Archimedes' Principle

• Understand Archimedes' principle and observe its effects on different objects having different densities

In physics, buoyancy is the upward force on an object produced by the surrounding liquid or gas in which it is fully or partially immersed, due to the pressure difference of the fluid between the top and the bottom of the object.

The net upward buoyancy force is equal to the magnitude of the weight of the fluid displaced by the body. This force enables the object to float or at least to seem lighter.

Buoyancy is important for many vehicles such as boats, ships, balloons, and plays an important role in different natural phenomena such as sedimentation. Archimedes’ principle is named after Archimedes of Syracuse, who first discovered the following law for a body immersed in a liquid.

"Any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.".

Pressure increases with depth below the surface of a liquid. Any object with a non-zero vertical depth will see different pressures on its top and bottom, with the pressure on the bottom being higher. This difference in pressure causes the upward buoyancy force.

Where ρ is the density of the fluid, h is the depth (negative height), and g is the standard gravity (9,8 N / kg on Earth).

An object with a higher average density than the fluid has less buoyancy than weight and it will sink. A ship floats because although it is made of steel, which is more dense than water, it encloses a volume of air and the resulting shape has an average density less than that of the water.

If the weight of an object is less than the weight of the displaced fluid when fully submerged, then the object has an average density that is less than the fluid and has a buoyancy that is greater than its own weight.

If the fluid has a surface, such as water in a lake or the sea, the object will float at a level where it displaces the same weight of fluid as the weight of the object. If the object is immersed in the fluid, such as a submerged submarine or air in a balloon, it will tend to rise.

If the object has exactly the same density as the fluid, then its buoyancy equals its weight. It will remain submerged in the fluid, but it will neither sink nor float. An object with a higher average density than the fluid has less buoyancy than weight and it will sink.

A ship will float even though it may be made of steel (which is much denser than water), because it encloses a volume of air (which is much less dense than water), and the resulting shape has an average density less than that of the water.

A ship floats thanks to buoyancy as described by the Archimedes’ principle.

• Cylinder graduated, tall form with spout. Capacity 100 x 1ml, borosilicate glass — 1226.15
• Rod with hook, diameter 10 mm, length 127 mm — 4113.53
• Tubular spring balance, plastic (dynamometer), range 2.5 N — 4110.21
• Cylindrical masses with hook (10, 25, 50 g) — 4110.08-10-12
• Food colouring — 4207.56

• What physical property does the buoyancy introduced by the Archimedes’ principle depend on?
• A body immersed in a liquid is said to be in equilibrium when buoyancy is greater, smaller or exactly equal to its weight?
• Buoyancy is a force having the same direction as the weight. What direction is it supposed to have?
• According to what you have observed in this experiment, and extending these considerations to fluids, bodies with smaller density with respect to the air density will float, will be buoyed up or will fall under the effect of gravity?