The Coriolis force is a fictitious force (or pseudo-force) that appears in a rotating reference frame. It deflects moving objects perpendicular to their velocity relative to the rotating frame. It notably explains the rotation of weather systems and the deflection of ocean currents.
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How to measure it in class
With the FizziQ app, it is possible to explore the Coriolis effect on a merry-go-round.
Steps:
- Position yourself on a merry-go-round or rotating platform with the smartphone
- Activate the FizziQ gyroscope and accelerometer
- Observe acceleration variations during radial movements (toward the center or outward)
- Compare with theoretical predictions
- Throw a ball toward the center of the merry-go-round and observe its deflection (filmed from a fixed point vs from the merry-go-round)
Scientific activities on this topic
- Experiment on a merry-go-round: https://www.fizziq.org/en/activities/centrifuge/
- Visualization of deflection with a Foucault pendulum
- Modeling of cyclones
Learn more
Origin of the force:
In a rotating reference frame (Earth, merry-go-round), a moving object appears to be deflected because the reference frame rotates beneath it. This “deflection” is described by the Coriolis force, perpendicular to velocity.
Direction of deflection:
- Northern Hemisphere: deflection to the right of motion
- Southern Hemisphere: deflection to the left
Effects at Earth scale:
| Phenomenon | Coriolis explanation |
|---|---|
| Cyclones | Rotation in cyclonic direction |
| Trade winds | Westward deflection |
| Gulf Stream | Eastward deflection |
| Foucault pendulum | Rotation of oscillation plane |
Order of magnitude:
On Earth (omega approximately equals 7.3 x 10 to the -5 rad/s), the Coriolis force is very weak for human-sized objects. It does NOT affect the direction of sink drainage (that is a myth!).
Formula
Coriolis force: F_c = -2m x omega x v (vector cross product)
In magnitude (horizontal motion at latitude lambda): F_c = 2 x m x omega x v x sin(lambda)
Coriolis acceleration: a_c = 2 x omega x v x sin(lambda)
where:
- m: object mass (kg)
- omega: angular velocity of rotation (rad/s)
- v: object velocity in the rotating frame (m/s)
- lambda: latitude (for Earth)
Application examples
- Long-range snipers must correct for Coriolis deflection
- Cyclones rotate in opposite directions in the two hemispheres
- The Foucault pendulum at the Pantheon completes a full rotation in 32 hours
- Intercontinental ballistic missiles incorporate Coriolis correction
FAQ
Q: Does water really rotate differently in the two hemispheres? A: No, that is a myth! The Coriolis force is far too weak at sink scale. The rotation direction depends on the basin geometry and initial disturbances.
Q: Can the Coriolis effect be detected with FizziQ in daily life? A: With difficulty. On Earth, the effect is too weak to be measured with smartphone sensors. However, on a fast-spinning merry-go-round, the effect becomes detectable.
Q: Why is it called a “pseudo-force”? A: It only exists in the rotating reference frame. An external observer (in an inertial frame) does not see it: the object follows a straight trajectory, it is the reference frame that rotates.
Q: How to visualize the Coriolis effect in class? A: Place a sheet on a turntable and draw a straight line with a marker. Viewed from above, the line appears straight, but on the sheet it is curved.
Related concepts
Rotating Reference Frame - Centrifugal Force - Fictitious Forces - Earth Rotation - Meteorology - Foucault Pendulum