4.6 Torque and Moment of Inertia | Rotational And Circular Motion | Physics 11

Introduction

In linear motion, mass is the measure of an object's resistance to a change in its state of motion. In rotational motion, the equivalent concept is the moment of inertia ( $I$ ). It is a measure of an object's resistance to a change in its state of rotation. Just as it is harder to push a more massive object, it is harder to start or stop the rotation of an object with a larger moment of inertia.

Inertia vs. Moment of Inertia

Concept	Inertia (Linear)	Moment of Inertia (Rotational)
Definition	Resistance to change in linear motion.	Resistance to change in rotational motion.
Depends on	Mass only.	Mass and the distribution of that mass relative to the axis of rotation.
Formula	Not a calculated quantity; it is mass ( $m$ ).	For a point mass: $I = m r^{2}$ . For a rigid body, it is the sum of $m r^{2}$ for all particles.

Formula for a Point Mass

The moment of inertia ( $I$ ) for a single point mass ( $m$ ) rotating at a distance ( $r$ ) from an axis is:

$I = m r^{2}$

This formula shows that the moment of inertia increases with both mass and, more significantly, the square of the distance from the axis. Distributing mass farther from the axis of rotation dramatically increases the moment of inertia.

Moment of Inertia for a Rigid Body

A rigid body is a collection of many particles. The total moment of inertia of the body is the sum of the moments of inertia of all its individual particles.

$I = \sum_{i} m_{i} r_{i}^{2}$

For continuous objects, this sum becomes an integral over the entire body:

$I = \int r^{2} d m$

In practice, pre-calculated formulas for common shapes are used:

Object	Axis of Rotation	Moment of Inertia ( $I$ )
Thin Hoop or Ring	Through center, perpendicular to plane	$M R^{2}$
Solid Cylinder or Disc	Through center, along axis	$\frac{1}{2} M R^{2}$
Solid Sphere	Through center	$\frac{2}{5} M R^{2}$
Thin Rod	Through center, perpendicular to rod	$\frac{1}{12} M L^{2}$

Radius of Gyration

The radius of gyration ( $k$ ) is the distance from the axis of rotation to a point where the entire mass of the body could be concentrated without changing its moment of inertia. It is given by:

$I = M k^{2} ⟹ k = \frac{I}{M}$

Analogy Between Linear and Rotational Quantities

The moment of inertia plays the same role in rotational mechanics that mass plays in linear mechanics.

Linear Quantity	Angular Quantity	Relationship
Displacement ( $s$ )	Angular Displacement ( $θ$ )	$s = r θ$
Velocity ( $v$ )	Angular Velocity ( $ω$ )	$v = r ω$
Acceleration ( $a$ )	Angular Acceleration ( $α$ )	$a = r α$
Mass ( $m$ )	Moment of Inertia ( $I$ )	—
Force ( $F$ )	Torque ( $τ$ )	$τ = r F$

Torque (Moment of Force)

Torque is the rotational equivalent of force — it is the turning effect of a force about an axis of rotation. For a force $F$ applied at position vector $r$ from the axis:

$τ = r \times F$

The magnitude is:

$τ = r F sin θ$

where $θ$ is the angle between $r$ and $F$ . Torque is maximum when $θ = 9 0^{\circ}$ (force perpendicular to the moment arm).

SI unit: N·m (Newton-metre) — same dimensions as energy $[M L^{2} T^{- 2}]$ but physically distinct.

Newton's Second Law for Rotation

The rotational equivalent of Newton's Second Law ( $F_{n e t} = ma$ ) directly involves the moment of inertia.

Formula:

$τ_{n e t} = I α$

Derivation:

Start with Newton's Second Law for a point mass: $F = ma$ .
The tangential acceleration is related to angular acceleration by $a_{t} = r α$ . So, $F = m (r α)$ .
Torque is defined as $τ = r F$ . Substitute the expression for $F$ : $τ = r (m r α) = (m r^{2}) α$
Since $I = m r^{2}$ for a point mass, we get: $τ = I α$

This law states that the net torque on an object is directly proportional to its angular acceleration, and the constant of proportionality is the moment of inertia. A larger moment of inertia means a smaller angular acceleration for the same applied torque.

Practical Examples

Truck Tires and Steering Wheels: Large, heavy truck tires have a high moment of inertia, which contributes to their stability once they are rotating. The large steering wheel is a practical application of torque ( $τ = r F$ ); its large radius ( $r$ ) allows the driver to apply the necessary torque to turn the wheels with less force ( $F$ ).
Ice Skater Spin: An ice skater controls their spin speed by changing their moment of inertia. With arms outstretched, $I$ is large and the spin is slow. Pulling their arms in concentrates their mass closer to the axis of rotation, decreasing $I$ and, by conservation of angular momentum, increasing their rotational speed.

Possible Questions and Answers

Q: Why is it easier to balance a long pole than a short pencil on your fingertip?

