A 69-kg man stands on a spring scale in an elevator. Starting from rest, the ele
ID: 1871849 • Letter: A
Question
A 69-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 0.78 s. The elevator travels with this constant speed for 5.0 s, undergoes a uniform negative acceleration for 1.6 s, and then comes to rest.
(a) What does the spring scale register before the elevator starts to move? N
(b) What does the spring scale register during the first 0.78 s of the elevator's ascent? N
(c) What does the spring scale register while the elevator is traveling at constant speed? N
(d) What does the spring scale register during the elevator's negative acceleration? N
Explanation / Answer
a)
F = ma
F = 69 kg x 9.81 m/s^2
F = 676.89N
b)
a = dv/dt
a = 1.20/0.78
a = 1.54 m/s^2
F = F_a + ma
F = 676.89 + 69*1.54
F = 783.15 N
c) 676.89 N
d)
a = dv/dt
a = -1.20/1.6
a = -0.75 m/s^2
F = F_a + ma
F = 676.89 + (69*-0.75)
F = 625.14 N
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