Homework Help: physics

Posted by Amanda on Sunday, February 15, 2009 at 11:32pm.

A hollow plastic sphere is held below the surface of a freshwater lake by a cord anchored to the bottom of the lake. The sphere has a volume of 0.600m^3 and the tension in the cord is 840N . What is the mass of the sphere?
is this how i would find the mass?
(4/3)X3.14X(.600)^3?
and to find the buoyant force exerted by the water on the sphere.
(.600)(9.8)?
please just tell me if im on the right track

physics - drwls, Monday, February 16, 2009 at 1:17am
Your buoyant force is
Fb = (water density)(volume)x g,
but the water density in SI units is 1000 kg/m^3, not 1. So Fb = 5886 N

If the mass of the sphere is M g, a vertical force balance requires that

M*g + T = Fb

M = (1000kg/m^3*0.6*m^3*g - 840 N)/g
= 600 kg - 840/g = 514 kg

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