Cylinder height from sphere volume equality: A circular cylinder has the same radius as a sphere and their volumes are equal. The height of the cylinder is equal to what multiple of its radius?

Difficulty: Easy

Correct Answer: 4/3 times its radius

Explanation:

Introduction / Context:Equating cylinder and sphere volumes with common radius yields a direct relationship between cylinder height and radius. This is a standard mensuration identity.

Given Data / Assumptions:

  • Common radius r.
  • V_cylinder = π r^2 h; V_sphere = (4/3) π r^3; volumes equal.

Concept / Approach:

  • Set π r^2 h = (4/3) π r^3 and solve for h in terms of r.

Step-by-Step Solution:

π r^2 h = (4/3) π r^3 ⇒ cancel π r^2 (r>0) to get h = (4/3) r.

Verification / Alternative check:

Pick r=3 ⇒ sphere V = (4/3)π*27 = 36π; cylinder with h=4 gives V=π*9*4=36π, equal.

Why Other Options Are Wrong:

  • 2/3 r or r: Too short to produce equal volume.
  • Diameter (2r): Too tall; volume would exceed the sphere volume.

Common Pitfalls:

  • Dropping the 1/3 in sphere’s volume.
  • Confusing diameter and radius when interpreting options.

Final Answer:

4/3 times its radius

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