A jar filled partially with water has an object floating in it. The jar is open. Now, if the jar is closed and a vacuum pump is used to pump the air out, what will happen to the floating object? will it rise up more or sink or ...? and why?
Buoyancy is equal to the weight of fluid displaced by the object. So indeed with the air removed the buoyancy created by the air is also removed and the object sinks. Also, the loss of the weight of the air would decrease the density of the water, requiring the object to sink a little to displace more weight.
The part that's causing my head to hurt is the unknown effect of the loss of air pressure on the object floating. It's density may change drastically (a balloon) or negligibly (an iron sphere). A balloon would rise as the pressure drops. Its increasing size means it would sit less deep in the water to displace the same weight.