This is of course of the water body which occupies the same volume as that of the cylinders (the sm

Here we take the experience of the finger dipped into a container filled with water and placed on a balance initially at equilibrium, it indicates an overload when the finger sinks in water. This new clip allows us, with numerical values, a quantitative approach to the phenomenon.
When dipped his finger in a container filled with water and placed on a balance, it previously equilibrated leva bates with marked masses, accusing overload (Figure 1a): the finger, which has no contact with the walls the container seems to transmit its weight side of the balance, hence the name given to this video.
In fact this experience is explained by the existence leva bates of buoyancy, and may also illustrate the principle of action and reaction, widely used in physics.
To quantitatively interpret this experience, we have achieved a new video by replacing the pan balance with a kitchen scales with digital leva bates display, and the finger by cylinders of known masses
Pour water into the container without filling. Place the container on the weighing pan and tare: the balance directly indicate overload, the weight of the container no longer involved.
Cling aluminum cylinder gallows for the raising or lowering very gently. leva bates Immerse it completely in the water in the container, without touching the bottom or sides. The balance indicates an overload of the image of the cylinder placed directly on the balance gives his actual mass (Figure 2a).
Replace the aluminum cylinder by a cylinder of the same dimensions but made of brass, and the dive-in the same manner in the container of water: the balance again indicates an overload, while the mass of the cylinder is (2b ).
The two cylinders have in common that their dimensions to the diameter to the height (Figure 3). They have the same volume, which is easily calculated: The density of aluminum is worth, is that of brass can be checked with these values that the masses of the two cylinders are those displayed by the balance when placed directly above. A scale does not measure volume, but it displays weight in units of mass, that is to say, in kilograms or grams. Overload obtained for each cylinder can not be explained by their mass, but by that of a body of which the volume is: which gives a density of
This is of course of the water body which occupies the same volume as that of the cylinders (the small difference is explained by the fact that the balance that we used to achieve this include a video display in two two grams).
Now the weight of liquid with the same volume as a solid immersed leva bates in it has a name: the buoyancy exerted by the fluid on the immersed solid. Why then is it down, while all the manuals say it occurs from the bottom up? It's just under reciprocal actions:
Buoyancy exerted on the solid is facing up (its origin comes from the pressure difference between the base and the top of the solid, and it is this that makes any body immersed in a fluid seems lighter); but under the law of action and reaction (also known as Newton's third law), the solid water exerts on a force equal and opposite to the surge, which is of the same magnitude but pointing down. If the fluid is a liquid, so incompressible against this full-thrust exerted on the bottom of the container, which in turn passes it to the weighing plate: it is the origin of overloading the balance displays , after converting en masse.
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