A single mass m1 = 4.6 kg hangs from a spring in a motionless elevator. The spri

Question

A single mass m1 = 4.6 kg hangs from a spring in a motionless elevator. The spring is extended x = 10 cm from its unstretched length. 1) What is the spring constant of the spring? 450.8 N/m Submit 2)masseshangingfromsprings2smaller Now, three masses m1 = 4.6 kg, m2 = 13.8 kg and m3 = 9.2 kg hang from three identical springs in a motionless elevator. The springs all have the same spring constant that you just calculated above. What is the force the top spring exerts on the top mass? 270.48 N Submit 3) What is the distance the lower spring is stretched from its equilibrium length? 20 cm Submit 4) Now the elevator is moving downward with a velocity of v = -2.2 m/s but accelerating upward with an acceleration of a = 4.1 m/s2. (Note: an upward acceleration when the elevator is moving down means the elevator is slowing down.) What is the force the bottom spring exerts on the bottom mass? 127.88 N Submit 5) What is the distance the upper spring is extended from its unstretched length? 85.1429 cm Submit 6) Finally, the elevator is moving downward with a velocity of v = -2.8 m/s and also accelerating downward at an acceleration of a = -2.6 m/s2. The elevator is: speeding up slowing down moving at a constant speed Submit 7) Rank the distances the springs are extended from their unstretched lengths: x1 = x2 = x3 x1 > x2 > x3 x1 < x2 < x3 Submit 8) What is the distance the MIDDLE spring is extended from its unstretched length? i cant figure out number 8 all the other answers are correct. the answer ist 71

Explanation / Answer

Let s1 is attached to the ceiling (elevator) and m1 hangs from it;
s2 is attached to m1; m2 hangs from s2;
s3 is attached to m2; m3 hangs from s3.

Since we're not given the masses of the springs, I'm assuming that their masses are negligible.

1) F = k * x (Hooke's Law)

Fg = m * g = k * x ==>> k = m1*g/x = 4.6*9.8/0.10 = 450.8 N/m

2) Since the top spring s1 is supporting all 3 masses, it is exerting a force equal to gravity's downward pull but in the opposite direction:
Fg = (m1 + m2 + m3) * g = 27.6 * 9.8 = 270.48 N

3) The lower spring s3 is supporting only m3; applying Hooke's law:
x = Fg / k = m * g / k
Notice, however that m3 = 2 * m1, therefore:
x3 = 2 * x1 = 2 * 10 = 20 cm

4) The downward velocity has no effect on the force situation, it is only changes in velocity (plus, of course, gravity, which is always there) that require a force. At constant velocity, the bottom spring s3 is supporting its mass m3 to balance gravity.
As the elevator slows, though, it also ends up slowing down the spring arrangement, too. However, because the stretching takes time, it means that some damped harmonic motion will be set up in the spring chain.
When the motion has finally damped out, the net force the bottom spring s3 exerts on m3 has two components--that of gravity and of the deceleration of the elevator:
F3,net = m3 * (g + a)
= 9.2 * (9.8 + 4.1) = 127.88 N

5) Assuming this question applies to the decelerating elevator (to make the question different from #2.), the upper spring s1 is not only balancing the force of gravity on all 3 masses but also counteracting the decelerating force:

F = (m1 + m2 + m3) * (g + a)
= 383.64 N
Again, using Hooke's law:
x = 383.64 /450.8 = 85.1 cm

8) Assuming again that this question applies to the decelerating elevator, the middle spring s2 is not only balancing the force of gravity on m2 and m3 but also counteracting the decelerating force:

F = (m2 + m3) * (g + a)
= (13.8 + 9.2) * (9.8 + 4.1) =
Also, x = F / k
= 319.7/450.8 = 0.7092 m
= 70.92 cm

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