138
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Accepted
If I pull a metal bar for long enough with a constant small force, will it eventually break?
Yes, the rod will ultimately break—barring any other failure mechanism that occurs first.
(Depending on the material and conditions, you may need to wait a very, very long time, but your mentions of &...
- 14.6k
108
votes
Accepted
If water is nearly as incompressible as ground, why don't divers get injured when they plunge into it?
Adding another perspective to the existing answers:
In your usual diving scenario, water is not confined to the points in space it occupied before, while a slab of ground is – on account of water ...
- 5,005
84
votes
Accepted
Why is the vibration in my wire acting so oddly?
Your wire is not quite round (almost no wire is), and consequently it has a different vibration frequency along its principal axes1.
You are exciting a mixture of the two modes of oscillation by ...
- 117k
83
votes
Accepted
Why is filling a balloon from your mouth much harder initially?
I think that most of the answers here are incorrect since it has nothing to do with decreasing resistance of rubber. In fact, the force required to stretch the balloon increases, not decreases while ...
- 954
74
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If water is nearly as incompressible as ground, why don't divers get injured when they plunge into it?
In simple terms, water (or any fluid) will move out of the way; concrete won’t (unless it is hit very hard). The important properties are viscosity and elasticity rather than compressibility.
- 35.5k
70
votes
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Is it correct to say that it is theoretically impossible for perfect rigid bodies to exist?
You are right. Perfectly rigid bodies are an idealization, like point particles or massless frictionless pulleys. They do not exist.
But they are useful. Plenty of objects exist that are so rigid that ...
- 27.7k
66
votes
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Clarifying the actual definition of elasticity. Is steel really more elastic than rubber?
There are two separate concepts here:
the Young's modulus, which determines the force needed to stretch the material
the elastic limit, aka yield strain, which determines how far the material can be ...
- 334k
53
votes
Clarifying the actual definition of elasticity. Is steel really more elastic than rubber?
Both the OP and John Rennie have well illustrated the imperfections in the usage of the word "elastic" in physics and how the word can create confusion between "stiffness" and a material's ability to ...
- 85.4k
43
votes
Accepted
Why do my experimental data for Hooke’s law not match the expected data?
The coils of the spring are touching one another and the spring is initially under self-compression so it takes a finite force to move all the coils away from one another and for the spring to behave ...
- 79.7k
43
votes
Accepted
Why does a rubber band become a lighter color when stretched?
Colour can come from pigment particles embedded in the translucent rubber matrix absorbing light. When you pull the band the particles become separated by a longer distance, but being themselves ...
- 28k
36
votes
Why is filling a balloon from your mouth much harder initially?
Take a strip of balloon rubber and pull it. It will get harder the more you pull. So why is it that inflating the balloon gets easier (at least long before the breaking point)?
The balloon starts ...
35
votes
Why does a rubber band become a lighter color when stretched?
Rubber bands are made of polymers (more specifically elastomers). A given polymer in the band can either be aligned with other polymers around it, or it can be misaligned. Therefore, you can end up ...
- 53.8k
29
votes
Why do my experimental data for Hooke’s law not match the expected data?
There is no "systematic error" in the experimental results. I have done this experiment as well when teaching a high school physics class. The effect you are seeing is due to the fact that the ...
- 11.5k
29
votes
If I pull a metal bar for long enough with a constant small force, will it eventually break?
This paper describes scientific creep tests with some specific steel.
In a low stress (but relatively high temperature) regime, they report creep of 200 micrometers per year, for a test specimen of ...
- 2,460
28
votes
Accepted
Why does a flat sheet roll back into a cylinder when having rolled it once?
Great question!
First, I would note that not all flat surfaces are represented by a sheet of paper. Flat rubber sheets for example, if rolled, will usually unroll themselves flat again.
The answer ...
- 2,436
26
votes
Accepted
Why is stress defined in the way as it is?
I think your perplexity is understandable, and it comes from the clash between the notion of stress, which belongs to continuum mechanics, and the molecular description. Let me arrive in a roundabout ...
- 3,245
25
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If water is nearly as incompressible as ground, why don't divers get injured when they plunge into it?
So why isn't the impact of diving into water equivalent to that of diving on hard concrete?
Water is rather incompressible, but it is not very hard. It deforms rapidly under shear stress, unlike ...
- 69.1k
22
votes
Why is filling a balloon from your mouth much harder initially?
When in doubt, use mathematics.
Imagine the balloon as a sphere (close enough for this answer) of initial radius $r_0$ and thickness $t$. Let's inflate it just a little bit from the uninflated state (...
- 117k
17
votes
Why is filling a balloon from your mouth much harder initially?
Like Dev said above, the material your typical round balloon is made from has a non-linear stress strain curve. When just starting to inflate it is fairly stiff, but then as it starts to blow up the ...
- 860
16
votes
Why is filling a balloon from your mouth much harder initially?
The volume of a balloon grows linear, while the surface (which you actually stretch) doesn't. So although you're blowing the same amount of air into a balloon, you don't stretch the surface as much as ...
- 269
16
votes
Why does a metal block make a shrill sound but not a wooden block upon hammering?
Is it that the wooden block vibrates with lesser frequency than the
metal block? If so, why is that?
'Yes', to the first question.
Metal is stiffer than wood and produces higher frequencies (...
- 34.5k
15
votes
Why does a rubber band become a lighter color when stretched?
In general the rubber from elastic bands scatters light. This explains why rubbers are opaque, but still reflect light quite a lot, in the same way as milk looks white.
In the presence of pigments, ...
- 669
14
votes
Accepted
Why does a metal block make a shrill sound but not a wooden block upon hammering?
The metal block has a relatively low level of internal damping, however the wooden block has a high level of internal damping: Much of the energy imparted to the wooden block is dissipated internally ...
- 2,662
13
votes
Why is the vibration in my wire acting so oddly?
UPDATE :
After looking again at the video, I agree that Floris' explanation seems to be correct and my explanation below is wrong. Slightly different frequencies of vibration in two perpendicular ...
- 26.5k
13
votes
Accepted
Can Hooke's law be derived?
Yes, we can derive Hooke's Law from more basic continuum conditions, provided that the material be stable and at equilibrium, so that the strain energy is smoothly minimized with respect to the ...
- 14.6k
13
votes
Accepted
How does a twisted piece of string/yarn wind back on itself? What kinds of forces are responsible for this?
It is much easier to explain with a diagram than it is with a chart or equations.
Imagine threading tiny weightless disks along your length of blue wool. Twist it and hold it stretched out ...
- 1,721
13
votes
Why does a rubber band become a lighter color when stretched?
Anders Sandberg explained the main effect (in fact, decreased per-area pigment concentration and the band becoming thin are the same effect).
There are two more effects that acts even for non-...
- 6,113
13
votes
Accepted
In a collision shouldn't objects of different mass have same acceleration?
If you model the colliding objects as rigid bodies, then the collision will take zero time. Since the collision has zero duration, there is no issue with the objects having different accelerations. ...
- 5,115
13
votes
If water is nearly as incompressible as ground, why don't divers get injured when they plunge into it?
Incompressible doesn't mean that it has to keep the same shape.
But, due to viscosity, water can be "slow" to change its shape under external influence. So when a diver arrives too fast, ...
- 4,323
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