Physics of Cooking Corn

Michelle Bradley

When cooking food in a pot over the stove, you have to be sure to use the right amount of water so that it does not overflow.  Corn can be cooked to perfection in boiling water in this manner. I thought it would be interesting to map out the amount of buoyant force would be exerted on a single ear of corn as it rests in a pot of water.  Since the water is in equilibrium with the corn, the buoyant force directly opposes and equals the force of gravity. A free-body diagram is pictured below, with the corn modeled as the gray circle with a mass M:

I took the corn to be about 5 kg. The volume of the corn is equal to the volume of the fluid displaced (in this case water), since the corn is completely submerged in the water while it is cooking.  Through estimation, I was able to find the fluid displaced. The starting height of the water was about .5 meters and the final height of the water, when the corn was added, was about .6 meters. I also estimated the radius of the pot to be .2 meters:

V_o= (pi)r²h = (pi)(.2)²(.5) = 0.06 m³                                                V_{fluid displaced}= 0.02 m³

V_f= (pi)r²h = (pi)(.2)²(.6) = 0.08 m³

By using this number, and the density of water which is 1000 kg/m³, I was able to find the buoyant force that acts on an ear of corn:

F_B=ρ_fluidV_{fluid displaced}g = (1000)( 0.02 )(9.8) = 196 N

The final formula for the buoyant force is interesting because it appears as if the mass of the corn is not included.  However, the mass of the object is included since it is what is making the fluid displace. This makes sense because if there was a bigger mass, the volume of the fluid displaced would be greater, and therefore the buoyant force needed to keep it floating would also be larger.