Calculate the net energy of this system (mass and Slinky in an elevator)

AI Thread Summary
The discussion revolves around calculating the net energy of a system involving a slinky and a mass in an elevator. It highlights that during the slinky's contraction, the mass experiences weightlessness while moving upward at a constant speed of 1 m/s. Participants question the assumptions about the mass and energy calculations, emphasizing the importance of considering the slinky's mass and the effects of gravitational potential energy. The conversation also touches on the relationship between initial and final energy states, suggesting that potential energy is gained as the mass rises. Overall, the thread seeks clarity on the energy dynamics of the system as the slinky contracts and the mass moves.
leafy
Messages
73
Reaction score
8
Homework Statement
Supposed we have a massless elevator as shown. Inside the elevator we have a hanging slinky and a 1kg mass attached to the slinky. We will give the elevator a initial constant speed of 1m/s upward. Then we cut the top of the slinky. The slinky is designed to fully contract in 1 second.
Relevant Equations
E=mgh
The slinky is designed to fully contract in 1 second. During this one second, the mass is weightless and move up at constant speed of 1m/s. After 1 second the mass gain 1m height in potential energy.

EA735C5B-8CAE-4298-93D3-16C500CBA09A.jpeg
Am I missing something?
 
Physics news on Phys.org
leafy said:
During this one second, the mass is weightless
You mean something else ? ##mg## is not switched off during one second !

And, uh, what is the problem in the problem statement ? (I don't see a question there...)

##\ ##
 
You assume, of course, that the slinky has mass. Why do you think you are missing something? Suppose you are in another elevator also moving up at constant speed of 1 m/s looking at the slinky. What would you see? Answer: What you see when you release the slinky standing on solid ground in the lab frame.

Why does it bother you that the mass is moving up at 1 m/s in the elevator picture and it doesn't bother you that the mass is temporarily at rest while the CM of the slinky accelerates as the slinky contracts in the lab frame picture?

Here is a nice video of what's going on for those unfamiliar with the falling slinky.
 
Last edited:
It bother me because I don't understand the answer.

E(initial) = E(final) ---- should be

E(initial) = 1/2mv^2 + E(slinky stretch) + E(mass linky x g x height slinky)
E(final) = (1/2mv^2 + mgh) + E(kinetic energy from slinky stretch) + E(kinetic energy from slinky height)

E(final) - E(initial) = mgh (potential energy of the mass)
 
Kindly see the attached pdf. My attempt to solve it, is in it. I'm wondering if my solution is right. My idea is this: At any point of time, the ball may be assumed to be at an incline which is at an angle of θ(kindly see both the pics in the pdf file). The value of θ will continuously change and so will the value of friction. I'm not able to figure out, why my solution is wrong, if it is wrong .
Thread 'Voltmeter readings for this circuit with switches'
TL;DR Summary: I would like to know the voltmeter readings on the two resistors separately in the picture in the following cases , When one of the keys is closed When both of them are opened (Knowing that the battery has negligible internal resistance) My thoughts for the first case , one of them must be 12 volt while the other is 0 The second case we'll I think both voltmeter readings should be 12 volt since they are both parallel to the battery and they involve the key within what the...
Thread 'Trying to understand the logic behind adding vectors with an angle between them'
My initial calculation was to subtract V1 from V2 to show that from the perspective of the second aircraft the first one is -300km/h. So i checked with ChatGPT and it said I cant just subtract them because I have an angle between them. So I dont understand the reasoning of it. Like why should a velocity be dependent on an angle? I was thinking about how it would look like if the planes where parallel to each other, and then how it look like if one is turning away and I dont see it. Since...
Back
Top