1. Miss Sauer’s 7th Grade Science
Buoyancy
Miss Sauer’s 7th Grade Science

2. Do Now: Objects that are MORE DENSE than water will _________________.
Objects that are LESS DENSE than water will _________________.

3. What About Boats?

4. What About Boats? Metal is more dense than water.
If we change the shape of the metal, like a bowl or a ship, it will float because of the air in the bowl or ship. AIR IS LESS DENSE THAN WATER.
The new shape has a volume of metal and a great deal of air.
The new shape has a new density. It is the density of the steel as well as the air. Air has a low density.
Altogether, the density is less than the density of water. The shaped steel floats on water.

5. Vocabulary
Volume: length x width x height; measured in cubic centimeters
Density: mass/volume; measured in grams per cubic centimeter
Pressure: measured in Newtons per square centimeter
Floats: an object less dense than water
Sinks: an object more dense than water

6. Changes in Density
The deeper we go in water, the more water there is above us.
The mass of water above us gets greater and greater.
Therefore, the pressure becomes greater and greater.
This pressure squeezes the water, making it more dense.
Some objects are too dense to float on water’s surface.
These objects sink down to deeper water.
The objects will continue to sink until they reach water that is as dense as they are
There, the objects will float.

7. Videos:
The Science of Water Pressure | History (3:24)
The effects of underwater pressure on the body – Neosha S Kashef (4:02)

8. Buoyancy Definition:
In science, buoyancy (also known as upthrust) is an upward force exerted by a fluid that opposes the weight of an immersed object.
In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid.

10. Forces of Buoyancy & Gravity:

11. The water is pressing downward on the top surface.
The water is pressing upward on the top surface.

12. The water is less dense at the top of the container.
The pressure of the water at the top of the container is less than the pressure on the bottom of the container.
The pressures are not equal.
The water is less dense at the top of the container.
The water is more dense at the bottom of the container.

13. Pressure is less at the top / more at the bottom

14. Packing of Molecules
When molecules are packed closely together, it makes the object heavier.
If we compare the molecules of a stone to water, the stone’s molecules are packed more closely together. Thus, making it more dense.

15. What About Submarines?

16. What About Submarines?
A submarine has special tanks called ballast tanks.
Ballast tanks are used to make the submarine heavier or lighter.
When the ballast tanks are filled with seawater, the submarine submerges, or goes underwater.
When the water is forced out of the ballast tanks, the submarine rises and surfaces.

17. Liquids & Gases
Liquids and Gases do not keep their shape, so they can sort themselves out by density.

18. What About Solids?
Solids keep their shape, so they can not sort themselves out by density.

19. Vocabulary
Positive Buoyancy: floats; the force that keeps an object afloat.
Neutral Buoyancy: doesn’t sink or float.
Negative Buoyancy: sinks

20. Brain Pop Video:

21. Closure Questions What is buoyancy?
What is the difference between positive, negative, and neutral buoyancy?
How do submarines fight the buoyancy and gravity forces?
How does density effect buoyancy?