Does Tying a Knot in a Plastic Bag Double its Weight?

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If you did that experiment in a vacuum (or, I suppose, in a fluid with a density similar to the plastic bag), you'd be able to see the difference.In summary, the conversation discusses the possibility of tying a knot in a plastic baggy to double its weight from 1 gram to 2 grams. However, it is determined that the mass and gravity do not change, so the weight remains the same. The idea of allowing air to enter the bag and trapping it to increase the weight is also brought up, but it is uncertain if this would actually work. The conversation ends with a discussion about the pressure of air and the possibility of conducting the experiment in a vacuum to eliminate the buoyancy factor.
  • #1
J23Smith
I know this will seemingly sound stupid but I have had multiple people argue with me about it. They claim that if they tie a knot in a plastic baggy it doubles the weight from 1 gram to 2 grams. I tell them there is no way because the mass doesn't change. I've tried to bet them any amount of money they were willing to bet and I have had no takers. So is there any way the weight changes without changing the mass or gravity?
 
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  • #2
W=mg...Putting the knot in a bag won't change m or g so the weight of the knot is the same.

Unless they meant that you weigh plastic bag that contains the knot.
 
  • #3
rock.freak667 said:
W=mg...Putting the knot in a bag won't change m or g so the weight of the knot is the same.

Unless they meant that you weigh plastic bag that contains the knot.

maybe they mean that they allow air to enter the plastic bag, then tie the knot so as to trap air...hence weight of the "bag" increses
 
  • #4
praharmitra said:
maybe they mean that they allow air to enter the plastic bag, then tie the knot so as to trap air...hence weight of the "bag" increses

They didn't intend on any air or anything else that might increase the weight entering the bag. They are just dumb as I suspected they were in this matter. Thanks guys.
 
  • #5
J23Smith said:
They didn't intend on any air or anything else that might increase the weight entering the bag.
Are you sure? Because this would actually work. I can see there being some intuitive confusion around this.

You weigh an open baggie on a scale, you tie a knot in it and you are weighing the baggie, plus any air trapped in it.

(A question arises: do you need to do the weighing in a vacuum chamber to get correct readings?)
 
  • #6
praharmitra said:
maybe they mean that they allow air to enter the plastic bag, then tie the knot so as to trap air...hence weight of the "bag" increses

If that actually works (and I'm not sure if it does), the 1 gram increase is in the right ballpark. Air's density is very roughly 1/1000 that of water, or 1 gram / liter. A 1 liter volume would contain about 1 gram of air.

What I do know is that a 1 liter bag of air (1 atm, room temperature), if weighed in a vacuum, would be about 1 gram heavier than an empty bag.

Edit:
Just occurred to me that this can be easily tested. If you weight 15 bags together, the increased weight (if it happens) would amount to 15 grams or about 1/2 ounce. A scale used for weighing food or post letters would show this difference.
 
  • #7
DaveC426913 said:
Are you sure? Because this would actually work. I can see there being some intuitive confusion around this.

You weigh an open baggie on a scale, you tie a knot in it and you are weighing the baggie, plus any air trapped in it.

(A question arises: do you need to do the weighing in a vacuum chamber to get correct readings?)

This doesn't totally make sense to me. Air is obviously creating pressure on everything, all the time. Say you have a bag that can enclose 10cm3 of air. If the bag is laying flat, there's still that much air above it. The layers on the scale would be scale>layer of plastic>layer of plastic>miles of air. If you blow up the bag and set it on the scale, you have scale>layer of plastic>10cm3 air>layer of plastic>miles of air minus 10cm3air. Any way you look at it, the same amount of "weight" or pressure from the air is going to be showing on the scale.

Where am I going wrong here?

(I haven't had anything to do with physics beyond SciAm articles for a good five years now, so forgive layman's terminology and such)
 
  • #8
Taika said:
This doesn't totally make sense to me. Air is obviously creating pressure on everything, all the time. Say you have a bag that can enclose 10cm3 of air. If the bag is laying flat, there's still that much air above it. The layers on the scale would be scale>layer of plastic>layer of plastic>miles of air. If you blow up the bag and set it on the scale, you have scale>layer of plastic>10cm3 air>layer of plastic>miles of air minus 10cm3air. Any way you look at it, the same amount of "weight" or pressure from the air is going to be showing on the scale.

Where am I going wrong here?

(I haven't had anything to do with physics beyond SciAm articles for a good five years now, so forgive layman's terminology and such)
Agreed. I'm not sure either. That's why I qualified it with the last line: it would work in a vacuum chamber, because you'd eliminate the bouyancy factor.
 

FAQ: Does Tying a Knot in a Plastic Bag Double its Weight?

Does tying a knot in a plastic bag actually double its weight?

Yes, tying a knot in a plastic bag can double its weight due to the added mass of the knot and the compression of the bag's material. However, the amount of weight added may vary depending on the type and thickness of the plastic bag.

How does the knot affect the weight of a plastic bag?

The knot on a plastic bag adds weight by compressing the material and creating a small bulge, which adds mass to the bag. The added weight is also due to the knot itself, which adds more material to the bag.

Will tying a knot in a plastic bag make it stronger?

Yes, tying a knot in a plastic bag can make it stronger by creating tension and preventing the bag from opening. This can be useful when carrying heavy objects in the bag to prevent it from tearing or spilling.

Does the type of plastic bag matter when it comes to doubling its weight?

Yes, the type of plastic bag can affect the amount of weight added by tying a knot. Thicker and sturdier bags may only add a small amount of weight, while thinner bags may experience a more noticeable increase in weight.

Are there any other factors that may affect the weight of a plastic bag when tying a knot?

Yes, factors such as the tightness of the knot and the amount of air trapped inside the bag can also affect the weight. A tighter knot and less air trapped inside the bag may result in a greater increase in weight.

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