Practical: Pressure in liquids
By p = ρ × g × h the pressure of a liquid depends only on the liquid’s density (ρ) and depth (h), not on the shape of the container. That is why the jet from a lower hole shoots farther than one from a higher hole. Sea water is denser than fresh water (about 1030 kg/m³), so at the same depth the pressure in the sea is slightly higher. A dam is built thicker at the bottom because pressure increases with depth. A hydraulic machine has two pistons of different areas: pressure applied to the small piston is passed on equally through the liquid to the large one, so F2 ÷ F1 = S2 ÷ S1; a small force produces a large force, but the small piston moves correspondingly farther. Jacks, car brakes and dentist’s chairs work on this rule; a water tower creates pressure to push water to tall buildings.
Experiment: an adult makes 3 holes 2 cm apart in a vertical line in the side of a 1.5 L plastic bottle (using a not-too-sharp awl; never a heated nail). Seal the holes with plasticine, fill the bottle with water and over a tray remove the plasticine; watch and sketch how far each jet shoots. Safety: only an adult makes holes; do not spill water (slipping); keep the bottle away from the table edge.