Identifying Density Differences Via Buoyancy Difference
For the current purposes characterize a body by mass and volume (mass divided by volume gives the mass density of the body). In a gravitational (or other body force) field, and in a vacuum, the body experiences a transfer of momentum giving its weight. If the vacuum is substituted by a medium (water, air), then the body loses the weight of the displaced medium's volume (otherwise that volume of medium replaced into the remaining medium would not remain force-free). Reconsider the body and the body's volume made up by medium: Subtracting the respective vacuum-weights (body minus medium) gives the resultant, effective, net-weight of the body in the medium. Thus, any body immersed in a medium will show a weight smaller than its own vacuum-weight. Bodies with densities smaller than that of the medium will have a negative effective weight and experience a net pull against the force field. (The negative vacuum-weight of the medium is called buoyancy. Buoyancy can reduce the net-weight and even make it negative, causing objects to swim on the surface of the medium, like ships do on water.)
In a gravitational force field, in a vacuum, a pair of scales, balanced, shows that the samples have the same weight. If the volumes differ, so will the mass densities. When immersed in some fluid like air or water, different densities will experience different buoyancy-forces, because the displaced fluid amounts have the same density, but different volumes. And thus the scales will come out of balance.
The relation between sample and fluid densities, using the equality of the sample masses, allows to calculate one missing density value.
- Dasymeter
Read more about this topic: Measuring Principle
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