# Friction free system

1. Jul 26, 2012

### moatasim23

If we some how assume a friction less surface,and drive our automobile on it...We know that automobile moves by the reaction force of friction...the wheels push the ground in the backward direction and the friction moves us fwd..(correct me if I am wrong)..In friction less system..how the motion of our automobile be like?

2. Jul 26, 2012

### Staff: Mentor

the wheels would spin and there would be no motion. Consider a car on a smooth patch of ice.

One caveat about the car is that the driving wheels may be connected to a differential so that if one tire slips the other tire wont spin but I guess you get the same result no motion forward.

Last edited: Jul 26, 2012
3. Jul 26, 2012

### Staff: Mentor

There would be no change in its motion. It can glide around, without changing its velocity (unless you throw something away, care about air resistance, or whatever).

4. Jul 26, 2012

### Danger

If you're persistent, and especially if you have deep-lug tires like on a 4x4, you might eventually get some tiny motion from the displacement of air by the wheels. I'm pretty sure that it would be random, though.

5. Jul 26, 2012

### Staff: Mentor

I guess if its a frictionless surface then the tire lugs wouldn't be able to dig into the surface to get a hold so you might bounce up and down and back and forth a bit but still not go anywhere.

6. Jul 26, 2012

### Danger

I was actually thinking of the tires acting like little paddle-wheels against the atmosphere.

7. Jul 26, 2012

### sophiecentaur

Or the exhaust gases could give you some jet thrust?

8. Jul 26, 2012

### moatasim23

Why do we slip backwards while moving in a slippery surface..I mean a less frictional surface.?

9. Jul 26, 2012

### sophiecentaur

Do we? I think you'd have to describe the scenario more clearly. On the face of it, I'd say that statement is not right.

10. Jul 26, 2012

### moatasim23

Yes according to my observation if I walk on a wattery surface..I slip backwards..

11. Jul 26, 2012

### Danger

Good point. That would probably have more effect than the tires would.
I agree with your later observation as well. The only way that slipping would cause rearward motion is if there's an incline or if the victim's centre of gravity is rearward. (Given the level of morbid obesity in the USA, the latter is quite possible. )

12. Jul 26, 2012

### sophiecentaur

Do you mean that You or your Feet slip backwards? Or do You fall over backwards?
This isn't a frivolous question. Your answer is important for a proper comment.

13. Jul 26, 2012

### Danger

I meant the feet slipping forward, with the result of the victim falling on his ***, which I believe would result in a small net motion to the rear. Living in a very icy area, I've never seen feet slip backward. (I don't know that it doesn't happen, but I've never encountered it.)

14. Jul 26, 2012

### the_emi_guy

A little bit of propeller action from radiator fan too.

15. Jul 26, 2012

### jbriggs444

It's the first step you take onto the icy area that gets you. Not the last one you take before leaving it.

Plant heel, heel slips. And if you don't pull a foot back and plant it firmly behind you darned quick, your butt will plant itself on the sidewalk instead.

That's not backward movement, by the way. That's forward movement. Together with backward rotation.

16. Jul 26, 2012

### sophiecentaur

Rearward rotation, yes, but there can't be rearward motion unless there's a force (from somewhere) towards the rear. If the ice is totally smooth, there is no force.

17. Jul 26, 2012

### Danger

You underestimate the climate in my province. It's all ice.
I'm well acquainted with the mechanics of doing an ***-plant. At the risk of sounding braggy, I used to have almost superhuman reflexes. Even so, I hit the pavement a few times. Whether you call it movement or rotation or whatever, there was negative progress along my direction of travel. That might just be due to the particular method that I use to get back up, which is to pull my legs back under myself. That places my feet behind where they were when I flipped. If I were to roll my upper body forward instead, I would gain ground.

18. Jul 27, 2012

### sophiecentaur

@Danger
I think that using your observations of yourself is clouding the issue, rather. If you start off perfectly stationary on frictionless, horizontal ice, nothing that you can do can make your centre of mass move in any particular direction. (Ignore the air for this argument).

If you step onto this same ideal piece of ice, your CM will continue forwards at the same velocity as your original step. Whether you manage to stay upright or fall over, your translational velocity cannot change. Newton's Laws can't be suspended for the special case of deep Canadian winters. It seems that even Canadian ice must have some vestigial friction - when combined with your shoes, if your observations are truly accurate and unbiassed. Skates, otoh, can all exert some force, laterally to the blades) on ice so you can get any result you want to from an experiment with skates.