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JEE Advanced 1981 · Units and Measurements

A gas bubble, from an explosion under water, oscillates with a period TT proportional to padbEcp^{a}d^{b}E^{c}. Where 'PP' is the static pressure, 'dd' is the density of water and 'EE' is the total energy of the explosion. Find the values of aa, bb and cc.

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Written solution

Step 1: Identify the given relation.

The period of oscillation TT is proportional to the static pressure pp, density of water dd, and total energy EE.

T∝padbEcT \propto p^{a}d^{b}E^{c}

Step 2: Determine the dimensions of each physical quantity.

Find the dimensional formulas for time period, pressure, density, and energy.

[T]=M0L0T1[T] = M^{0}L^{0}T^{1} [p]=[F][A][p] = \frac{[F]}{[A]} [p]=MLT−2L2[p] = \frac{MLT^{-2}}{L^{2}} [p]=ML−1T−2[p] = ML^{-1}T^{-2} [d]=[m][V][d] = \frac{[m]}{[V]} [d]=ML3[d] = \frac{M}{L^{3}} [d]=ML−3[d] = ML^{-3} [E]=[F][x][E] = [F][x] [E]=(MLT−2)(L)[E] = (MLT^{-2})(L) [E]=ML2T−2[E] = ML^{2}T^{-2}

Step 3: Set up the dimensional equation.

Substitute the dimensional formulas into the proportionality relation.

[T]=[p]a[d]b[E]c[T] = [p]^{a}[d]^{b}[E]^{c} M0L0T1=(ML−1T−2)a(ML−3)b(ML2T−2)cM^{0}L^{0}T^{1} = (ML^{-1}T^{-2})^{a}(ML^{-3})^{b}(ML^{2}T^{-2})^{c} M0L0T1=Ma+b+cL−a−3b+2cT−2a−2cM^{0}L^{0}T^{1} = M^{a+b+c}L^{-a-3b+2c}T^{-2a-2c}

Step 4: Equate the exponents of MM, LL, and TT.

Compare the powers on both sides of the equation.

a+b+c=0a + b + c = 0 −a−3b+2c=0-a - 3b + 2c = 0 −2a−2c=1-2a - 2c = 1

Step 5: Solve the system of equations.

Simplify the third equation to find a relation between aa and cc.

a+c=−12a + c = -\frac{1}{2}

Substitute this relation into the first equation to solve for bb.

−12+b=0-\frac{1}{2} + b = 0 b=12b = \frac{1}{2}

Substitute the value of bb into the second equation.

−a−32+2c=0-a - \frac{3}{2} + 2c = 0 −a+2c=32-a + 2c = \frac{3}{2}

Express aa in terms of cc.

a=−12−ca = -\frac{1}{2} - c

Substitute this expression into the modified second equation.

−(−12−c)+2c=32-\left(-\frac{1}{2} - c\right) + 2c = \frac{3}{2} 12+3c=32\frac{1}{2} + 3c = \frac{3}{2} 3c=13c = 1 c=13c = \frac{1}{3}

Solve for aa using the value of cc.

a=−12−13a = -\frac{1}{2} - \frac{1}{3} a=−56a = -\frac{5}{6}

Answer: a=−56a = -\dfrac{5}{6}, b=12b = \dfrac{1}{2}, c=13c = \dfrac{1}{3}

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