Question

Ram has a semicircular disc. He rotates it about its diameter by 360 degrees. When he rotates the disc, a volume of air in his room gets swept. What is the name of the object/shape that exactly occupies this volume?

• A. Sphere
• B. Hemisphere
• C. Cone
• D. Cylinder

Answer 1

The correct option is A

Sphere

Let us consider a semicircle. The diagram below shows a semicircle and its diameter which is its axis of rotation (AB). We rotate the semicircle about AB by 360 degrees.

It is clear that by seeing the $2^{nd}$ diagram that the shape obtained is a sphere. If it was rotated by 180 degrees instead of 360 degrees, we get a hemisphere. The line segment AB will become the diameter of the sphere formed.