Why does a raindrop or a skydiver stop speeding up and fall at a steady 'terminal' velocity?
As the drop speeds up, the upward viscous drag of the air (which grows with speed) and buoyancy increase until they exactly balance gravity.
As the drop speeds up, the upward viscous drag of the air (which grows with speed) and buoyancy increase until they exactly balance gravity. With zero net force, acceleration stops and the speed stays constant — the terminal velocity. For a small sphere this is v = 2r²(ρ−σ)g/9η, so bigger, denser drops fall faster.
Key point
Terminal velocity = the speed where drag + buoyancy balance weight (net force 0).
Common mistake
thinking a falling body keeps accelerating all the way down.
Memory tip
drag rises with speed until it cancels gravity → constant speed.
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