Container tilted 25°: water surface vs axis

A container partly filled with water is tilted 25° 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 25° → axis makes 25° with the horizontal
   The angle between the axis and the horizontal is the tilt angle
3. Angle between the lines = tilt angle = 25°
   The angle between the (horizontal) water surface and the bottle axis equals the bottle's tilt

Why it works

Water at rest has a horizontal free surface, so the angle between that surface and the container axis is exactly the angle by which the axis is tilted from the vertical.

Common mistakes

• Answering 65° by subtracting 25° from 90°.
• Believing the shape or width of the container changes the angle.

Related

tilted-container-surface-axis-angle tilted-bottle-water-surface-angle tilted-bottle-water-surface-60-degree