When we place a straw (a hollow cylindrical tube) in water, the water inside the straw rises above the surface level outside and a miniscus (curvature of the surface) forms. Consider the column of water in the straw from the height of the water outside the straw to the top of the column. (See figure, where the column is shown shaded.) Let P1 be a point in the straw at the bottom of the column and let P2 be a point inside the water column at the top.
The force due to surface tension for a maximally stretched surface is given by Fsurf =γL, where γ is the coefficient of surface tension (which depends only on the substance) and L is the length of the line of contact between an object and the fluid. For this problem use the following:
ρ = density of water
γ = surface tension of water
r = radius of the straw
h = height of the column
Patm = atmospheric pressure
g = acceleration due to gravity
What expression gives the magnitude of the force due to pressure on the top of the column?