Mathematically explain why ? who weighs 105 lb prefers to sit on 25x4 in block wood rather than on pointy tack

Question

not sure if there's a formula for this one, prof. wants me to explain mathematically

Answer 1

It's because the block has a lot more surface area than the tack. This means the person's weight is concentrated on a smaller area.

Also, tacks hurt.

Answer 2

There is less surface area on pointy tack, this means that there is more pressure going to certain areas of the body.

p (pressure) = F(force)/A(surface area)

This would make sitting on tack more bothering and/or painful to the user of the seat.