Required inquiry: mini-table on sand. Same area + larger force sinks deeper; same force + smaller area sinks deeper — that ratio is solid pressure p = F/S (in Pa). Add bricks or flip the tabletop in the 3D lab and match F, S, p to well depth.
Memory cue: watch sand wells · p = F/S · raise p ↓S or ↑F · lower p ↑S or ↓F.
Key formulas
p = F/S
1 Pa = 1 N/m²
How to use
1Read Start here and the Related knowledge map.
2Place the table upright; add bricks (same area) and compare sink depth.
3Flip tabletop-down (similar force, larger area); read p = F/S as wells get shallow.
Read next
Related knowledge
Separate force from weight, inquire with a mini-table on sand, then meet pressure p = F/S and how to raise or lower it.
Mind map
1. What is pressure force
A force acting perpendicular to an object's surface is called a pressure force (contact force).
It need not come from weight, nor point straight down (e.g. a thumbtack pushed into a wall).
At rest on a horizontal surface, the force on the surface equals the object's weight (F = G).
2. Inquiry: effect of pressure
Goal: what affects the effect of a contact force? Materials: mini-table, bricks (or weights), fine sand (or sponge / foam).
Conclusion: same area → larger force → stronger effect.
Conclusion: same force → smaller area → stronger effect.
Place the mini-table on sand (or sponge); watch how deep the legs sink.
Add bricks: same contact area, larger force — sink deeper.
Flip the table (top down) with the same bricks: similar force, larger area — sink shallower.
3. Pressure: idea and formula
Pressure is the ratio of contact force to contact area — force per unit area. Larger pressure means a clearer mechanical effect.
1 Pa is tiny — about 0.5 Pa for a newspaper lying flat on a desk.
p = F/Sp in pascals (Pa), F in N, S in m².
1 Pa = 1 N/m²Named after Blaise Pascal; symbol Pa.
4. Typical pressure values
Newspaper flat on a desk: ~0.5 Pa.
Watermelon seed flat on a desk: ~20 Pa.
Person standing: ~1.5×10⁴ Pa.
Ballet dancer on pointe: up to ~5×10⁵ Pa.
Water-jet cutter: ~10⁸–10⁹ Pa.
Exam tip: a dancer (~5.0×10⁵ Pa) can exceed an elephant (~2.5×10⁵ Pa) because contact area is tiny.
5. Increasing & decreasing pressure
From p = F/S: change pressure by changing force or contact area.
Raise p: increase F or decrease S — sharp blades, thumbtack tips, pointed window hammers.
Lower p: decrease F or increase S — railroad ties, wide tracks, wide straps, skis.
6. Pitfalls
On a flat surface at rest F = G, but force is not always weight and not always downward.
Use p = F/S for solids; p = ρgh for liquids inside — do not mix them.
S is the real contact area (e.g. total area of four legs), not a nominal face size.
Applications
Skis and tracks increase area to lower pressure on snow or soft ground.
Sharp blades and nails reduce area so the same force cuts more easily.
FAQ
What shows the pressure effect?
How deep the sand (or sponge) is dented. Deeper wells mean a stronger effect — set by both force and area, written as p = F/S.
Is the contact force always equal to weight?
On a horizontal surface at rest, yes in magnitude — but force need not come from weight and need not point straight down.
How do you raise or lower pressure?
Raise: increase F or decrease S. Lower: decrease F or increase S.
Are force and pressure the same?
No. Force is a push; pressure is force per area.
Why does standing create higher pressure than lying down?
Smaller contact area → larger p = F/S for the same weight.
