A bar is allowed to slide on a vertical rail system, pulling a film of soap as it slides (as shown in the figure above). It is found that with a length of ?= 25 cm and a film height of ?= 20 cm, the bar can be made to remain suspended without sliding. If the surface tension of the soap film is ?= 0.4 N/m, find the mass of the slide bar. (a) If the bar was pulled down until = 30, could it be made to remain stationary at that position? A. No. A larger ? will increase the pull on the bar. B. Yes. The balance of forces doesn't depend on ?. C. Yes. A larger ? will decrease the pull on the bar. D. No. More energy is required to pull a larger film.
under equilibrium condition
w = ?(2L)
mg = ?(2L)
m = ?(2L)/g
=0.4 (2 (0.25)/9.8
=0.0204 kg
=20.4 grams
the slider is beyond equilibrium position so it will not be in balanced state . as a large film will require more energy to strectch , hence when released the slider will move upwards .
hence option(D) is correct
No. More energy is required to pull a larger film. is the correct answer
A bar is allowed to slide on a vertical rail system, pulling a film of soap...