Pressure and its units
The effect of a body on its support depends both on the pressing force F and on the area S over which it acts. The ratio of the force acting perpendicular to a surface to the area of that surface is called pressure: p = F : S. The unit of pressure is the pascal (Pa): 1 Pa = 1 N/m², the pressure when a force of 1 N presses on 1 m². In practice kPa (1 kPa = 1000 Pa) and MPa (1 MPa = 1 000 000 Pa) are used. With the same force, the smaller the area, the greater the pressure: knives, needles and nails go in easily because of their sharp points. Conversely, heavy machines have wide wheels or tracks (tractors, tanks), building foundations are made wide, and skis reduce pressure on snow. Use S in m² and F in N (1 cm² = 0.0001 m²).
“Pressure and sand”: put the same book on soft plasticine or damp sand, first flat and then on its edge, and compare the depth of the dent. Safety: do not use needles or nails; use books and lids instead of sharp things.