   Chapter 4, Problem 92AP

Chapter
Section
Textbook Problem

A 72-kg man stands on a spring scale in an elevator. Starting from rest, the elevator ascends, attaining its maximum speed of 1.2 m/s in 0.80 s. The elevator travels with this constant speed for 5.0 s, undergoes a uniform negative acceleration for 1.5 s, and then comes to rest What does the spring scale register (a) before the elevator starts to move? (b) During the first 0.80 s of the elevator’s ascent? (c) While the elevator is traveling at constant speed? (d) During the elevator’s negative acceleration?

(a)

To determine
The spring scale reading before the elevator starts to move.

Explanation

Given info: Mass of the man is 72 kg. Elevator attains maximum speed of 1.2 m/s in 0.80 s. The elevator travels with constant speed for 5.0 s and undergoes uniform negative acceleration for 1.5 s.

The formula for normal force is,

N=m(a+g) (I)

• m is the mass of the man.
• g is the acceleration due to gravity.
• a is the acceleration of the elevator.

When the elevator is about to move, acceleration is zero.

Substitute 72 kg for m, 9.8ms2 for g and 0ms2 for a in Equation (I) to get N

(b)

To determine
The spring scale reading during the first 0.80 s.

(c)

To determine
The spring scale reading when the elevator travels with constant velocity.

(d)

To determine
The spring scale reading during the elevator’s negative acceleration.

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