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Let me preface this as I am not a physics expert but this is something I was wondering as I would love a technical explaination.

For example: say a person is walking with a filled plastic grocery bag and you can tell that the handle might rip, just due to the pressure between your fingers/handles being great. However, it does not rip and you are making it.

UNTIl--you decide you raise your hand holding the bag and place it onto the counter when you get home. As you do so, the bag rips.

If no additional weight was placed upon the bag, just the movement of raising your hand, then what caused the bag to experience a greater force that caused it to break?

Please feel free to explain in a technical manner but assume descriptions would be helpful!

Thank you & GOD Bless!

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    $\begingroup$ In order to accelerate the bag upwards, you have to exert a greater upward pull than just keeping it at a fixed height (the magnitude of the pull for fixed height is equal to the weight). The act of pulling it upwards, due to the greater pulling force, thus makes it even more likely to rip the bag. If you want to prevent the breaking, you slowly let it move towards the ground. $\endgroup$ Commented Nov 29, 2023 at 7:03
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    $\begingroup$ As stated above, you are applying more force in order to move it up, that force if larger than maximum tension tolerable by the plastic bag can tear it. $\endgroup$
    – Ashutosh
    Commented Nov 29, 2023 at 7:05
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    $\begingroup$ note, a bag can rip "while it is stationary" if the weight of its contents exceeds the maximum force the handles or film support. For example, if the breaking tension is 0.9mg, then when you go to pick up the bag with force ~1.1mg, you'd find yourself holding the torn off handles while the contents remain on the ground. This happens occasionally with paper bags with the glued on handles, and causes less of a mess than the 1.1mg breaking tension which raises your contents before failure. $\endgroup$
    – Sam
    Commented Nov 29, 2023 at 17:35
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    $\begingroup$ I feel like all the answers are missing the point. If the problem was actually the acceleration, then the bags would break right when you started trying to raise them -- in reality they often break when you get past the middle. The reason that happens is that when your arms are straight, you can hold the bags symmetrically so that equal weight is on each handle. When you raise them to chest height, unless you're really careful you'll put more of the weight on one handle and that one will snap. $\endgroup$
    – knzhou
    Commented Dec 1, 2023 at 2:08
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    $\begingroup$ @knzhou you should promote this comment to an answer $\endgroup$ Commented Dec 1, 2023 at 22:12

5 Answers 5

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This is like holding a weight $mg$ with a string which can break with a tension of $1.1 mg$. When you are not moving the bag, or moving it at a uniform speed, the string's tension is constant at $mg$, and just high enough to support the weight of the bag, and low enough for it to not break. But 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 ($m \times 0.1g$). This increases the tension to $1.1 mg$ and string breaks.

So as long as you don't accelerate the bag, that is not move it upwards, the tension remains under the breaking limit. But as soon as you move it upwards, that increases the tension and the handle can break. If you want to prevent this from happening, reduce the upward acceleration (say $0.01g$), which increases the tension only by a small amount ($mg+0.01mg=1.01mg<1.1mg$), and hence the string wont break. So move up the bag slowly next time, it won't break. Or, of course, use a stronger bag or distribute weight between two bags :)


Notation
$m :$ mass of the bag
$g :$ acceleration due to gravity

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    $\begingroup$ @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. $\endgroup$
    – DKNguyen
    Commented Nov 29, 2023 at 20:34
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    $\begingroup$ @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. $\endgroup$
    – DKNguyen
    Commented Nov 29, 2023 at 20:39
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    $\begingroup$ @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. $\endgroup$
    – Ben
    Commented Nov 30, 2023 at 3:19
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    $\begingroup$ 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. $\endgroup$
    – TecBrat
    Commented Nov 30, 2023 at 5:40
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    $\begingroup$ And if your other hand is free, use it to support the bottom of the bag at the same time. $\endgroup$
    – nasch
    Commented Nov 30, 2023 at 21:08
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You move the bag upwards with an acceleration. If you were to pull the bag up with infinitesimal acceleration, it would not rip then, because the force required for that would approach zero. But since there is an acceleration, you will have to account for the pseudo force as well (in the reference frame of your hand) while making FBD. That pseudo force, combined with the earth's gravitational force, exceeds the breaking stress of the bag.

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    $\begingroup$ Is there any need to involve pseudoforces and frames of reference here? In order to raise the bag, your hand applies a real force to the bag which is greater than the bag's weight, and the bag's handle is put under a tension equal to that greater force. $\endgroup$ Commented Nov 29, 2023 at 15:52
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    $\begingroup$ The OP won't know what FBD stands for either.. $\endgroup$ Commented Nov 29, 2023 at 18:24
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    $\begingroup$ @TannerSwett Multiple answers can be given to a single question but only a single answer is accepted, which an OP finds most useful to them. So there is no necessity for a resistance to answers with different approaches, which can be understandable by broader set of audience. $\endgroup$ Commented Nov 29, 2023 at 19:50
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(An attempt to explain this with non-technical language).

There're two forces acting on the bag - the force from your hand, and the Earth's gravitational force (the weight). When you lift the bag, you need to exert a larger force. In fact, the bag will not rise unless you exert a force that's larger than the weight. E.g., when the bag is on the ground and you lift it up, the reason the bag is lifted off the ground is because you exerted a force that's greater than its weight.

It should not be surprising that larger forces cause the bag to break. If you want the bag to not break, Newton's laws say that the way to do it is to lift the bag s-l-o-w-l-y, with as small an acceleration as possible.

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  • $\begingroup$ Thank you so much for your explanation! Could you please elaborate on what Neuton's Law says specifically about lifting the bag "slowly"? $\endgroup$
    – user385891
    Commented Mar 28 at 3:29
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    $\begingroup$ @user385891 when you lift the bag slowly, the acceleration is small, and by Newton's 2nd law the force is small. $\endgroup$
    – Allure
    Commented Mar 28 at 3:45
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It is because of the bag's inertia. It takes force to accelerate any mass, like a bag, and if you start moving it up then that force is added on top of the force the bag already exerts on the handles due to its gravitational attraction to the Earth.

While the force being exerted due to the bag's weight is not enough to rip the handles, it seems that adding just a bit more by moving the bag upwards is enough, and then it rips apart.

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  • $\begingroup$ As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center. $\endgroup$
    – Community Bot
    Commented Dec 1, 2023 at 6:28
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Gravitational pull acting on the strength and integrity of the bag, combined with a short, quick pull as the bag is lifted, straining the integrity further still.

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