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A heap of grain forms a cone of base radius \( 1.5 \) m and height \( 0.90 \) m. The angle of repose is

  • A.45 degrees
  • B.26 degrees
  • C.31 degrees
  • D.59 degrees

Answer: C

Correct answer: 31 degrees

The heap is a right circular cone, so the angle of repose is the base angle between the sloping surface and the horizontal:

\[ \tan\theta = \frac{h}{r} = \frac{0.90}{1.5} = 0.60 \]

\[ \theta = \arctan 0.60 = 30.96^\circ \approx 31^\circ \]

Use the base radius, not the diameter: taking 0.90/3.0 would give 17 degrees. A value near 30 degrees is typical of free-flowing dry cereal grain, and it rises as moisture and cohesion increase - which is why hopper walls are pitched well steeper than the measured repose angle.

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