I put my thumb on my stomach and push in about an inch. I feel pressure, but no pain. But if I put something sharp there and push in the same amount, I feel pain. Why does a small object pressing into you excite the pain neurons and large objects do not? Why doesnt my thumb feel like a dozen needles?
There may be multiple reasons, but the most important is that nerves register not total pressure, but pressure per unit of area. Your thumb has the same surface area as hundreds of needles, so it may activate many different nerves, but each one has a small amount of activation. A needle puts the pressure on just a few nerves, but they have a high activation.
I can also think of a few secondary reasons. A needle can very easily penetrate past the surface layers of skin whereas your thumb won’t; even a nonpainful amount of pressure with a needle may be going past the outer, dead layer of skin cells. Part of it may be psychological as well. Your brain expects that needles will hurt much more than thumbs. Sometimes the anticipation of pain is as bad as the pain itself.
You mean, “not force, but force per unit area (i.e. pressure).” Pressure per unit area doesn’t have any real meaning.
Organic life is nothing if not a study in gradients. The gradient between most effected and uneffected is steep enough to trigger the perception of pain with the sharp object, not so the blunt one.
Think of it like the difference between a snow shoe and a high-heeled ladies’ shoe, as the lady walks across wet ground.
With the snow shoe, the force is spread out over a large area, and the lady’s foot doesn’t sink.
With the high-heeled shoe, the force is concentrated on a tiny area and the heel will punch down into the soil.
A troubleshooting question related to the OP would be:
Are you, in fact, only applying a small amount of pressure with that sharp object?
Perhaps you THINK you are applying the same/less PSI than you are with your thumb, but your ability to measure/control that may be flawed. You may, in fact, be applying a great deal more pressure than you think.
Time for some sort of scientific rig!