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Re: Question from grade 11 student



I'm doing a project on archimede's law of hydrostatic. Since I'm a scuba
diver, I went underwater a different depths with a object . My goal is to
try to figure out if the object's weight changes in proportion with the
depth.

These were my results:

Depth Lenght of my spring (Hook's law F: k * l

0 feet 25,2 cm
10 23,9
20 24,5
50 25,2


I understand that my object is lighter at 10 feet, but why does it get
heavier as I go further deep down ?

Marc Desgroseilliers,
enseignant,
chef de groupe a la vie etudiante,
ecole secondaire Dorval,
commission scolaire du Lakeshore,
636-0115
____________,

mdesgroseilliers@musicm.mcgill.ca

Dear Marc,
Your spring reading measures essentially the difference between the
body's weight and the bouyant force on the body. Any significant variation
in the bouyant force will be reflected in the spring reading. The bouyant
force is equal to the weight of water displaced by the body, which in turn
depends on the volume of the body and density of the water. If the volume
of the body is fixed (rigid body), then any variations in water density
(for example due to local variations in salinity or temperature) will
therefore be reflected in your spring reading.
Apart from this, I'm not sure that small fluctuations in your
readings might not be just due to errors in the accuracy of your scale
readings. One should be sure that your variations are significantly larger
than the accuracy or precision errors before ascribing the variations to
other causes.

Hope this helps.

Vince


Vince Santarelli santarel@andromeda.rutgers.edu
Physics Department, Smith Hall, Rm.366 OFFICE: (201) 648-5072
Rutgers University, Newark Campus DEPT: (201) 648-5250
Newark, NJ 07102 FAX: (201) 648-1434