# KVPY (Kishore Vaigyanik Protsahan Yojana) Stream SB-SX (Class 12 & 1st Year B.Sc.) Physics: Questions 202 - 204 of 268

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## Question number: 202

MCQ▾

### Question

A parent nucleus X is decaying into daughter nucleus Y which in turn decays to Z. The half-lives of X and Y are 6000 years and 30 years respectively. In a certain sample, it is found that the number of Y nuclei hardly change with time. If the number of X nuclei in the sample is the number of Y nuclei present in it is –

### Choices

Choice (4) Response

a.

b.

c.

d.

## Question number: 203

MCQ▾

### Question

A stream of photons having energy 4.5 eV each impinges on a magnesium surface. The work function of magnesium is 3.66 eV. The emerging photo – electrons are slowed down by copper plates placed 10 mm away. If the potential difference between the two metal plates is 1 V, the maximum distance the electrons can move away from the magnesium surface before being turned back is –

### Choices

Choice (4) Response

a.

8.3 mm

b.

5.0 mm

c.

7.0 mm

d.

3.5 mm

## Question number: 204

Appeared in Year: 2015

MCQ▾

### Question

A horizontal disk of moment of inertia 4.25 kg-m2 with respect to its axis of symmetry is spinning counter clockwise at 15 revolutions per second about its axis, as viewed from above. A second disk of moment of inertia 1.80 kg-m2 with respect to its axis of symmetry is spinning clockwise at 25 revolutions per second as viewed from above about the same axis and is dropped on top of the first disk. The two disks stick together and rotate as one about their axis of symmetry. The new angular velocity of the system as viewed from above is close to.

### Choices

Choice (4) Response

a.

18 revolutions per second and counter clockwise

b.

3 revolutions per second and clockwise

c.

3 revolutions per second and counter clockwise

d.

18 revolutions per second and clockwise

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