PHYSICS 1403, QUIZ 5, Chapter 10, August 3, 2006
Partial credit will be LIBERAL, provided that you show your work!
USE BACK IF NECESSARY (Please tell me you have done so!)
Name: ______________________
1. (20 points) See Figure. A straw, length L = 0.5 m & circular cross sectional area A = 1.0  10-4 m2
is put vertically into an oil of density  = 4.0  103 kg/m3. A finger is put over the top so that no
air can get in or out, & the straw is lifted vertically out of the fluid. It retains the oil so that the
distance from the finger bottom to the top of the oil is h = 0.1 m. The air at the bottom & outside
the straw exerts a pressure PA = 1.0  105 N/m2 which produces an upward force FA on the oil.
The air in the space between the finger & the oil top exerts an unknown pressure P which
produces a downward force F on the oil. The oil in the straw remains STATIC. (Hint: This
problem doesn’t use Archimedes’ Principle!! Note: The numbers in this problem aren’t realistic!)
a. Compute the mass m of the oil in the straw. (Hint: The volume V of a cylinder is equal to the
cylinder height times the cross sectional area.) Compute the weight of the fluid in the straw.
b. Compute the upward force FA on the oil due to the air pressure at the bottom of the straw.
c. Compute the downward force F at the top of the oil. What Physical Principle did you use to do
this?
d. Compute the air pressure P at the top of the oil.
2. (10 points) See Figure. A box has mass m = 2,200 kg & volume V = 4.0 m3. When
placed in a certain fluid, the box floats, not moving, with exactly half (!!) of it
submerged below the fluid surface.
a. Compute the buoyant force FB on the box. What Physical Principle did you use to
do this calculation?
b. Compute the density ρF of the fluid. (Hint: This is NOT the same as the density of the box; ρF ≠ m/V!!
You CANNOT use just the definition of density in terms of mass and volume to answer this! Answers
attempt to use only this definition will receive ZERO credit!). What Physical Principle did you use to
do this calculation?