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21.

A freshly prepared sample of a radioisotope of half-life 1386 s has activity 103  disintegrations per second. Given that ln 2=0.693, the fraction of the initial number of nuclei (expressed in nearest integer percentage) that will decay in the first 80s after preparation of the sample is

A) 4

B) 5

C) 6

D) 2

22.

A particle of mass 0.2 kg moving in one dimension under a force that delivers a  constant power of 0.5W to the particle. If the initial speed (in ms-1)  of the particle is zero, the  speed (in ms-1) after 5s  is

A) 4

B) 5

C) 8

D) 6

23.

A bob of mass m,  suspended by a string of length l1,  is given a minimum velocity required to complete a full circle in the vertical plane, At the highest point, it collides elastically with another bob of mass m  suspended by a string of length l2, which is initially at rest, Both the strings are massless and inextensible. If the second bob. After collision acquires the minimum  speed  required  to  complete  a full circle in the vertical  plane, the ratio  $\frac{l_{1}}{l_{2}}$  is

A) 2

B) 3

C) 4

D) 5

24.

The work functions   of silver and sodium are 4.6 and 2.3 eV, respectively The  ratio of the slope of the stopping potential versus frequency plot for silver to that of sodium is

A) 1

B) 4

C) 2

D) .3

25.

The uniform circular disc of mass 50 kg and radius 0.4 m is rotating with an angular velocity of 10 rad/s about its own axis, is vertical. Two uniform circular rings, each of mass 6.25 kg and radius 0.2 m, are genty placed symmetrically on the disc in such a manner that they are touching each other along the axis of the disc and are horizontal. Assume that the friction is large enough such that the rings are at rest relative to the disc and the system rotates about the original axis.  The new angular velocity (in rad s-1) of the system is

A) 4

B) 8

C) 6

D) 5

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