# Is frictional force really a force?

1. Aug 8, 2009

### ngjingyi

Is frictional force really a force since a force is a push or pull? Does frictional force give a push or a pull if its really a force?

Thanks

2. Aug 8, 2009

### D H

Staff Emeritus
A force is any thing that results in a change some thing's momentum.

3. Aug 8, 2009

Yes and yes.

4. Aug 8, 2009

thanks

5. Aug 8, 2009

### markkens

Friction is a transfer of momentum, expressed and measured as a force.

6. Aug 8, 2009

### rcgldr

A force can also be applied with no change in momentum, such as the compression force of a vice on a spring.

7. Aug 8, 2009

### Cyrus

Or static friction.

8. Aug 8, 2009

### Staff: Mentor

I don't know if all are a push of pull, but I would think so -- friction can either be a push or a pull.

9. Aug 8, 2009

### Phrak

Oh, oh, oh. It took me reading all eight posts, and especially watters' before I understood your question. Maybe I can rephrase what I think you are asking.

Friction opposes an applied force. As the applied force can be in either direction or even zero, the force of friction varies too. So is it a force?

10. Aug 8, 2009

### Naty1

So pick a definition of force and see if friction fits the definition.

Newton said a body remains at rest or in a state of motion unless compelled to change by FORCES. so friction would seem to fit that definition. It always oppose motion and never aids it. KE is converted to heat.

It's an experimentally determined force (I think) and nobody has been able to calculate frictional coefficients (mu) from first principles; in other words, I know of no theory that provides the friction force between different materials.

Keeps me from falling every time I take a step, so I like it for that.

11. Aug 8, 2009

### pgardn

It would seem to me friction is ultimately an electromagnetic force. Anytime two surfaces come in close proximity there will be repulsion between electrons. If you have a rough surface then you might encounter more of these forces that might either resist or help start the motion of an object in contact with a surface because some of these interactions will be parallel to the motion or lackthereof. Seems reasonable to me anyway. It may be much more complicated, but this appears as a reasonable cursory explaination.

Last edited: Aug 9, 2009
12. Aug 8, 2009

### ngjingyi

yep thanks for rephrasing

13. Aug 9, 2009

### nooma

Whilst i think you might struggle to define friction as either a push or a pull, i think from its effects it is one or the other. I.e. on a sloope does friction push or pull you back up the slope, or both?

However I think it is most important to realize that friction is a reaction force and only exists in the presence of another push or pull.