A nucleus disintegrates into two smaller parts, which have their velocities in the ratio . The ratio of their nuclear sizes will be . The value of is:
JEE Physics 2023 Question with Solution
Answer
Correct answer:2
Step-by-step solution
Standard Method
Given: A nucleus disintegrates into two parts with velocity ratio
Find: The value of if the ratio of nuclear sizes is .

Using conservation of momentum for the two fragments:
Therefore,
Since nuclear mass density is constant, mass is proportional to volume. Hence,
So,
Thus,
Taking cube root,
Matching with the given form , we get
Therefore, the required value is .
Direct Proportionality Trick
Given:
Find: The value of in the size ratio expression.
From momentum conservation in two-body breakup,
so masses are inversely proportional to velocities. Hence,
For nuclei, radius varies as the cube root of mass:
Therefore,
Comparing with gives
The correct answer is .
Common mistakes
Using is incorrect. From momentum conservation, mass is inversely related through . First write the momentum equation, then form the correct mass ratio.
Assuming radius is directly proportional to mass is wrong. Nuclear radius follows because nuclear density is approximately constant. Convert mass ratio to radius ratio using cube root, not direct proportion.
Comparing the obtained ratio with carelessly can reverse numerator and denominator. After finding the radius ratio, match the inside of the bracket exactly before reading off .
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