NVAEasyJEE 2023Nuclear Fission & Fusion

JEE Physics 2023 Question with Solution

A nucleus disintegrates into two smaller parts, which have their velocities in the ratio 32\frac{3}{2}. The ratio of their nuclear sizes will be (x3)1/3\left(\frac{x}{3}\right)^{1/3}. The value of xx is:

Answer

Correct answer:2

Step-by-step solution

Standard Method

Given: A nucleus disintegrates into two parts with velocity ratio

v1v2=32\frac{v_1}{v_2} = \frac{3}{2}

Find: The value of xx if the ratio of nuclear sizes is (x3)1/3\left(\frac{x}{3}\right)^{1/3}.

Two daughter nuclei move in opposite directions after disintegration, labeled m1 with velocity v1 to the left and m2 with velocity v2 to the right, with v1 by v2 equal to 3 by 2.

Using conservation of momentum for the two fragments:

m1v1=m2v2m_1 v_1 = m_2 v_2

Therefore,

m2m1=v1v2=32\frac{m_2}{m_1} = \frac{v_1}{v_2} = \frac{3}{2}

Since nuclear mass density is constant, mass is proportional to volume. Hence,

m1m2=43πr1343πr23\frac{m_1}{m_2} = \frac{\frac{4}{3}\pi r_1^3}{\frac{4}{3}\pi r_2^3}

So,

(r2r1)3=m2m1\left(\frac{r_2}{r_1}\right)^3 = \frac{m_2}{m_1}

Thus,

(r2r1)3=32\left(\frac{r_2}{r_1}\right)^3 = \frac{3}{2}

Taking cube root,

r2r1=(32)1/3=(23)1/3\frac{r_2}{r_1} = \left(\frac{3}{2}\right)^{1/3} = \left(\frac{2}{3}\right)^{-1/3}

Matching with the given form (x3)1/3\left(\frac{x}{3}\right)^{1/3}, we get

x=2x = 2

Therefore, the required value is 22.

Direct Proportionality Trick

Given: v1v2=32\frac{v_1}{v_2} = \frac{3}{2}

Find: The value of xx in the size ratio expression.

From momentum conservation in two-body breakup,

m1v1=m2v2m_1 v_1 = m_2 v_2

so masses are inversely proportional to velocities. Hence,

m2m1=v1v2=32\frac{m_2}{m_1} = \frac{v_1}{v_2} = \frac{3}{2}

For nuclei, radius varies as the cube root of mass:

rm1/3r \propto m^{1/3}

Therefore,

r2r1=(m2m1)1/3=(32)1/3\frac{r_2}{r_1} = \left(\frac{m_2}{m_1}\right)^{1/3} = \left(\frac{3}{2}\right)^{1/3}

Comparing with (x3)1/3\left(\frac{x}{3}\right)^{1/3} gives

x=2x = 2

The correct answer is 22.

Common mistakes

  • Using m1m2=v1v2\frac{m_1}{m_2} = \frac{v_1}{v_2} is incorrect. From momentum conservation, mass is inversely related through m1v1=m2v2m_1 v_1 = m_2 v_2. First write the momentum equation, then form the correct mass ratio.

  • Assuming radius is directly proportional to mass is wrong. Nuclear radius follows rm1/3r \propto m^{1/3} because nuclear density is approximately constant. Convert mass ratio to radius ratio using cube root, not direct proportion.

  • Comparing the obtained ratio with (x3)1/3\left(\frac{x}{3}\right)^{1/3} carelessly can reverse numerator and denominator. After finding the radius ratio, match the inside of the bracket exactly before reading off xx.

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