A soap bubble is blown to a diameter of 7 cm. Then 36960 erg of work is done in blowing it further. If the surface tension of the soap solution is 40 dyne cm⁻¹, then the new radius is cm. (Take π=(22)/7)

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7. Given: initial diameter 7 cm, so r₁=3.5 cm; W=36960 erg; T=40 dyne cm⁻¹. Erg, dyne and centimetre belong to the same CGS system, so no conversion is needed. Two surfaces on the bubble: W=8π T(r₂²-r₁²). 8π T=8×(22)/7×40=1005.7 dyne cm⁻¹, so r₂²-r₁²=(36960)/(1005.7)=36.75 cm². r₂²=36.75+12.25=49 cm². r₂=7 cm.

A soap bubble is blown to a diameter of 7 cm. Then 36960 erg of work is done in blowing it further. If the surface tension of the soap solution is 40 dyne cm⁻¹, then the new radius is ______ cm. (Take π=(22)/7)

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