• Zach, whose mass is 80 kg, is in an elevator descending at 10 m/s. The elevator takes 3.0 s to brake to a stop at the first floor. What is Zach’s weight before the elevator starts braking? What is Zach’s weight while the elevator is braking?

Explanation:

mass, m = 8 kg

initial velocity, u = 10 m/s

time, t = 3  s

final velocity, v = 0 m/s

Let the acceleration is a

use first equation of motion

v = u + at

0 = 10 - a x 3

a = 3.33 m/s² downward

As the elevator is moving with constant velocity so the weight is the true weight = m x g = 80 x 9.8 = 784 N

Now the elevator is moving down with an acceleration is 3.33 m/s², let the weight is R.

Use Newton's second law

mg - R = ma

R = m ( g - a)

R = 80 ( 9.8 - 3.33)

R = 517.6 N