Honors Buoyancy

I noticed that many of you used an equation that said delta mg = FB. Since it was incorrect in the quiz problem, it took me a while to figure out where this came from and then I realized it came directly from our lab. The reason it is incorrect in the problem is that in the problem the scale is holding the object up ( teh object is hanging from the scale) and so the force the scale has to exert in the problem is REDUCED by the bouyant force pushing up and now the scale and FB are both togther countering Mobject g. i.e. FB + Fscale = Mg.

In our lab, the string was holding the object up and when we place the object in the beaker the fluid pushed up reducing the force the string had to apply. However the scale reading INCREASED because since the water pushed up with FB there was a force from the object DOWN on the water = FB so in the lab the final scale reading equal weight of water and beaker + FB, and so FB = deltaMg in our lab.