QUIZ 12

1. A U-shaped wire apparatus is holding a film of water as shown in figure. How much work is needed to pull the wire 0.6mm to the right? Take the surface tension of water as 0.072N/m.

Solution. We find the work from                 W = gDA = gDxL = (0.072N/m)(0.6 x 10-3m)(0.0125m) = 5.4 x 10-7J.

2. A 500-g block is hanged at the end of a light spring. When the block vibrates the frequency is 0.25Hz. What is the spring constant? Assume SHM and neglect the air resistance.

Solution. We find the spring constant from

          f = 1/T = 1/[2p(M/K)^1/2],

         0.25Hz = 1/[2p(0.5kg/K)^1/2],

    which gives K = 1.23N/m.