You have to pull the trigger on a water pistol to get the water to squirt out.
To make the water to come out faster, you have to pull the trigger harder.
Fluids resist flow. This phenomenon is known as viscosity.
Newton devised a simple model for fluid flow that could be used to relate how hard you have to pull the trigger to how fast
the liquid will squirt out of the pistol. Picture a flowing liquid as a series of layers of liquid sliding past each other.
The resistance to flow arises because of the friction between these layers.
If you want one layer to slide over another twice as fast as before, you'll have to overcome a resisting force that's twice
as great, Newton said. The slower one layer slides over another, the less resistance there is, so that if there was no difference
between the speeds the layers were moving, there would be no resistance.
Fluids like water and gasoline behave according to Newton's model, and are called Newtonian fluids.
But ketchup, blood, yogurt, gravy, pie fillings, mud, and cornstarch paste DON'T follow the model. They're non-Newtonian fluids because doubling the speed that the layers slide past each other does not double the resisting force. It may less than
double (like ketchup), or it may more than
double (as in the case of quicksand and gravy). That's why stirring gravy thickens it, and why struggling in
quicksand will make it even harder to escape.
For some fluids (like mud, or snow) you can push and get no flow at all-
until you push hard enough, and the substance begins to flow like a normal liquid.
This is what causes mudslides and avalanches.
Author: Fred Senese firstname.lastname@example.org