QUIZ 12

1. The force needed to just lift a circular platinum ring of radius 2.8cm from the surface of a liquid is 8.4 x 10^-1 N. What is the surface tension of the liquid? Neglect the net force due to the weight and the buoyant force.

    Solution.

   For the two surfaces , inner and outer circumferences, we have

                   F = g2L = g2(2pr);

                  8.4 x 10^-1 N = g4p(0.028m), which gives g  = 2.4N/m.

2. A small fly of mass 0.15g is caught in a spider’s web. The web vibrates with a frequency of 4.0Hz. What is the effective spring constant k of the web?

  Solution.

 We find the spring constant k from

                 f = (k/m)^1/2/2p; 

               4.0Hz = [k/(0.00015kg)]^1/2/2p, which gives k = 9.5 x 10^-2N/m.