Is the Coriolis force a real force?

It is an inertial force — the term −2mΩ×v you must add to make Newton’s laws work in a rotating frame such as the Earth’s surface. Seen from a non-rotating frame, the object simply moves in a straight line (or under real forces) while the ground turns underneath it. The two turntable views show exactly these two descriptions.

Why are things deflected to the right in the northern hemisphere?

In the north the vertical part of Earth’s spin, Ω sinφ, points up, and the horizontal part of −2Ω×v always points to the right of the motion. In the south it points down, so deflection is to the left. At the equator sinφ = 0 and horizontal motion is not deflected, but vertical motion still feels an east–west Coriolis force, so dropped objects drift east even there.

How long does a Foucault pendulum take to turn around?

The swing plane precesses relative to the floor with period sidereal day/|sinφ|: 23.93 hours at the North Pole, about 31.8 hours at the Paris Panthéon (48.85°N) or 11.3° per hour, and not at all at the equator. Seen from above it turns clockwise in the north and counter-clockwise in the south.

Do toilets and sinks drain the other way in the southern hemisphere?

Essentially no. A sink has a Rossby number U/(fL) of about 10³ and a Coriolis acceleration around 10⁻⁵ m/s², far smaller than leftover currents from filling and the shape of the basin. Only careful experiments like Shapiro’s 1962 large round tank, left to settle for a day and drained very slowly, reliably show the northern counter-clockwise vortex.

Why do hurricanes spin counter-clockwise in the north?

Air flows toward the low-pressure centre and the Coriolis force turns it to the right in the north, producing an inflow that spirals counter-clockwise; in the south it is the reverse. Hurricanes are hundreds of kilometres across and last days, so their Rossby number is around 1 and Coriolis really does control them. Near the equator f is too small, which is why hurricanes rarely form within about 5° of it.

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🌀Coriolis Effect Simulator - Rotating Turntable, Eastward Deflection, Foucault Pendulum

The Coriolis force −2mΩ×v is the inertial force that appears in a rotating frame of reference. This tool first shows the same ball side by side on a turntable — a straight line seen from the ground, a curved path seen from the disk — with the Coriolis and centrifugal forces drawn in. It then moves to Earth for long-range shell deflection, the eastward drift of objects dropped from towers, cyclone spin and inertial circles, simulates Foucault pendulum traces at different latitudes, and uses the Rossby number to explain why sink drains don’t care which hemisphere they are in.

Free · No sign-up · Runs in your browserLast updated: 2026-10-29

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How to use Coriolis Effect & Foucault Pendulum

1On Turntable, set the spin, launch speed and direction, press Play and compare the paths and force vectors in both frames
2Under Deflection on Earth, choose a projectile, a tower drop or cyclones, and adjust the latitude to read the deflection and inertial period
3On Foucault pendulum, pick Paris, the North Pole, the Equator, Beijing or Sydney, exaggerate Earth’s rotation and press Play to watch the rosette
4On Drain myth, pick a scenario or enter a speed and size to see whether the Rossby number makes Coriolis matter

FAQ

Is the Coriolis force a real force?: It is an inertial force — the term −2mΩ×v you must add to make Newton’s laws work in a rotating frame such as the Earth’s surface. Seen from a non-rotating frame, the object simply moves in a straight line (or under real forces) while the ground turns underneath it. The two turntable views show exactly these two descriptions.
Why are things deflected to the right in the northern hemisphere?: In the north the vertical part of Earth’s spin, Ω sinφ, points up, and the horizontal part of −2Ω×v always points to the right of the motion. In the south it points down, so deflection is to the left. At the equator sinφ = 0 and horizontal motion is not deflected, but vertical motion still feels an east–west Coriolis force, so dropped objects drift east even there.
How long does a Foucault pendulum take to turn around?: The swing plane precesses relative to the floor with period sidereal day/|sinφ|: 23.93 hours at the North Pole, about 31.8 hours at the Paris Panthéon (48.85°N) or 11.3° per hour, and not at all at the equator. Seen from above it turns clockwise in the north and counter-clockwise in the south.
Do toilets and sinks drain the other way in the southern hemisphere?: Essentially no. A sink has a Rossby number U/(fL) of about 10³ and a Coriolis acceleration around 10⁻⁵ m/s², far smaller than leftover currents from filling and the shape of the basin. Only careful experiments like Shapiro’s 1962 large round tank, left to settle for a day and drained very slowly, reliably show the northern counter-clockwise vortex.
Why do hurricanes spin counter-clockwise in the north?: Air flows toward the low-pressure centre and the Coriolis force turns it to the right in the north, producing an inflow that spirals counter-clockwise; in the south it is the reverse. Hurricanes are hundreds of kilometres across and last days, so their Rossby number is around 1 and Coriolis really does control them. Near the equator f is too small, which is why hurricanes rarely form within about 5° of it.

Features

Synchronised inertial and rotating-frame animations with Coriolis, centrifugal and velocity vectors
Full 3D −2ω×v RK4 trajectories: right in the north, left in the south, plus the westward shift from vertical motion
Eastward deflection of a dropped object (analytic ⅓ωgt³cosφ vs numerical), with Reich’s mine-shaft experiment and more
Animated cyclones in both hemispheres and inertial-circle period and radius
Exact linear Foucault solution: precession period = sidereal day/|sinφ|, petal and star traces
Rossby number calculator comparing sinks, tornadoes, hurricanes and ocean gyres

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