# Pushing against fabric vs wall

1. Nov 16, 2004

### UrbanXrisis

If a piece of fabric was stretched out, and I pushed my finger into the fabric with a force of 5N, the fabric will push back the 5N because of its potential energy. correct?

So what if I did the the same thing and pushed my finger agaist a wall? The wall doesnt stretch but it still pushs back with 5N. How does it know to puch back 5N on my finger?

2. Nov 16, 2004

### cdhotfire

hmm, seems to me that if it didnt push back at 5 N it would either throw you back or you would break the wall.

3. Nov 16, 2004

### UrbanXrisis

hahaha, I never knew I was that strong. Why does the fabric have to stretch to produce the 5N?

4. Nov 16, 2004

### Staff: Mentor

I assume you are asking: What is going on with the fabric that allows it to push with 5N of force? When you push the fabric, you are stretching the molecular bonds--like little springs--which resist that distortion.

Ah, but the wall compresses. When you push on the wall, you are compressing those molecular "springs" that constitute the surface of the wall.

5. Nov 16, 2004

### Sirus

Pretend it's a horizontal spring on a table. If you push on it (horizontally) with a constant force, it will be compressed, but eventually it will push back with enough force to keep your hand in equilibrium. This is because $F_{spring}=-kx$, where x is displacement from equilibrium position. I believe the fabric works the same.

Last edited: Nov 16, 2004