Science Puzzle
Balance in a Vacuum
A large polystyrene block and a small lead weight are placed on a balance scale in air and perfectly balanced: the scale reads equal.
A bell jar is placed over the scale and the air is pumped out to create a vacuum. What happens to the balance?
The Answer
The polystyrene side sinks. In air, both objects experience an upward buoyant force equal to the weight of air they displace. The polystyrene block has a much larger volume than the lead weight, so it displaces far more air and receives far more buoyant lift.
When the objects were balanced in air, that buoyancy was already accounted for: the scale showed equal apparent weights, not equal true masses. The polystyrene actually has a greater true mass to compensate for its extra buoyant lift.
Remove the air and both objects lose their buoyancy. But the polystyrene loses proportionally much more upward lift than the lead does. The polystyrene therefore becomes relatively heavier, and its side of the balance descends.
This is the same principle by which a balloon deflates when it is carried to a place with less air, and why precision weighing in laboratories must account for atmospheric buoyancy.
The principle: Atmospheric buoyancy. Air exerts an upward buoyant force proportional to an object's volume, not its mass. Large light objects receive more buoyant lift, so removing the air tips a previously balanced scale toward the larger object.