# B Why do we slip on ice??

1. Sep 10, 2016

### terryds

Why do we slip when we try to walk on ice? I know that it's because the friction is small.
But, I still don't get it. The kinetic friction or static friction?
Is the friction force direction opposite or same direction as the walk?

2. Sep 10, 2016

### Simon Bridge

Consider that a surface of water is very slippery ... you do not have a problem understanding why it is so hard to stand on a liquid right?
Ice is less dense than water, when you stand on it you compress it, which causes a thin layer to turn to water and you slide on the water.
That help?

Try googling "why is ice so slippery" and see what you find.

3. Sep 10, 2016

### terryds

But, what about a slippery floor or a wet ground? We tend to slip on that surface, why?

What does the small friction do with this?
I mean, is it due to the Newton 1st law?
But, if it's true, that means there are friction force exerting on my foot when I just stand on the floor?
And, what forces are actually acting on my foot right after I step my foot on the floor?
Does the friction force just exerts when I walk or also when I stand up?
I'm so confused.

4. Sep 10, 2016

### davenn

I'm assuming you haven't googled "why is ice so slippery" as suggested by @Simon Bridge

how about doing a little research

Dave

5. Sep 10, 2016

### Simon Bridge

Oh I see.
When you stand up, if you were perfectly balanced you would not slip even on a frictionless surface.
However, you are never perfectly balanced, instead there is always some lateral force at you feet that is balanced by friction, hopefully.
Ifvthe static friction cannot be high enough, your feet slide.

You can see this with a force diagram... try different models.

6. Sep 10, 2016

### terryds

I've watched it's called regelation but she said that "scientists haven't fully understood yet". Is it right?

7. Sep 11, 2016

### Simon Bridge

Depends on what is meant by "fully understood". We do not have a theory of everything yet so nothing can be fully understood.
Usually what happens is that we have a bunch of neat ideas work very well, extremely in some cases.
When someone asks for an explanation we do need some consensus about what counts as an "explanation" for that person.

For instance, iirc the dminant effect re slipping on ice is thought to be to do with surface water on the ice - the probem under investigation is how that water gets there when the temperatures are very low: see overview -
http://dujs.dartmouth.edu/2013/04/what-causes-ice-to-be-slippery/#.V9O-LKJ97VM
... add that you know you can get stuck to ice, and you can wear shoes with lots of little spikes to help you walk on ice, so the physics of slipping over can get complicated.

But weren't you asking about the physics of slipping over at all - on any surface, not just ice?
Like you can slide on concrete flooring, especially if it is polished, especially if you are in socks ... concrete is not a liquid, so that does not work.

Is your question about slipping itself or about how friction works at the microscopic level or what?
The goalposts seem to be moving about. Please pose a coherent question and stick to it.

8. Sep 12, 2016

### Khashishi

A large part of ice's slipperiness is because it's smooth. So-called black ice is just smooth ice on top of a black road. Since it's smooth, it doesn't scatter light much, so you just see ground. And it's very slippery.

9. Sep 12, 2016

### hmmm27

Black ice is (more) slippery because it isn't smooth [edit: my bad, it's smooth, just not completely flat] : it's hugging the irregularities of the ground. Applying a planar compression thus results in hot spots, so overall more phase change happens, ie: it produces more water to slide on than regular ice.

Last edited: Sep 12, 2016
10. Sep 12, 2016

### Khashishi

The top side is smooth. That's the side you slip on. Melting isn't a factor if it's really cold out. Compression hot spots won't result in melting but in cracking.

11. Sep 12, 2016

### hmmm27

smooth, not flat.

Check out the specific and latent heat values for ice. Doesn't matter (much) what temperature it is, most of the energy goes to the actual phase change.

12. Sep 12, 2016

### Khashishi

Most of what energy?

13. Sep 12, 2016

### hmmm27

squish shoe onto surface, backed by weight, make heat. Release foot from surface, make cold. See terryds' vid. A non-flat surface provides less surface area, so more pressure.

A smooth surface provides little static friction (your point). But a surface doesn't have to be flat to be smooth: a line vs a sine wave: both smooth, and neither would be frictive. Apply planar compression to a flat surface and a pebbly surface (like asphalt for instance) : the tops of the "pebbles" (or whatever rounded stone term is applicable) will be under more pressure per unit of areal measurement than the flat surface.

Last edited: Sep 12, 2016
14. Sep 12, 2016

### FactChecker

Why we slip on ice:
Normal walking uses static friction (which is greater than kinetic friction). Our learned reflexes of walking is based on the foot not slipping on the floor. Ice has such a low static friction (as described above by others) that it is immediately overcome and kinetic friction applies. Our reflexes are not tuned to kinetic friction. If they were, we would do a lot better.

Regarding the direction of the force:
The forces are equal and opposite (Newton's Third Law). As we walk forward, friction allows our foot to push the Earth backward. The Earth is relatively large and does not move backward much. The same friction allows the Earth to resist our foot with an equal forward force. We are relatively small and move forward.