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Oldfart
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Inspired by something I saw today in the New Yorker Magazine, I cannot resist asking: Will a Slinky move down an up eccalator forever?
Oldfart said:Inspired by something I saw today in the New Yorker Magazine, I cannot resist asking: Will a Slinky move down an up eccalator forever?
How? Ideally, an escalator is identical to an infinitely long stairway.Oldfart said:I think it's incorrect to compare the escalator to a staircase, the dynamics are different.
I only used the ball because it is easier to visualize a simple ball descending over an arbitrarily long distance than a slinky, which might intuitively seem like it's going to run out of energy. No one has trouble envisioning why a ball could bounce all the way down a staircase of arbitrary height. It's just a short step to recognize that a slinky can do the same thing.Oldfart said:Dave's bouncing ball is also dynamically different than a Slinky, but if you prefer, try it with the ball. I suspect that if the ball hops "down" forever, the Slinky will also.
DaveC426913 said:How? Ideally, an escalator is identical to an infinitely long stairway.
Sorry, not seeing it. Are you saying that the ball/slinky can "tell" the difference between a staircase and an escalator? If I constructed them and put you on one of them in a darkened room, what tests would you conduct to determine which one you're on? (Ignoring real-world limits such as altitude and gravitational gradient).AlephZero said:There is a very fundamental difference. On an infinitely long stairway, the energy that keeps the slinky moving is coming from its weight (acting over the whole of the spring) as it descends. On the escalator, the energy is coming from the reaction from the ends of the spring and the steps (and ultimately from the motor driving the escalator upwards, of course).
You've lost me. Why would you want to counteract horizontal movement? You want it to go over each step.AlephZero said:...there DOES seem to be a problem stopping the ball moving horizontally in one direction and therefore descending the escalator whether or not it is moving upwards. I can't see how to counteract that without some "backspin" on the ball, and it's not clear to me how the spin could be maintained indefinitely.
From his post, it sounded like he might be referring to real life. In real life, it would go for about 10s then fall over.DaveC426913 said:How? Ideally, an escalator is identical to an infinitely long stairway.
AlephZero said:There is a very fundamental difference. On an infinitely long stairway, the energy that keeps the slinky moving is coming from its weight (acting over the whole of the spring) as it descends. On the escalator, the energy is coming from the reaction from the ends of the spring and the steps (and ultimately from the motor driving the escalator upwards, of course).
AlephZero said:There is a very fundamental difference. On an infinitely long stairway, the energy that keeps the slinky moving is coming from its weight (acting over the whole of the spring) as it descends. On the escalator, the energy is coming from the reaction from the ends of the spring and the steps (and ultimately from the motor driving the escalator upwards, of course).
gsal said:I think that a stairway and an escalator provide TOTALLY different experience...for one, an escalator has continuous input of energy; whereas the stairway is just there. As mentioned before, an escalator would slap from below a bouncing ball, whereas a stairway wouldn't.
No. Once you or the ball has the bit of horizontal motion imparted upon it, it has no reason to stop moving horizontally.gsal said:P.D. by the way, just to add that the reason I see for developing a back spin at the escalator is because the escalator step is moving at an angle but but the ball is coming down vertically...think about it, try to land on a moving surface while your center of gravity is coming down...you will feel that your feet are going away from under you...right?
Well said. This is what I'm trying to convey.Michael C said:Look at this from a point of view of Galilean Relativity. A staircase is at rest in the inertial frame defined by the Earth. You can also define an inertial frame in which the steps in the centre portion of the escalator are at rest. The force of gravity will be the same in both frames. For the Slinky, there is no difference.
DaveC426913 said:No. Once you or the ball has the bit of horizontal motion imparted upon it, it has no reason to stop moving horizontally.
Think of horizontal people movers at the airport. Once it has brought you up to walking speed, you are moving inertially. You don;t feel a constant pull backwards. And if you bounced a ball while on this people mover, it would go straight up and down just as if you weren't moving at all.
I'm not changing your exercise. You were not specific.gsal said:Hold on...aren't you changing MY exercise? or forgetting my initial condition?...you said "once the ball has horizontal motion"...that's a bit afterwards or a different initial condition...I said, if I drop the ball vertically!
"Stationary" with respect to what reference point - exactly?gsal said:If I am dropped from a stationary position into a stairway or an escalator,
Michael C said:Here's a video of a Slinky on an escalator:
That was taken with a fixed camera and a moving escalator, but you could get the same effect with an escalator at rest and a moving camera.
lorrad said:Ok 2 cents
First an escalator is not an infinitely long stairway
An escalator has angular velocity whereas a stairway is static
DaveC426913 said:I'm not changing your exercise. You were not specific.
In both the frame of the reference of the stairs and in the frame of reference of the escalator, a ball dropped vertically will come to rest on the step and have no horizontal motion. So, this test does not distinguish the difference.
I suspect that when you were speaking of "a ball being dropped vertically", what you meant (but did not specify) is with respect to the warehouse that the escalator/stairs are housed in. i.e. outside the system being examined.
Dave said:I don't see why not.
A bouncing ball will.
DaveC426913 said:How does the ball know that?
Again, in a dark warehouse, what test could you do to tell whether you are on a set of stairs or on an escalator?
Michael C said:The main part of the escalator has no angular velocity: it is moving at a steady speed in a straight line.
BryceCo said:I'd check for a slight breeze, indicating I was on an escalator.
No, eventually the Slinky will come to a stop due to friction and other external forces acting on it.
The Slinky moves on the escalator due to a combination of gravity and the motion of the escalator steps. The downward pull of gravity causes the Slinky to stretch out and the motion of the escalator steps helps to keep it moving.
The Slinky may move for a longer period of time on some escalators compared to others, but it will eventually come to a stop on all types of escalators due to friction and other external forces.
As the Slinky moves on the escalator, it may experience changes in direction and speed due to variations in the escalator's motion or the Slinky's own elasticity. However, it will ultimately come to a stop.
The Slinky may continue to move for a longer period of time on a moving staircase or conveyor belt due to the constant motion and lack of friction. However, it will eventually come to a stop due to other external forces acting on it.