MCAT Physical : Submersion
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Example Question #1 : Other Fluid Principles
Which of the following is a possible unit of measurement for surface tension?
Surface tension is a phenomenon observed in fluids, and is defined as the resistance of a fluid to outside force (or a resistance to increased surface area). Surface tension is measured in force per unit length.
The SI unit for force is Newtons and the SI unit for length is meters; therefore, the surface tension is usually measured as 
Surface tension can also be measured in energy per unit area. Energy is measured in Joules and area is measured in square meters; therefore, the units for surface tension can also be 
We can see that these units are equivalent by looking at the conversion of Joules and Newtons:
Example Question #463 : Mcat Physical Sciences
Which of the following is false regarding surface tension?
Surface tension prevents water molecules in a droplet from spreading
Compared to an object that is completely submerged in water, an object floating on water has less resistance to motion
The interaction between molecules at the surface is stronger than the interaction between molecules below the surface
Surface tension forces water droplets to form spheres because they has the lowest surface area to volume ratio
Compared to an object that is completely submerged in water, an object floating on water has less resistance to motion
Consider a beaker containing water. The molecules at the surface of water interact with fewer molecules than the molecules at the center of the beaker. The surface molecules have no water molecules above them, whereas the molecules in the center of the solution interact with molecules in all directions. Since the surface molecules have fewer interactions, the interactions between these molecules are much stronger than the interactions of a center molecule; fewer interactions have opposing counterparts, resulting in stronger forces.
Recall that surface tension can be defined as the resistance to increasing surface area. This means that the surface area is minimized due to surface tension. Of all the geometrical shapes, spheres have the lowest surface area to volume ratio; therefore, surface tension forces water droplets to take spherical shapes. Surface tension occurs due to the intermolecular forces between liquid molecules. These intermolecular forces between molecules are cohesive, preventing the spreading of water molecules.
It is harder to move an object that is floating on water than an object submerged in water. This occurs because the interactions between the molecules at the surface are stronger than the interactions below the surface. Moving an object involves moving the water molecules around it. Since it is harder to move molecules at the surface (stronger interactions), it is harder to move an object at the surface. An object floating on water will have more resistance to motion.
Example Question #1 : Other Fluid Principles
Increased surface tension __________ the surface area of the molecule because it __________ the amount of molecules at the surface.
decreases . . . decreases
increases . . . increases
decreases . . . increases
increases . . . decreases
decreases . . . decreases
Surface area is directly related to the number of molecules at the surface. An increase in surface area will lead to an increase in the number of molecules at the surface. Since surface tension is the ability of a fluid to resist an increase in surface area, the surface area decreases due to increased surface tension. This means that the amount of molecules at the surface also decreases due to surface tension.
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