Coriolis effect
Launch a ball across a spinning turntable. From the lab frame, the path is a straight line. But stand on the turntable — the rotating frame — and that same path curves. No real force deflects it. The Coriolis effect is a fictitious force that appears only because the observer is rotating. It is also what steers hurricanes, trade winds, and ocean currents on a spinning Earth.
What the Coriolis effect is
In a rotating reference frame, a moving object appears to be deflected by a fictitious force perpendicular to its velocity: F = −2m(ω × v). This is the Coriolis force. It doesn’t exist in the inertial frame — there, the object moves in a straight line. The deflection is entirely a consequence of observing from a rotating platform.
Two frames, one motion
Toggle between the inertial and rotating frames. In the inertial (lab) frame, the ball travels straight while the turntable spins beneath it. In the rotating frame, you spin with the turntable and the ball appears to curve. Same physics, different descriptions. The Coriolis force is the mathematical term that makes Newton’s second law work in the rotating frame.
Cyclones and trade winds
Earth rotates, so the atmosphere is a rotating reference frame. Air rushing toward a low-pressure center is deflected by the Coriolis force — rightward in the Northern Hemisphere, leftward in the Southern. The result: cyclonic rotation. Counterclockwise in the North, clockwise in the South. The trade winds, jet streams, and the Ekman spiral in ocean currents are all consequences of the same fictitious force acting on mass moving across a rotating sphere.