Timeline for Why does a plastic bag rip when it is raised to a higher position when being held but not when it is stationary?
Current License: CC BY-SA 4.0
15 events
| when toggle format | what | by | license | comment | |
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| Mar 28 at 3:26 | vote | accept | user385891 | ||
| Mar 28 at 3:28 | |||||
| Mar 28 at 3:26 | comment | added | user385891 | Thank you for this comment and explanation! It is greatly appreciated! | |
| Dec 7, 2023 at 15:45 | history | edited | AlphaLife | CC BY-SA 4.0 |
made notation clear, even though its obvious
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| Dec 2, 2023 at 22:34 | comment | added | Agnius Vasiliauskas | @An_Elephant Why can't everything just move up being accelerated instead of persisting to cancel out extra force ? Basically due to Newton third law - for every action there's a reaction. You pull bag UP, and it's handle tension pulls your hand DOWN. | |
| Dec 1, 2023 at 13:22 | comment | added | H2ONaCl | @TecBrat If you have a seemingly immovable 100 tonne block of concrete on a concrete surface and a human tries to push it horizontally, the human's horizontal force is balanced by just as much horizontal force generated on the surface under the block. The more force the human applies, the more force will be generated in the opposite direction. (This is unlike the air around the bag, which will not do this.) If the human is replaced by a machine that can produce more force, the inertia is overcome, when some of the concrete deforms at a microsopic level as it is ground into a powder. | |
| Dec 1, 2023 at 13:09 | comment | added | H2ONaCl | @TecBrat Inertia is the phenomenon that velocity is constant in the absence of unbalanced force. Since the question posits that an unbalanced force will be applied, it is a given that inertia will be "overcome". So I wouldn't have mentioned it either. Since the bag is subject to only gravity, the person holding it, and the surrounding air which applies a balanced pressure on all sides, introducing even a small upward force will overcome inertia if you have the instrument that measures velocity (and a change in velocity) accurately. | |
| Nov 30, 2023 at 21:08 | comment | added | nasch | And if your other hand is free, use it to support the bottom of the bag at the same time. | |
| Nov 30, 2023 at 19:29 | comment | added | setholopolus | TLDR; To avoid ripping the bag, lift it onto the counter very slowly. | |
| Nov 30, 2023 at 5:40 | comment | added | TecBrat | I'm surprised that the word "inertia" hasn't been mentioned in any of these comments. I'm not a physicist, so I can't give you details and formulae, but I can say that in order to move the bag up, the pull has to overcome the inertia of the contents. It's even worse if the bag was moving down, even a little bit, before you try to lift it. | |
| Nov 30, 2023 at 3:19 | comment | added | Ben | @An_Elephant "Why can't everything just move up being accelerated" The bag won't start to move up unless a force is applied to accelerate it. If there were some uniformly-applied force (like an electric field, if the bag and its contents were charged) then indeed everything could just accelerate together. But that's not how the force to accelerate the bag is applied; it's applied by you pulling on the handles, hoping the body of the bag will follow. That only happens because it increases the tension on the plastic, which will then either break or pull upwards on the body of the bag. | |
| Nov 29, 2023 at 20:39 | comment | added | DKNguyen | @An_Elephant For example, if you were pulling up on a mass connected by a bungee cord, while the cord tension was not enough to cancel out the net force, your hand and the mass would be moving up but at different accelerations and velocities. The mass would be moving slower than your hand and this stretches out the bungee cord and increases the tension on the cord until it does cancel out. At that point the velocity and acceleration of the mass matches that of your hand. | |
| Nov 29, 2023 at 20:34 | comment | added | DKNguyen | @An_Elephant Because, assuming the bag is inelastic, internal forces. Your hand is moving up, the bag is moving up, and the contents of the bag are moving up at the same velocity. Therefore they are fixed relative to one another so forces must cancel out, otherwise they would be moving relative to each other. | |
| Nov 29, 2023 at 19:46 | comment | added | An_Elephant | "as soon as you accelerate it with 0.1g acceleration, the tension now needs to balance both the weight of the bag (mg ) and the force due to the acceleration you are providing (m×0.1g ). " Why it should ? Why should there be equal force to cancel out net force ? Why can't everything just move up being accelerated instead of persisting to cancel out extra force ? | |
| Nov 29, 2023 at 7:12 | history | edited | AlphaLife | CC BY-SA 4.0 |
added 542 characters in body
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| Nov 29, 2023 at 7:06 | history | answered | AlphaLife | CC BY-SA 4.0 |