A student sits on a frictionless rotating stool while holding two dumbbells at arm’s…

PHYSICS · Single correct · A Level · Physics · Engineering physics · Engineering physics

Problem

A student sits on a frictionless rotating stool while holding two

2.0\,\text{kg}

dumbbells at arm’s length,

0.80\,\text{m}

from the rotation axis. The stool and student (excluding the dumbbells) have a combined moment of inertia of

2.5\,\text{kg}\cdot\text{m}^2

. Initially the system rotates at

2.5\,\text{rad/s}

. The student then pulls the dumbbells inward to a radius of

0.25\,\text{m}

. How much work did the student do in pulling the dumbbells inward? (a)

0\,\text{J}

(b)

6.5\,\text{J}

(c)

13\,\text{J}

(d)

-13\,\text{J}