Moment of Inertia Apparatus

Quick Answer: The Moment of Inertia Apparatus is a physics laboratory instrument for studying rotational inertia, the resistance of a turning body to changes in its angular velocity. Learners apply a known torque, measure the angular acceleration that results and see how moving mass farther from the axis makes a body harder to speed up or slow down.

Mass, Distance and Rotation

In straight-line motion, mass measures how hard it is to change an object’s velocity. In rotation that role is played by the moment of inertia, I, which depends not only on how much mass a body has but on how far that mass lies from the axis. Each small piece of mass contributes its mass multiplied by the square of its distance from the axis, so mass on the rim counts far more than mass near the centre.

The rotational form of Newton’s second law is τ = Iα: torque equals moment of inertia times angular acceleration. A typical experiment with a moment of inertia apparatus winds a cord around an axle, hangs a mass from it and times the mass as it falls through a measured height. From the fall time the angular acceleration is calculated, and from the known torque the moment of inertia follows.

School and college instruments with this name come in several forms, including flywheels with a falling mass, rotating bars carrying movable weights, and platforms on which rings and discs can be compared. The form of this apparatus is not described in its title, so its design and accessories should be confirmed before ordering.

Specifications

Applications

• Measuring the moment of inertia of a flywheel or other rotating body
• Showing that mass far from the axis increases rotational inertia
• Verifying the rotational form of Newton’s second law
• Linking to flywheels, figure skaters, gyroscopes and rolling objects

Care & Handling

• Check that the axle turns freely and that the bearings are clean before timing any run.
• Wind the cord in a single neat layer so the effective radius stays constant.
• Place a soft pad under the falling mass and keep feet clear of its path.
• Tighten any movable masses firmly before spinning, and store the apparatus with the cord slack.

Why Choose LabEquip

Undergraduate mechanics labs and senior physics classes buy this apparatus because rotational dynamics is far easier to grasp with a measured torque and a timed fall. LabEquip offers it in the STEM kits range, and the Moment of Force Apparatus covers the static turning effects that come first. Use the contact page to specify the design required.

Frequently Asked Questions

What is the difference between mass and moment of inertia?

Mass measures resistance to changes in straight-line motion and does not depend on shape. Moment of inertia measures resistance to changes in rotation and depends on how the mass is arranged around the axis. The same object can have different moments of inertia about different axes.

Why does moving mass outward make the body harder to spin?

Each part of the body contributes its mass times the square of its distance from the axis. Doubling the distance of a mass makes its contribution four times larger, so the same torque produces a smaller angular acceleration.

How is the moment of inertia found with a falling mass?

Time the mass falling a known height from rest to find its linear acceleration, then divide by the axle radius to get the angular acceleration. The cord tension times the axle radius gives the torque, and dividing torque by angular acceleration gives the moment of inertia.

Why does a figure skater spin faster with arms pulled in?

Pulling the arms in reduces the skater’s moment of inertia. With no outside torque, angular momentum, the product of moment of inertia and angular velocity, stays constant, so the angular velocity must increase.

Which rolls down a slope faster, a hollow or a solid cylinder?

The solid cylinder wins. A hollow cylinder has more of its mass at the rim and therefore a larger moment of inertia for its mass, so more of its energy goes into spinning and less into forward motion.

What are the main sources of error?

Friction in the axle bearing takes some of the torque, so the calculated moment of inertia comes out slightly too large unless friction is measured and allowed for. Reaction time in timing, a cord that overlaps on itself and a mass that swings as it falls add further uncertainty.

Last Updated: September 2026