Spider-Man holds the bottom of an elevator with one hand. With his other hand, h
ID: 1459253 • Letter: S
Question
Spider-Man holds the bottom of an elevator with one hand. With his other hand, he holds a spider cord attached to a 40-kg box of explosives at the bottom of the cord.
a)Determine the force that the cord exerts on the box if the elevator is at rest.
b)Determine the force that the cord exerts on the box ifthe elevator accelerates up at 3.8 m/s2 .
c)Determine the force that the cord exerts on the box if the upward-moving elevator's speed decreases at a rate of 3.8 m/s2 .
d)Determine the force that the cord exerts on the box if the elevator falls freely.
Explanation / Answer
Mass of the box m = 40 kg
(a). The force that the cord exerts on the box if the elevator is at rest F = Weight of the box
= mg
= 40 x9.8
= 392 N
b)The force that the cord exerts on the box ifthe elevator accelerates up at 3.8 m/s2 is F ' = m( a+ g )
Where a = accleration = 3.8m/s 2
Substitute values you get F ' = 40 ( 3.8 + 9.8 )
= 40 (13.6)
= 544 N
c) The force that the cord exerts on the box if the upward-moving elevator's speed decreases at a rate of 3.8 m/s2 is
F = m( g + a)
Where a = -3.8 m/s 2
F = 40 ( 9.8 -3.8)
= 40 (6)
= 240 N
d) The force that the cord exerts on the box if the elevator falls freely F = m( g - a)
In freely falling body a = g
So, F = m ( g - g )
= m( 0)
= N
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