KCSE 2025 Physics P1 Q4 — Pascal's Principle (Toothpaste Tube)
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The Question
“When a tube of toothpaste is squeezed at any point along its body, the paste comes out through the open nozzle. State and explain the principle that this demonstrates.”
Identify the principle at work
The paste inside the sealed tube is an enclosed fluid. Squeezing any part of the tube raises the pressure inside it. The behaviour that follows is described by Pascal's principle, which deals with how pressure spreads through a trapped fluid.
State how the pressure is transmitted
Pascal's principle says that pressure applied at any point of an enclosed fluid is transmitted equally and undiminished to all other parts of the fluid. So the squeeze does not stay where your fingers are — the extra pressure reaches the nozzle just as strongly, and because the nozzle is open, the paste is pushed out there.
Final Result
The principle is Pascal's principle: pressure applied to an enclosed fluid is transmitted equally to all parts of the fluid. Squeezing the tube raises the pressure throughout the paste, and this pressure drives it out through the open nozzle.
Why this method works
A fluid cannot support a shearing force, so when you push on one part of a trapped liquid the molecules simply pass the push on from neighbour to neighbour until it is felt everywhere at once. This is why the pressure increase is the same at the nozzle as under your fingers, no matter where you press. The paste then escapes at the nozzle for a simple reason: everywhere else the tube walls push back and contain the pressure, but the open nozzle offers no such resistance, so that is the one place the fluid can move to relieve it. The same principle, transmitting pressure equally through an enclosed liquid, is what makes hydraulic brakes and hydraulic presses work.
The everyday test: squeeze the tube anywhere — the middle, the bottom, the side — and paste still emerges from the nozzle, confirming the pressure reaches it equally from any point.