Archimedes' principle
- Archimedes’ principle states that:
An object submerged in a fluid at rest has an upward buoyancy force (upthrust) equal to the weight of the fluid displaced by the object
- The object sinks until the weight of the fluid displaced is equal to its own weight
- Therefore, the object floats when the magnitude of the upthrust equals the weight of the object
- The magnitude of upthrust can be calculated by:
- Where:
- F = force in newtons (N)
- ρ (Greek letter rho) = density in kilograms per metre cubed (kg m-3)
- g = gravitational field strength in newtons per kg (N kg-1)
- V = volume in metres cubed (m3)
- Since m = ρV, upthrust is equal to F = mg which is the weight of the fluid displaced by the object
- Archimedes’ Principle explains how ships float:
Upthrust on a boat
Boats float because they displace an amount of water that is equal to their weight
Worked example
Icebergs typically float with a large volume of ice beneath the water. Ice has a density of 917 kg m-3 and a volume of Vi.
The density of seawater is 1020 kg m-3.
What fraction of the iceberg is above the water?
A. 0.10 Vi B. 0.90 Vi C. 0.97 Vi D. 0.20 Vi
Answer:
Step 1: List the known quantities
- Density of ice, ρi = of 917 kg m-3
- Volume of ice = Vi
- Density of seawater, ρw = 1020 kg m-3
- Volume of seawater = Vw
Step 2: Consider Archimedes' Principle
- According to Archimedes' Principle the force of upthrust is equal to the weight of the seawater displaced by the iceberg
- Buoyancy force is the weight of the displaced water
Step 3: Equate the forces of weight and upthrust
- Since the iceberg is floating, its weight is exactly equal to the buoyancy force
Step 4: State the density equation and rearrange for mass
Step 5: Substitute ρV for mass in the mg equivalence
Step 6: Determine the ratio of densities
- Cancelling g:
- Dividing by
Step 7: Solve for the volume of ice submerged underwater
- This means that 90% of the iceberg's volume is submerged underwater
- The correct answer is A
Examiner Tip
Don't get confused by the two step process to find upthrust.
- Step 1: You need the volume of the submerged object, but only because you want to know how much fluid was displaced
- Step 2: What you really want to know is the weight of the displaced fluid.
A couple of familiar equations will help;
- m = ρV to get mass
then
- W = mg to get weight
If you are feeling particularly mathematical, you can combine your equations, so that W = ρVg