What is Newton's law of viscosity?

Imagine pouring syrup from a jar into your pancakes. The thicker, stickier it is, the harder you have to push to get that smooth flow. That pushing effort against the fluid’s resistance is exactly what Newton's law of viscosity describes! It tells us how much "gooeyness" or internal friction a liquid has when layers slide past each other.

Think about two flat cards stacked on top of one another, with a thin layer of water between them. If you slide the top card forward, the water right next to it moves too. But the bottom card stays still because its contact surface is fixed. This creates a velocity gradient, meaning the speed changes gradually from the stationary bottom to the moving top.

Newton discovered that for many common fluids (called Newtonian fluids), the force needed to slide these layers is directly related to how fast they move apart and how thick the fluid is. If you double the speed of your sliding card, you need twice as much force! This relationship is captured in a simple formula: Shear Stress = Viscosity × Shear Rate.

You can test this at home with honey and water. Viscosity is basically a measure of that internal friction. Honey has high viscosity; it resists flowing because its layers grip each other tightly. Water has low viscosity; its layers slide easily over one another like skaters on ice.

So, Newton's law isn't about grand cosmic forces, but simple, everyday sliding. It explains why oil lubricates engines (it slides well between metal parts) and why ketchup sometimes refuses to come out of the bottle until you shake it hard enough to overcome its initial resistance. Next time you stir your tea, remember: you are actively battling viscosity!