Density and Buoyancy
8. All objects experience a buoyant force when immersed in a fluid. As a basis for understanding this concept:
8.c Students know the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid the object has displaced.
- 2 regular drinking glasses
- 2 small corks
1. Fill the first glass halfway with water and float the cork on the water surface: observe!
(The cork should be attracted towards the sides of the glass, push the cork to the center a few times to watch it move to the outside)
2. Fill the second glass to the brim (almost overflowing) and float the cork on the water surface: observe!
(The cork should float close to the center, push it towards the sides a few times to see how it moves again to the center)
Prior knowledge & experience:
- Students may or may not be familiar with the cork material.
- Students may need to learn or review the term "meniscus" when looking at the water in the the half full cup.
- If using this event for buoyancy, it would be important for students to know about adhesion and cohesion.
1. Would a cork floating in each of these glasses float the same even if the water levels were different? (Have the students write down their answer and their reasoning)
During the Demonstration
1. According to your observations how does the cork float in the half-filled glass? The full glass? (Record or state observations: cork floats at the sides of the half-filled glass and at the center of the full glass)
1. Where is the water level the highest in the half-filled glass? (At the sides of the glass or circumference of the concave meniscus)
2. Where is the water level the highest in the full glass? (At the center of the glass or the top of the convex meniscus)
3. Why is the cork moving towards and sticking to the side of the glass in the half-filled glass? (Adhesion causes the water level at the sides of the glass to be higher than at the center)
4. Why is the cork floating towards the center of the glass in the full glass? (Cohesion causes the water at the center of the glass to be higher than the sides)
5 Why can the glass be filled more than full without overflowing the water? (Because of the surface tension caused by cohesion).
Students will be surprised to find that the cork floats at the highest point in the water. This counter-intuitive as most students will assume that gravity will pull the cork to the lowest point in the water.
Any object feels an upward force called "buoyancy" from the fluid it sets in. Objects in water feel the upward buoyancy force from the water. You feel an upward buoyancy force from the air, as does a helium balloon. Every object also feels a downward force from gravity: every object feels its weight. When an object weighs more than the upward buoyancy, the object sinks. When weight is less than buoyancy, the object floats.
A floating object rises until only a small part of it is in the water. This makes less of the object be in the water, so that upward buoyancy force is smaller. The object rises until the buoyancy and weight are equal. Because that buoyancy force from the water is upward, the cork moves upward in the water until it reaches the top.
Adhesion is the attraction between unlike objects. Cohesion is the attraction of like molecules for each other. There are differences in the cohesive forces of solids, liquids, and gases. Water is affected by both adhesion and cohesion. Water is attracted to many substances (adhesion), and there is also an attraction among water molecules for each other (cohesion)
When the glass is half filled with the water, the circumference of the meniscus (at the sides of the glass) is the highest level. The water is highest here because of the adhesive forces between the water and the glass molecules. When the glass is brimful filled with water, the cohesive forces between the water molecules and surface tension form a film on the surface of the water, making it possible to fill the glass more than full. The highest level of the meniscus is now in the center of the glass. The cork floats in each of the water levels accordingly because buoyant objects tend to float at the highest spots of the meniscus.
Tips and Tricks:
- When the glass is half-filled with water, the circumference of the meniscus is the highest level.
- This is caused by the adhesive forces between the water and the glass molecules.
- As a result of this high level at the side of the glass, the cork moves to that place.
- When the glass is brimful filled with water, the surface tension and the cohesive forces between the water molecules form a film on the surface of the water, making it possible to fill the glass more than full.
- The highest level of the meniscus is now in the center of the glass, which is where the cork will float.
- As floating objects tend to float at the highest spots of the meniscus, the center would be the spot where the cork would float when the glass is full.
Students will assume that the cork will float in the lowest point in the water. The buoyant force is greater than the force produced by gravity.
Photographs and Movies
Surface Tension and Floating
Where Does the Cork Float?