Apparent Weight and Accelerating in a Fluid

A scale reads tension, not weight. Hang a block from a spring scale in air, and tension equals weight because nothing else pulls on the block. Dunk it in water, and now three forces act: weight down, buoyancy up, tension up. If the block hangs still, these three add to zero, so tension equals weight minus buoyancy. That smaller tension is the apparent weight, and it is exactly what the scale shows.

Buoyancy depends only on the fluid's density and the submerged volume, not on what the object is made of. So the fraction of weight that buoyancy cancels depends on how the object's density compares to the fluid's. An aluminum block, much denser than water, loses only a modest fraction of its weight. A cork, less dense than water, feels a buoyant force bigger than its weight entirely, so if nothing holds it down, the forces cannot balance. Newton's second law then gives a net upward force, and the cork accelerates toward the surface rather than sitting in equilibrium.

This is the same force-balance idea from Unit 8's buoyancy work, now paired with Newton's second law instead of stopping at equilibrium. It also previews why terminal behavior shows up in fluids generally: as an object speeds up through a fluid, drag grows and eventually cancels the leftover force, which is exactly why a rising balloon or a falling skydiver stops accelerating and coasts at a constant speed.