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Q7.32(d):
Read each statement below carefully, and state, with reasons, if it is true or false; (d) For perfect rolling motion, work done against friction is zero.

Solution :

Theoretical Foundation: Mechanics of Rolling Motion

To determine the validity of the statement "For perfect rolling motion, work done against friction is zero," we must analyze the interaction between the point of contact of a rolling body and the surface upon which it moves.

Step 1: Kinematic Analysis of the Point of Contact

In a state of perfect rolling motion (rolling without slipping), the instantaneous velocity of the point of contact ($v_{contact}$) of the rigid body with respect to the stationary surface must be zero. Let $v_{cm}$ be the velocity of the center of mass, $\omega$ be the angular velocity, and $R$ be the radius of the body.

The condition for no slipping is given by: $v_{contact} = v_{cm} - R\omega = 0$ [Per the condition of Pure Rolling]

Step 2: Analysis of Work Done by Friction

Work done ($W$) by a force ($F$) is defined by the line integral: $W = \int \vec{F} \cdot d\vec{s}$ [Per the Work-Energy Theorem]

In the context of a body rolling on a surface, the friction force acting at the point of contact is static friction ($f_s$). Since the point of contact is instantaneously at rest relative to the surface (as established in Step 1), the displacement ($d\vec{s}$) of the point of contact relative to the surface is zero during the infinitesimal time interval of contact.

Step 3: Logical Deduction

Since the displacement $d\vec{s} = 0$ at the exact point of application of the friction force, the instantaneous power delivered by friction ($P = \vec{f}_s \cdot \vec{v}_{contact}$) is zero.

Consequently, the total work done by friction over any finite displacement is: $W_{friction} = \int \vec{f}_s \cdot \vec{v}_{contact} \, dt = \int \vec{f}_s \cdot 0 \, dt = 0$

Visual Representation of Forces in Perfect Rolling

$\vec{v}_{cm}$ Point of Contact ($v=0$) Surface

Comparative Analysis of Energy Dissipation

Motion Type Point of Contact Velocity Work Done by Friction
Perfect Rolling $0$ $0$
Sliding $\neq 0$ Negative (Dissipative)

Final Solution: The statement is True. In perfect rolling motion, the point of contact is instantaneously at rest relative to the surface, resulting in zero displacement for the force of friction; therefore, no work is done against friction.


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