Science: Fluid Mechanics: Pressure and Buoyancy
Exploring pressure in fluids and the forces that make objects float or sink
Science: Fluid Mechanics: Pressure and Buoyancy
Exploring pressure in fluids and the forces that make objects float or sink
Physics - Grade 9-12
- 1
A force of 240 N is applied evenly over an area of 0.80 m^2. What pressure is produced?
Use the formula P = F/A.
The pressure is 300 Pa because pressure equals force divided by area, so 240 N divided by 0.80 m^2 equals 300 N/m^2, which is 300 Pa. - 2
A diver is 15 m below the surface of fresh water. If the density of water is 1000 kg/m^3 and g = 9.8 m/s^2, what fluid pressure is due to the water alone at that depth?
The fluid pressure due to the water is 147,000 Pa because pressure in a fluid is rhogh, so 1000 x 9.8 x 15 = 147,000 Pa. - 3
Explain why the pressure in a fluid increases with depth.
Think about the weight of the fluid above a point.
Pressure increases with depth because deeper layers of fluid must support the weight of more fluid above them. As a result, the force on a given area becomes greater lower in the fluid. - 4
A hydraulic lift has a small piston with an area of 0.020 m^2 and a large piston with an area of 0.40 m^2. If a force of 150 N is applied to the small piston, what force is produced on the large piston?
Set the pressures equal: F1/A1 = F2/A2.
The force on the large piston is 3000 N. By Pascal's principle, the pressure is the same in both pistons, so F2 = F1 x A2/A1 = 150 x 0.40/0.020 = 3000 N. - 5
State Archimedes' principle in your own words.
Archimedes' principle states that an object in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces. - 6
A block is fully submerged in water and displaces 0.025 m^3 of water. What buoyant force acts on the block? Use a water density of 1000 kg/m^3 and g = 9.8 m/s^2.
Find the weight of the displaced water.
The buoyant force is 245 N because the buoyant force equals the weight of the displaced water: F_b = rhoVg = 1000 x 0.025 x 9.8 = 245 N. - 7
An object weighs 60 N in air and has an apparent weight of 45 N when submerged in water. What is the buoyant force on the object?
The buoyant force is 15 N because buoyant force equals the loss of weight in the fluid, and 60 N minus 45 N equals 15 N. - 8
A metal cube has a density of 7800 kg/m^3. Will it float or sink in water? Explain your answer.
Compare the object's density to the fluid's density.
The cube will sink in water because its density is much greater than the density of water, which is about 1000 kg/m^3. Objects denser than the fluid sink if no other support force acts on them. - 9
A piece of wood has a density of 600 kg/m^3 and is placed in water. What fraction of its volume will be submerged when it floats?
The wood will have 0.60, or 60 percent, of its volume submerged. For a floating object, the fraction submerged equals the object's density divided by the fluid density, so 600/1000 = 0.60. - 10
Why does a steel ship float even though steel is denser than water?
Consider the density of the whole ship, not just the metal.
A steel ship floats because its overall average density, including the air-filled interior, can be less than the density of water. The ship displaces enough water for the buoyant force to balance its weight. - 11
A balloon contains helium, which is less dense than air. Explain why the balloon rises.
The balloon rises because the air around it exerts a buoyant force equal to the weight of the displaced air. If that buoyant force is greater than the total weight of the helium and the balloon material, the balloon accelerates upward. - 12
A submarine takes in water into its ballast tanks before diving. Explain how this changes its buoyancy.
Think about how ballast changes the submarine's average density.
Taking in water increases the submarine's mass and average density. This reduces its net buoyancy, and if its weight becomes greater than the buoyant force, the submarine sinks. Releasing water and replacing it with air lowers its average density and increases buoyancy.