A: A long pole has a much larger moment of inertia because its mass is distributed far from the pivot point (your finger). According to $τ = I α$ , a large $I$ means that any small, unwanted torque (from your hand shaking) will produce a very small angular acceleration ( $α$ ), giving you more time to react and make corrections.
Q: Do objects have a single, fixed moment of inertia?

A: No. The moment of inertia of an object depends on the chosen axis of rotation. A rod, for example, has a different moment of inertia when rotated about its center compared to when it is rotated about one of its ends.

Introduction

Inertia vs. Moment of Inertia

Concept	Inertia (Linear)	Moment of Inertia (Rotational)
Definition	Resistance to change in linear motion.	Resistance to change in rotational motion.
Depends on	Mass only.	Mass and the distribution of that mass relative to the axis of rotation.
Formula	Not a calculated quantity; it is mass ( $m$ ).	For a point mass: $I = m r^{2}$ . For a rigid body, it is the sum of $m r^{2}$ for all particles.

Formula for a Point Mass

The moment of inertia ( $I$ ) for a single point mass ( $m$ ) rotating at a distance ( $r$ ) from an axis is:

$I = m r^{2}$

Moment of Inertia for a Rigid Body

A rigid body is a collection of many particles. The total moment of inertia of the body is the sum of the moments of inertia of all its individual particles.

$I = \sum_{i} m_{i} r_{i}^{2}$

For continuous objects, this sum becomes an integral over the entire body:

$I = \int r^{2} d m$

In practice, pre-calculated formulas for common shapes are used:

Object	Axis of Rotation	Moment of Inertia ( $I$ )
Thin Hoop or Ring	Through center, perpendicular to plane	$M R^{2}$
Solid Cylinder or Disc	Through center, along axis	$\frac{1}{2} M R^{2}$
Solid Sphere	Through center	$\frac{2}{5} M R^{2}$
Thin Rod	Through center, perpendicular to rod	$\frac{1}{12} M L^{2}$

Radius of Gyration

The radius of gyration ( $k$ ) is the distance from the axis of rotation to a point where the entire mass of the body could be concentrated without changing its moment of inertia. It is given by:

$I = M k^{2} ⟹ k = \frac{I}{M}$

Analogy Between Linear and Rotational Quantities

The moment of inertia plays the same role in rotational mechanics that mass plays in linear mechanics.

Linear Quantity	Angular Quantity	Relationship
Displacement ( $s$ )	Angular Displacement ( $θ$ )	$s = r θ$
Velocity ( $v$ )	Angular Velocity ( $ω$ )	$v = r ω$
Acceleration ( $a$ )	Angular Acceleration ( $α$ )	$a = r α$
Mass ( $m$ )	Moment of Inertia ( $I$ )	—
Force ( $F$ )	Torque ( $τ$ )	$τ = r F$

Torque (Moment of Force)

Torque is the rotational equivalent of force — it is the turning effect of a force about an axis of rotation. For a force $F$ applied at position vector $r$ from the axis:

$τ = r \times F$

The magnitude is:

$τ = r F sin θ$

where $θ$ is the angle between $r$ and $F$ . Torque is maximum when $θ = 9 0^{\circ}$ (force perpendicular to the moment arm).

SI unit: N·m (Newton-metre) — same dimensions as energy $[M L^{2} T^{- 2}]$ but physically distinct.

Newton's Second Law for Rotation

The rotational equivalent of Newton's Second Law ( $F_{n e t} = ma$ ) directly involves the moment of inertia.

Formula:

$τ_{n e t} = I α$

Derivation:

Start with Newton's Second Law for a point mass: $F = ma$ .
The tangential acceleration is related to angular acceleration by $a_{t} = r α$ . So, $F = m (r α)$ .
Torque is defined as $τ = r F$ . Substitute the expression for $F$ : $τ = r (m r α) = (m r^{2}) α$
Since $I = m r^{2}$ for a point mass, we get: $τ = I α$

Practical Examples

Truck Tires and Steering Wheels: Large, heavy truck tires have a high moment of inertia, which contributes to their stability once they are rotating. The large steering wheel is a practical application of torque ( $τ = r F$ ); its large radius ( $r$ ) allows the driver to apply the necessary torque to turn the wheels with less force ( $F$ ).
Ice Skater Spin: An ice skater controls their spin speed by changing their moment of inertia. With arms outstretched, $I$ is large and the spin is slow. Pulling their arms in concentrates their mass closer to the axis of rotation, decreasing $I$ and, by conservation of angular momentum, increasing their rotational speed.

Possible Questions and Answers

Q: Why is it easier to balance a long pole than a short pencil on your fingertip?

A: A long pole has a much larger moment of inertia because its mass is distributed far from the pivot point (your finger). According to $τ = I α$ , a large $I$ means that any small, unwanted torque (from your hand shaking) will produce a very small angular acceleration ( $α$ ), giving you more time to react and make corrections.
Q: Do objects have a single, fixed moment of inertia?

A: No. The moment of inertia of an object depends on the chosen axis of rotation. A rod, for example, has a different moment of inertia when rotated about its center compared to when it is rotated about one of its ends.