# How do wheels decrease the amount of energy needed to move an object?

1. Jul 4, 2009

### kashiark

I was thinking about it, and the only thing I could come up with is that it reduces friction with the surface the object is sitting on and perhaps it distributes weight evenly?

2. Jul 4, 2009

### negitron

That's it; rolling friction is much less than sliding friction. It's the same reason a ball will roll down an inclined plane, rather than slide down.

3. Jul 4, 2009

### kashiark

Why is rolling friction less? It seems like common sense that it is, but why is it?

4. Jul 4, 2009

### Pengwuino

Edit: Nevermind, didn't understand the question.

Last edited: Jul 4, 2009
5. Jul 4, 2009

### atyy

But does rolling friction come into play in ideal rolling without slipping - in such a case the point of contact is always stationary?

6. Jul 4, 2009

### cosmo123

I wouldnt think so, no. If its completely perfect, then no rubbing means no friction.

7. Jul 4, 2009

### Naty1

In physics, mechanical work is the amount of energy transferred by a force acting over a distance. As noted already, with a rotating wheel you can use a lot less force to turn a wheel with a load on it relative to pushing the weight over the ground or carrying it yourself.
It's a type of mechanical advantage relative to brute force.

8. Jul 4, 2009

### negitron

Rolling friction occurs even when the wheel doesn't slip. Friction, as a general case, occurs when electrostatic forces act between the surface atoms of one body moving over the surface atoms of another. Sliding is not the only case in which this occurs. For a wheel, there is always some portion of it in contact with the surface over which it rolls, and as it turns, atoms on the wheel's leading surface are continually being mashed down onto the surface of travel where electrostatic forces weakly bind them then are pulled apart again at the trailing surface--the energy required to pull those weak bonds apart is what we call friction.

9. Jul 4, 2009

### A.T.

The question was why rolling friction is less than sliding friction. The area of contact of a rolling wheel is the same, as if the wheel slides. It is not the size of the contact area, but it's movement relative to the ground: In the rolling case the area of contact has no horizontal speed.

10. Jul 4, 2009

### negitron

You can immediately falsify this statement by asking yourself why brakes work, since the area of contact with the ground doesn't change, the wheels just stop turning.

11. Jul 4, 2009

### rcgldr

In the case of a wheel, the friction at the axle creates an opposing torque to movment of the wheel. The force required to overcome that force equals that torque divided by the radius of the wheel, and since the radius of the wheel is much larger than the radius of the axis, the amount of force at the rim of the wheel required to overcome the resistance of a plain bearing axis, is relatively small.

An alternative to a wheel is to use a set of moving rollers, which eliminates the axis issue completely, but then you'd need a constant supply of rollers to place under a moving platform.

Another way to take advantage of this is to place moving rollers (roller bearings) or spheres (ball bearings) between the axis and the inner hole of a wheel, to reduce the opposing torque compared to a simple bearing axis.

12. Jul 4, 2009

### Pengwuino

.... what are you talking about?

13. Jul 4, 2009

### negitron

Your post. It was wrong, or at best, highly unclear.

14. Jul 4, 2009

### sir_manning

I know that most macroscopic friction for non-deformable substances is caused by electrostatic forces, but I can't quite envision your description. Wouldn't the energy gained (in the form of the rotational KE of the wheel, I guess) due to the weak binding of atoms at the the leading surface equals the work required to pull the atoms apart at the trailing surface?

...I think I'm wrong, because I could say the same thing about sliding friction: the energy gained from an atom on surface A binding with a new atom on surface B equals the work required to pull the atom on surface A apart with an old atom on surface B - therefore sliding friction is nonexistent... But, that's incorrect.

15. Jul 4, 2009

### Pengwuino

Ah nevermind, I see what's wrong, I didn't understand the original question. I was thinking a rolling wheel vs. sliding something like a brick as opposed to rolling a wheel vs. sliding that same wheel. Well that makes it quite a different situation :rofl:

Last edited: Jul 4, 2009
16. Jul 4, 2009

### kashiark

I see what negitron is trying to say. If the wheels aren't turning, the friction increases regardless of the small contact area of the wheel touching the ground. However, could changing the atoms which are reacting with the ground decrease friction? Or am I just wrong?

17. Jul 4, 2009

### negitron

Exactly. The contact area does not change, but the friction rises dramatically. Ergo, the reason rolling friction is less than sliding friction has nothing to do with contact area.

I'm not sure what you mean here. You mean using a different material?

18. Jul 4, 2009

### kashiark

No, I mean when the wheel turns, new atoms are making contact with the ground. Could electrostatic forces build up from the same atoms keeping contact with the ground?

19. Jul 4, 2009

### negitron

Perhaps I'm dense today; I still don't quite understand what you're asking. Sorry.

20. Jul 4, 2009