Physics Test 219

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Physics Test 219

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Weekly Quiz Competition

Question 1
4 / -1

Which of the following pairs of physical quantities does not have same dimensional formula ?

A
Work and torque

B
Angular momentum and Planck's constant

C
Tension and surface tension

D
Impulse and linear momentum

Solution

Tension=[MLT⁻²]

Surface Tension = [ML⁰T⁻²]

Clearly these two have different dimension.
Question 2
4 / -1

A plank is held at an angle α to the horizontal on two fixed supports A and B. The plank can slide against the supports (without friction) because of its weight Mg. Acceleration and direction in which a man of mass mm should move so that the plank does not move is:

A
gsinα(1 + m/M) down the incline

B
gsinα(1 + M/m) up the incline

C
gsinα(1 + m/M) up the incline

D
gsinα(1 + M/m) down the incline

Solution

F.B.D. of man and plank are

For plank to be at rest, applying Newton's second law to plank along the incline

Mgsinα = f

and applying Newton's second law to man along the incline.

mgsinα + f = ma

a = gsinα(1 + M/m) down the incline
Question 3
4 / -1

A force is acting at a point The value of α for which angular momentum about origin is conserved is:

A
2

B
Zero

C
1

D
-1

Solution
Question 4
4 / -1

A simple pendulum of length ll is suspended from the ceiling of a cart which is sliding without friction on an inclined plane of inclination θ. What will be the time period of the pendulum?

A

B

C

D

Solution

Here, the point of suspension has an acceleration. (down the plane). Further, can be resolved into two components gsinθ (along the plane) and gcosθ (perpendicular to the plane).
Question 5
4 / -1

The frequency of a sonometer wire is f but when the weights producing tension are completely immersed in water the frequency becomes f/2 and on immersing the weights in certain liquid the frequency becomes f/3. The specific gravity of the liquid is

A
1.320

B
1.333

C
0.889

D
1.185

Solution
Question 6
4 / -1

A heat engine has an efficiency η. Temperatures of source and sink are each decreased by 100 K. Then, the efficiency of the engine.

A
Increases

B
Decreases

C
Remains constant

D
Becomes 1

Solution

where T₁ and T₂ are the temperatures of a source and sink respectively.

When T₁ and T₂ both are decreased by 100 K each, (T₁ - T₂) stays constant. T₁ decreases.

∴ η increases.
Question 7
4 / -1

Which of these is correct in regard to a magnet?

A
Geometric length = 0.8 times the magnetic length

B
Magnetic length = 0.8 times the geometric length

C
Magnetic length= Geometric length

D
Geometric length = 10/9 of magnetic length

Solution

Magnetic length is the distance between the two equal and opposite poles of the magnet.

Poles are not actually at the end of the magnet they are little inside from the end this makes the magnetic length of the magnet little less than the Geometric length.

Geometric length is the actual length of the magnet.

So when the relationship between these two measured it comes to be,

Magnetic length = 0.8× Geometric length
Question 8
4 / -1

The mean lives of radioactive substances are 1620 y and 405 y for α- emission and β- emission respectively. Find out the time during which three-fourth of a sample will decay if it is decaying both by α- emission and β- emission simultaneously.

A
449 y

B
399 y

C
549 y

D
579 y

Solution

Let at some instant of time tt, number of atoms of the radioactive substance are N. It may decay either by α - emission on by β- emission. So, we can write,
Question 9
4 / -1

When the velocity of an electron increases, its de-Broglie wavelength

A
increases.

B
decreases.

C
remains same.

D
may increase or decrease.

Solution

The de Broglie wavelength is given by

So if the velocity of the electron increases, the de Broglie wavelength decreases.
Question 10
4 / -1

A charge q is located at the centre of a cube. The electric flux through any face is

A

B

C

D

Solution