Q.1. Three sound waves of equal amplitudes have frequencies (v – 1), v, (v + 1). They superpose to give beats. The number of beats produced per second will be:

1. 1

2. 4

3. 3

4. 2

Sol: Variation of intensity due to superposition of waves having frequencies v -1 & v and v & v + 1 is as shown

Variation of intensity due to superposition of waves having frequencies v – 1 and v + 1 is as shown

Resultant of (a) and (b) be as shown So number of beats/s = 2

Answer: (4)

Q.2. The height at which the acceleration due to gravity becomes g/9(where g = the acceleration due to gravity on the surface of the earth) in terms of R, the radius of the earth is:

Q.3. One kg of a diatomic gas is at a pressure of 8x10⁴ N/m². The density of the gas is 4 kg/m³. What is the energy of the gas due to its thermal motion?

1. 8 x 10⁴ J
2. 3 x 10⁴ J
3. 5 x 10⁴ J
4. 6 x 10⁴ J

Sol: 

K = 5/2pV = 5/2 x 8 x 10⁴ x ¼ = 5 x 10⁴ J

Directions: Question numbers 34, 35 and 36 are based on the following paragraph.

Two moles of helium gas are taken over the cycle ABCDA, as shown in the P-T diagram.

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