Pressure in liquids and gases
Pressure shows how a force is spread over an area: pressure = force ÷ area (p = F ÷ S). Its unit is the pascal: 1 Pa = 1 N/m². The same force on a small area gives a large pressure (needle, knife) and on a large area a small one (snowshoes, a camel’s broad foot). Gas pressure comes from molecules hitting the walls (a balloon inflates). A liquid also presses on the walls and bottom of its container; inside a liquid pressure grows with depth and at one depth it is the same in all directions. The pressure of a liquid at rest is hydrostatic pressure: p = ρ × g × h (ρ – density, h – depth). For water pressure rises by about 100 000 Pa (about one atmosphere) for every 10 m of depth. Pressure applied to an enclosed liquid or gas is passed on equally in all directions (Pascal’s law): hydraulic presses, jacks and car brakes work this way. A manometer measures pressure. If a diver rises too fast from depth, gas bubbles form in the blood (decompression sickness), so slow ascent and special training are essential.
Experiment: an adult makes 3 small holes (low, middle, high) in a vertical line in the side of a plastic bottle; seal them with plasticine. Fill the bottle with water, and over a tray or sink remove the plasticine and watch which jet shoots farthest (the lowest). Safety: only an adult should make the holes, with a not-too-sharp awl; work over a tray and wipe up spills (slipping risk).