Question

When a toilet is flushed or a sink is drained, the water (and other material) begins to rotate about the drain on the way down. Assuming no initial rotation and a flow initially directly straight toward the drain, explain what causes the rotation and which direction it has in the Northern Hemisphere. (Note that this is a small effect and in most toilets the rotation is caused by directional water jets.) Would the direction of rotation reverse if water were forced up the drain?

a) Centrifugal force caused by Earth's rotation; clockwise
b) Coriolis effect caused by Earth's rotation; counterclockwise
c) Gravitational pull; clockwise
d) Frictional forces; counterclockwise

Answer 1

The rotation of water in a sink or toilet is caused by the Coriolis effect due to Earth's rotation. In the Northern Hemisphere, the rotation is counterclockwise.

Step-by-step explanation:

The rotation of water and other materials in a sink or toilet when they are drained is caused by the Coriolis effect due to Earth's rotation. The Coriolis effect is the apparent deflection of the path of moving objects due to the rotation of the Earth. In the Northern Hemisphere, the rotation caused by the Coriolis effect is counterclockwise.

If water were forced up the drain, the direction of rotation would reverse, as the Coriolis effect would act in the opposite direction, leading to a clockwise rotation in the Northern Hemisphere.