Does a bottle of water or a bottle of ice roll downhill faster? [Video]

  • #1
Before you watch the video, give your answer here.... and why you think it's correct.

 
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Answers and Replies

  • #2
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I think the bottle of ice will roll down faster. Water expands upon freezing, so the bottle will be bigger, so it will roll faster.

Edit:Ok, I thought it was a drinking bottle of water being rolled, so what I said is void.
 
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  • #3
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I think the bottle of water will roll faster, because the water in it doesn't have to rotate when it starts rolling.
 
  • #4
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I agree with willem2, in the ideal case; I don't know if the video would help.
 
  • #5
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I would think the ice rolls faster. If the bottles are both completely filled, the bottle of water will be heavier as water is denser than ice, and hence the water bottle should feel a greater force of friction and so move slower. Probably a lot more to it than that though, to do with fluid dynamics of the water and whatnot.
 
  • #6
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When you drop objects in free fall, what is the guiding principle? [Neglecting friction such as air resistance, all small bodies accelerate in a gravitational field at the same rate.]

So they should roll equally fast.
 
  • #7
cjl
Science Advisor
1,864
437
When you drop objects in free fall, what is the guiding principle? [Neglecting friction such as air resistance, all small bodies accelerate in a gravitational field at the same rate.]

So they should roll equally fast.
You're ignoring the fact that the water is a fluid, and thus, doesn't have to rotate at the same speed as the container.

(The correct answer is that it depends)
 
  • #8
BruceW
Homework Helper
3,611
119
If the slope is very gentle, then the water would act approximately like a non-viscous fluid. Therefore, it wouldn't rotate, only the plastic bottle would. Therefore the bottle of water will go down the slope faster than the bottle of ice. (since it takes energy to make the ice rotate).
But if the slope is steep enough that the water acts like a viscous fluid, friction of the water would cause the bottle to rotate less quickly. Then one of two things could happen: 1) the bottle start sliding instead of rolling, in which case it speeds up. 2) it keeps rolling, but the rotation is slower, so it goes slower.
I think the viscosity of water will be too low, so the bottle with water will go faster.
 
  • #9
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You're ignoring the fact that the water is a fluid, and thus, doesn't have to rotate at the same speed as the container.

(The correct answer is that it depends)
The last word there: depends.

I want to assume we pretend all kinds of things are not happening. Freezing water does not expand: and it does not burst the bottle. That the bottle is not plastic and grows wider. That water has no internal viscosity (so I prefer a cryostat of liquid helium and frozen helium, which is hard to come by).
 
  • #10
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The liquid water will be sloshing around in the bottle. The friction of the sloshing around will slow it down. Alot. That's how you can tell if an egg is raw or hard-boiled: by spinning it.

Frozen-water bottle should should roll down faster.
 

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