Why Pillows Feel Softer on Hard Floors: The Physics of Force and Cushioning

[BigMacnFries]

When I sit on a hard wooden floor I exert about 700N of force on the floor as it exerts this force back on me. If I have a pillow (or spring or carpet etc) between me and the floor I must still be exerting 700N of force, and I guess therefore it must exert 700N on me also. Why then does it feel softer?

 

[Gokul43201]

The feeling of hardness or softness of an object is related to its elastic/plastic modulus, not to the force exerted on you. If at all, the discomfort level experienced (which is related but different from the hardness) by you is a measure of the pressure exerted on you, not the force. A soft object provides a greater area of contact and hence reduces (and evens out) the pressure on you.

 

[BigMacnFries]

Yes I see how a greater surface area reduces pressure. Even so a wooden block moulded perfectly to my face would still feel harder than a pillow. I assume the pillow collapses under my weight until the atoms are compressed enough that it would require more than 700N of force to compress it more, therefore the pillow applies 700N to me. With equal surface area and equal force why is there a difference?

To put in another way if I sit on a bunch of thin but inflexible bits of metal I get sore. However if these metal bits are flexible and have flexible springs under them (eg an innersprung mattress) then I don't get sore. Once the springs stop compressing I am ontop of a bit of stationary metal, just like my inflexible metal, why does one hurt and not the other

 

[Mk]

To put in another way if I sit on a bunch of thin but inflexible bits of metal I get sore. However if these metal bits are flexible and have flexible springs under them (eg an innersprung mattress) then I don't get sore. Once the springs stop compressing I am ontop of a bit of stationary metal, just like my inflexible metal, why does one hurt and not the other

Or put it this way:

If you have one Bowie knife, and sit on it you will have one really deep cut.

If you have 50 Bowie knives and sit on them you will have 50 shallow cuts.

Which is... uhm better and softer.

 

[BigMacnFries]

Yes, as I said in my first reply I understand more surface area means the same amount of pressure gets divided up over more area and therefore there is less force per area.
In my inner sprung example you could have a grid of thick inflexible aluminium wires that you lie on with a surface area of that is exactly the same as the contact you make with the top of the innersprung mattress. (Imagine there is no cover on the mattress, just the wire frame, it would still be more comfortable than an inflexible frame)

 

[quark]

BigMac,

Your assumptions are not totally correct. The effects of pressure distribution vs comfort were studied more than 2000 years ago. Form your shape on wet clay and then dry it(this is the way to exactly map the surface area). You should feel the same comfort as if you are sleeping on a soft bed.

 

[Mk]

As Gokul said, its how much "give" the material has that provides the softness - the elastic modulus.

 

[BigMacnFries]

Is greater surface area sufficient to explain the softness, is elastic modulus a means to greater surface area or a source of softness itself?

 

[Gokul43201]

The elastic modulus is itself an indicator of softness. Actually, the real material property involved is something called...you guessed it...hardness. This is a property that tells you how much a particular material deforms plastically if you poker it with a, ummm... a poker. The property is a function of the atomic bonding and geometry and well as microstructural features in the material.

How hard/soft something feels has NOTHING to do with the force or pressure exerted on you.

 

[BigMacnFries]

Gokul
To quote from you above:
"If at all, the discomfort level experienced (which is related but different from the hardness) by you is a measure of the pressure exerted on you, not the force." (first reply)

"How hard/soft something feels has NOTHING to do with the force or pressure exerted on you" (last reply)

Please explain

Also if quark is right about clay shaped to your body being as comfortable as a bed then it does seem pressure is the factor that determines comfort and materials with a high elastic modulus are soft only because they deform and reduce the pressure.

 

[rcgldr]

The clay bed analogy is flawed. People (at least living ones) aren't motionless. They breath, and they move around just a bit. Any movement results in a lot of resisitve force from a hard surface, and much less with a soft surface.

 

[quark]

The theory is flawed when people are not motionless because, with hard substances, you have to have a different contour for every body posture. Soft material automatically form the contours and that is why you feel comfortable.

You can feel the difference in comfort levels when you stand, sit and lay flat on a soft material. The material is soft in all cases but you will be more comfortable when you lay flat on it, because your weight is distributed over maximum possible area.