# Opening this jar is made easier by cooling or heating?

• B
• robertphy
In summary, the wood on the lid of the jar is supposed to unscrew from the jar, but it's stuck because the glass jar has a higher coefficient of thermal expansion. He suggests two ways to make it easier to remove: heating the lid and cooling the lid. He also points out that the wood on the lid of the jar is supposed to unscrew from the jar, but it's stuck because the glass jar has a higher coefficient of thermal expansion. He suggests two ways to make it easier to remove: heating the lid and cooling the lid.

#### robertphy

I've found this question online and don't agree with the explanation given.
There’s a tight lid on a jar that you need to get loose.
The lid on this jar is made of wood and is supposed to unscrew from the jar — if only it wasn't stuck. The coefficient of thermal expansion of the glass jar is higher than that of the wood lid.
How can you make the lid easier to remove?

Explanation
If you heat the lid and the jar, then the jar will expand more than the lid does — which would make the lid of the jar even tighter and more difficult to open.

If you cool the lid and the jar, then the jar will contract more than the lid does, leaving the lid relatively large compared to the jar and easier to remove.

I disagree. If you heat the jar, the diameter of the top opening increases more than the diameter of the wooden lid, making it looser. The opposite happen if you cool the jar and the lid.
Thank you

robertphy said:
I disagree. If you heat the jar, the diameter of the top opening increases more than the diameter of the wooden lid, making it looser. The opposite happen if you cool the jar and the lid.
Why would the wood expand more than the glass if its CTE is less than that of glass?
robertphy said:
Thank you
The CTE of the glass is greater than that of glass, so it should expand more on heating and contract more on cooling.

robertphy and vanhees71
Which one has the female contact surface or thread?
That one has to expand, or the other one has to contract.

robertphy, russ_watters and 256bits
Lnewqban said:
Which one has the female contact surface or thread?
That one has to expand, or the other one has to contract.
Looks like the devil's in the details.
A cork in a bottle is similar in line to the OP reasoning.

russ_watters and Lnewqban
PeroK said:
Why would the wood expand more than the glass if its CTE is less than that of glass?

The CTE of the glass is greater than that of glass, so it should expand more on heating and contract more on cooling.
Thank you, now I understand the point.

hutchphd
Surprisingly, they're limited to opening only American jars:

swampwiz
renormalize said:
Surprisingly, they're limited to opening only American jars:
View attachment 327385
My high school English teacher would say that this a dangling modifier.

Interestingly, real wood has a different coefficient of thermal expansion with the grain than across the grain. This suggests that the lid shape is not preserved. So the real world experience could be that both heating and cooling make the lid more difficult to remove.