Bottle tilted 60°: water surface vs axis

A bottle partly filled with water is tilted 60° and left at rest on a horizontal table. What is the angle between the water surface and the axis of the bottle?

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 60° → axis makes 60° with the horizontal
   The angle between the axis and the horizontal is the tilt angle
3. Angle between the lines = tilt angle = 60°
   The angle between the (horizontal) water surface and the bottle axis equals the bottle's tilt

Why it works

The free surface of water at rest is always horizontal, while the axis is tilted 60° away from the vertical. The angle between the surface and the axis therefore equals the tilt angle.

Common mistakes

• Measuring the angle between the water surface and the table top, which is 0°.
• Halving the tilt angle to 30° as if it were an angle of incidence.

Related

tilted-bottle-water-surface-angle tilted-container-surface-axis-angle tilted-container-water-surface-25-degree