Spin ω
4.3 rpm
Unseen · No.140 — Fictitious forces
Coriolis — the force that isn't there.
Launch a puck in a dead-straight line across a slowly spinning disc. Watch from the ground and it stays straight. Watch from the disc and it bends — the same throw, the same physics, a curve that exists only because you're turning with it.
Click the disc to throw a puck →
4.3 rpm
Two truths, one throw
01 The straight line
In the ground-truth frame — standing still off the disc — the puck obeys Newton plainly. No sideways push acts on it, so it travels a perfectly straight line and the marked disc simply rotates underneath.
Nothing curves. There is no force to explain.
02 The bend
Step onto the disc and turn with it. Now the very same puck appears to peel sideways, tracing a curve. To keep Newton's laws working you must invent a force pushing it — the Coriolis force.
It deflects everything moving in the frame: right if the disc turns anticlockwise, left if it turns the other way.
03 The gap
The dashed gold line is where the thrower on the disc is still aiming. The green trail is where the puck actually went. The widening gap between them is the deflection — and it grows with time and with spin.
Toggle the frame and it vanishes. The force was only ever bookkeeping.
A parcel of air sliding toward a low-pressure centre is turned right in the Northern Hemisphere and left in the Southern. That single turn is why cyclones spiral anticlockwise up north and clockwise down south, why the trade winds lean west, and why long-range shells and ocean gyres all drift off their aim.
Earth's ω · 7.29 × 10⁻⁵ rad/s
One turn · every 23h 56m
Deflection · 2 Ω v sin(latitude)
The bookkeeping
Fcor = −2 m Ω × v
It is a correction term you add so that F = ma still balances inside a spinning frame. Change to a frame that isn't turning and the term is exactly zero — the puck was always going straight. That is what makes it a fictitious, or inertial, force: real in its consequences, absent from the world.