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Chapter 10: Problem 8

A circular saw takes a long time to stop rotating after the power is turned off. Without the saw blade mounted, the motor stops much more quickly. Why?

Short Answer

Expert verified
The saw takes longer to stop rotating when the blade is mounted due to a larger rotational inertia, which results from an increased mass and mass distribution from the axis of rotation. This larger rotational inertia means a greater resistance to changes in rotational motion, so the blade keeps spinning for a longer time after the power is cut.

Step by step solution

01

Understand the Principle of Inertia

The principle of inertia, as stated by Newton's first law, tells us that an object at rest will stay at rest and an object in motion will stay in motion unless acted on by an external force. So, when the power to the saw is turned off, it doesn鈥檛 stop immediately because of inertia.
02

Understand Rotational Inertia

Rotational inertia (or moment of inertia) is the rotational analog to linear inertia and it basically describes how difficult it is to change the rotational velocity of an object. In simple terms, the larger the mass distribution from the axis of rotation, the larger the moment of inertia and the slower the object will come to a stop.
03

Apply the Concepts to the Problem

When the saw blade is mounted on the motor, the combined system (motor plus blade) has a larger mass and mass distribution from the axis of rotation. Hence, the system has a larger rotational inertia compared to when the saw blade is not mounted. So, it will take a longer time for the blade to stop rotating when power is cut off because it has more inertia.

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