\left(-v_{0}\right)}{v-v_{s}}\right]
\]
Here,$\quad \nu=340 \mathrm{m} / \mathrm{s}, \nu_{0}=20 \mathrm{m} / \mathrm{s}$
\[
v_{s}=20 \mathrm{m} / \mathrm{s}, n=240 \mathrm{Hz}
\]
Hence, $n^{\prime}=240\left(\frac{340+20}{340-20}\right)$
\[
\begin{array}{l}
=240 \times \frac{360}{320} \\
=270 \mathrm{Hz}
\end{array}
\]
Note When the speed of source and observer are much lesser than that of sound, the change in frequency becomes independent of the fact whether the source is moving or the observer.
Question 7
4 / -1

Two circular coils 1 and 2 are made from the same wire, but the radius of the 1st coil is twice that of the 2nd coil. What is the ratio of potential differences applied across them, so that the magnetic field at their centres is the same?

A
3:1

B
4:1

C
6:1

D
2:1

Solution

\[

\begin{aligned}

B_{1} &=B_{2} \\

\frac{\mu_{0} l_{1}}{2(2 a)} &=\frac{\mu_{0} l_{2}}{2 a} \\

\Rightarrow \frac{1}{I_{2}} &=2

\end{aligned}

\]

For resistances of coils

\[

R_{1}=\rho \frac{2 \pi(2 a)}{A}

\]

\[

\mathrm{R}_{2}=\rho \frac{2 \pi a}{\mathrm{A}}

\]

$\begin{aligned} \Rightarrow \quad \frac{R_{1}}{R_{2}} &=2 \\ \frac{V_{1}}{V_{2}} &=\frac{I_{1} R_{1}}{I_{2} R_{2}}=\frac{I_{1}}{I_{2}} \times \frac{R_{1}}{R_{2}} \\ &=2 \times 2=4 \end{aligned}$
Question 8
4 / -1

A shell at rest, at the origin explodes into three fragments of masses 1 kg, 2 kg and m kg. The pieces weighing 1 kg and 2 kg fly off with a speed of 5 ms–1along x– axis and 6 ms–1along y– axis respectively. If the 'm' kg piece flies off with a speed of 6.5 ms–1, what must be the total mass of the shell?

A
4 kg

B
5 kg

C
3.5 kg

D
4.5 kg

Solution

We can realise the situation as shown in figure. Let the direction of mass $m$ makes angle $\theta$ with the $z$ -axis.

Resolve momentum $6.5 \mathrm{m}$ along $x$ and $y$ -axis and équate.
Therefore,

and
\[
\begin{array}{l}
6.5 m \cos \theta=5 \times 1 \\
6.5 m \sin \theta=6 \times 2
\end{array}
\]
or $(6.5 \mathrm{m})^{2}=(5)^{2}+(12)^{2}=(13)^{2}$
Or $m=\frac{13}{6.5}=2 \mathrm{kg}$
Therefore, total mass of the shell
\[
=1+2+2=5 \mathrm{kg}
\]
Question 9
4 / -1

For the given circuit to act as a full-wave rectifier, the AC input should be connectedacross ..i... and ..ii... and the DC output should appear across ...iii... and ...iv....

i, ii, iii and iv, respectively, are

A
B, D, A and C

B
B, A, C and D

C
C, A, B and D

D
C, D, B and A

Solution

For the given circuit which is acting as a full-wave rectifier, the AC input should be connected across B and D and the DC output should appear across A and C.
Question 10
4 / -1

If the velocity of light 'c', gravitational constant 'G' and Planck's constant 'h' are chosen as fundamental units, the dimensions of length 'L' in the new system are

A
[h1c1G-1]

B
[h1/2c1/2G-1/2]

C
[h1c-3G1]

D
[h1/2c-3/2G1/2]

Solution

Let $L=\left[h^{a} c^{b} G^{d}\right]$
$\therefore\left[L^{\prime}\right]=\left[\mathrm{ML}^{2} \mathrm{T}^{-1}\right]^{a}\left[\mathrm{LT}^{-1}\right]^{b}\left[\mathrm{M}^{-1} \mathrm{L}^{3} \mathrm{T}^{-2}\right]^{d}$
$\Rightarrow \quad a=\frac{1}{2}, b=-\frac{3}{2}, d=\frac{1}{2}$
Hence, $L=\left[h^{1 / 2} c^{-3 / 2} G^{1 / 2}\right]$

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Physics Test - 13

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Physics Test - 13

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SHARING IS CARING

Question 1

first increases then decreases

first decreases then increases

always increases

always constant

Solution

Question 2

5 x (10)6VM

5 x (10)-7VM

5 x (10)5VM

None of these

Solution

Question 3

3.2 sec

4.8 sec

8 sec

None of these

Solution

Question 4

v/4

2v

4v

\(-\frac{v}{2}\)

Solution

Question 5

all reflections

air water boundary

metal - air boundary

none of these

Solution

Question 6

270 Hz

240 Hz

268 Hz

360 Hz

Solution

Question 7

3:1

4:1

6:1

2:1

Solution

Question 8

4 kg

5 kg

3.5 kg

4.5 kg

Solution

Question 9

B, D, A and C

B, A, C and D

C, A, B and D

C, D, B and A

Solution

Question 10

[h1c1G-1]

[h1/2c1/2G-1/2]

[h1c-3G1]

[h1/2c-3/2G1/2]

Solution

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5 x (10)6 $\frac{V}{M}$

5 x (10)-7 $\frac{V}{M}$

5 x (10)5 $\frac{V}{M}$