Understanding Friction: The Impact of Contact on Object Movement

[lekh2003]

I've just been thinking, and I don't know why I couldn't figure this out on my own, but I got this question bugging me.

I was moving a sofa the other day. The sofa was against a wall. I started thinking why it was harder to push the sofa parallel to the wall when it was touching the wall rather then when it was not touching the wall. I hope you see what I mean.

My initial thought was that there was friction against the wall, but then I thought mathematically. What would the friction be? I am not exerting any force on the sofa pushing it towards the wall, so why is there friction?

Why is it harder to push an object when it is simply in contact with the wall, even though there is no force to act on the object to generate friction?

 

[A.T.]

My initial thought was that there was friction against the wall, but then I thought mathematically. What would the friction be?

Friction depends on the normal force. When you push the sofa against the wall, it deforms which requires a normal force.

 

[lekh2003]

Friction depends on the normal force. When you push the sofa against the wall, it deforms which requires a normal force.

But there isn't a real time force. It's just like that, still. And then I apply a force completely perpendicular to the normal force for friction.

And what if it was a flat hard box with no deformation allowed? It would still be easier to push if it wasn't against the wall.

 

[mfb]

The sofa can have some force simply from having contact with the wall (and being compressed due to that), and you pushing the sofa might contribute as well.
The effect should be small unless you push it in a very odd direction.

 

[A.T.]

But there isn't a real time force.

No idea what a "real time force" is. As long as parts of the sofa are compressed by the wall, there is a normal contact force between them.

And what if it was a flat hard box with no deformation allowed?

There is no such thing in real life. But aside from the friction that depends on the normal force, you can have stickiness that opposes sliding with zero or even negative normal force.

 

[lekh2003]

No idea what a "real time force" is. As long as parts of the sofa are compressed by the wall, there is a normal contact force between them.There is no such thing in real life. But aside from the friction that depends on the normal force, you can have stickiness that opposes sliding with zero or even negative normal force.

Ok so could i say that the amount of friction is proportional to the amount the object compresses. So a sofa might be harder to push than a box.

 

[A.T.]

Ok so could i say that the amount of friction is proportional to the amount the object compresses.

No, the normal force is usually non-linear.

So a sofa might be harder to push than a box.

Depends on many parameters which might be different between the two.

 

[lekh2003]

No, the normal force is usually non-linear.Depends on many parameters which might be different between the two.

Ok not directly proportional, but as the compression is higher, the friction must also increase.

I think I got it. Thanks.

 

[A.T.]

Ok not directly proportional, but as the compression is higher, the friction must also increase.

Yes, but the normal force also depends on the material properties. And the friction then depends on the normal force and the surface properties.

 

[lekh2003]

Yes, but the normal force also depends on the material properties. And the friction then depends on the normal force and the surface properties.

Yes, I understand that. Thanks.