What is moment of inertia?

• A. The difficulty in getting a body or object to rotate around an axis - depends on position, size, speed of performer
• B. The force that attracts a body toward the earth's center
• C. The energy stored due to motion
• D. A turning or twisting force

Answer: (A)

Moment of inertia is the resistance an object has to starting or changing its rotation around an axis. It depends on how the object's mass is distributed relative to that axis: the farther mass is from the axis, the greater the moment of inertia. Because of this, more torque is needed to achieve the same angular acceleration when the mass is spread out compared with when it’s closer in. A practical example is a figure skater: with arms extended, the mass is farther from the axis and the moment of inertia is larger, making it harder to spin up; pulling the arms in lowers the inertia and allows faster spins with less torque. The speed of rotation itself doesn’t change the moment of inertia for a rigid body; angular velocity can change, but the inertia value is set by mass distribution and the rotation axis. The other concepts—the pull toward Earth (gravity), the energy of motion (kinetic energy), and the turning force (torque)—are different ideas.