Tilted container: water surface vs axis

A container partly filled with water is tilted 45° and rests on a horizontal table. What is the angle between the water surface and the axis of the container?

Step-by-step

1. The water surface stays horizontal (gravity does not change direction)
   A liquid at rest always has a horizontal free surface
2. Bottle tilted 45° → axis makes 45° with the horizontal
   The angle between the axis and the horizontal is the tilt angle
3. Angle between the lines = tilt angle = 45°
   The angle between the (horizontal) water surface and the bottle axis equals the bottle's tilt

Why it works

The free surface of still water is always horizontal. The axis is tilted 45° away from the vertical, so the angle between the surface and the axis is exactly the tilt angle.

Common mistakes

• Answering the angle between the water surface and the table, which is 0°.
• Believing a wider or narrower container would change the angle.

Related

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