Archimedes' Force: Floating Bodies and Balloons
Why a heavy steel ship floats while a small coin sinks
- explain why a fluid pushes a body upward,
- compute the Archimedes' force with FA = ρgV,
- state Archimedes' principle in words,
- predict whether a body floats, sinks, or hovers,
- explain how ships and balloons work.
1. Why a fluid pushes upward
You already know from the earlier topic that pressure in a liquid grows with depth: the deeper you go, the harder the liquid presses. Now imagine a block sitting under water. The liquid presses on every face of the block.
The top face is closer to the surface, so the water there presses down with a smaller force. The bottom face is deeper, so the water there presses up with a larger force. The side pushes cancel each other out, but the up push on the bottom beats the down push on the top. What is left over is a steady upward force. That leftover push is the buoyant force.
This is why a bucket of water feels lighter while it is still under the surface of a pool, and why you can lift a friend easily in a swimming pool but not on dry land.