© Department of Physics, University of Guelph
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1. Which of the following represents a sketch
of terminal velocity vs. diameter of the sedimenting particle?
2. When a sedimenting particle reaches its terminal velocity, which of the following is true?
(Fg = weight of particle; Fb = buoyant force; Fv = frictional force)
(a) Fg + Fb = Fv
(b) Fg + Fv = Fb
(c) Fg > Fv + Fb
(d) Fg = Fv +Fb
(e) Fg + Fb < Fv
3. A protein of molar mass 6.0 x 106
g mol-1 and density 1.33 x 103 kg m-3
is in an ultracentrifuge tube (filled with water) at a distance of 10 cm
from the rotation axis. The rotor is spinning at 60000 rpm. What is the
centrifugal "force" on a protein molecule?
4. A centrifuge tube is filled with a cesium chloride solution prepared
in such a way that there is density gradient down the tube, i.e., the solution
is more dense towards the bottom of the tube. The figures on the diagram
indicate the specific gravity at various points.
Some polysaccharide of density 1280 kg m-3 is added at the top and the tube is spun at very high speed for a day.
(a) Using the equation
sketch a graph of the terminal speed of the polysaccharide vs. depth in the tube. (Notice that rlchanges linearly with depth.) Assume "g" is constant, independent of depth.
(b) What value of rl reduces vT to zero?
(c) Show on the diagram where the polysaccharide will be found after the day's spinning. Why will it be found there?
(d) The amount of settling of the
cesium chloride in the solution is negligible. Why?