Learn about the physics of fluid motion through a challenging animated physics simulator.

Density is a property of both fluids and solids. Density is defined as mass per unit volume, or how closely packed the molecules are inside an object. When molecules are tightly packed together, we say that the object is dense. When molecules are spread far apart, we say that the object is light.

Density does not change with the size of an object.  A big wooden oar and a tiny wooden popsicle stick are both made out of the same material, wood, and so have the same density.

When we talk about fluids, density is really important, much more important than weight.  By comparing the density of a solid object to the density of fluid, we can determine whether the object sinks or floats.  Whether the object sinks or float has nothing to do with its size, but everything to do with the material it is made out of and the density of that material.

Inertia is basically a measure of stubbornness. Mass is stubborn, it doesn’t want to move when it’s resting. And once you get it going, it doesn’t want to stop. The more mass an object has, the more stubborn it is. Therefore, denser objects have more mass and are more stubborn about moving around.

You can think of mass as a classroom of hyperactive 2-year-olds. Those of you with younger siblings know what I’m talking about. Getting these 2-year-olds to do what you want is really hard, because they are so stubborn.  The more kids in the classroom, the harder it is to get them to do what you want.

Fluid also have mass, and therefore inertia.  A denser fluid has more force when it hits an wall than a less dense fluid, since there are more molecule hitting the ball in the dense fluid.  But it also take more energy to get a dense fluid moving as fast as a lighter fluid, because it has more mass to accelerate, and that mass is stubborn.

When a fluid hits an object, its mass has to get diverted around an object.  But because mass doesn’t want to move around an object.

Gravity always pulls objects down. So why, then, do some objects like wood float on top of water? Or if you release wood while submerged in water, why will it float upwards? Is gravity pushing the wood up?

The wood is sitting in a fluid (water). We know that gravity pulls everything down, including the fluid. If we dump water out of a container, it falls on the floor because gravity pulls it downward. Now, when we have a piece of wood sitting in water, gravity is trying to pull down both the piece of wood and the water. So what happens? If the force of gravity is stronger on the wood, then the wood sinks through the water. If the force of gravity is stronger on the water, then the water gets pulled downward under the wood, and as a consequence, the wood floats upward. So the wood only goes up because gravity is pulling water downward.

So, what make the force of gravity stronger in wood or water? This is a property known as density. Since gravity acts on mass (F=mg) when something has more mass per volume (or density) than something else, the force of gravity is greater on that object.

Now, onto the real topic here, buoyancy. Buoyancy is a measure of whether an object is more or less dense than the fluid it is sitting in. For this reason, buoyancy is a property of an object that can change, depending on its surroundings. For example, wood is negatively buoyant in air (i.e. more dense than air), but positively buoyant in water (i.e. less dense than water). Air is less dense than water. If you trap an air bubble in a cup and release it under water, the air will float to the surface. This also explains why water will sit nicely in a tank, and not float up and out of it. The water is heavier than the air which fills up the rest of the room that the tank is sitting in.

So, objects float up because gravity pulls the fluid that the objects are sitting in downward, which describes what we commonly call positive buoyancy.

Viscosity is friction in fluids. Fluid are composed of molecules, but those molecules are linked to each other by bonds.  Those bonds can be broken, but it takes effort to break those bonds. Those bonds between molecules are what makes fluids sticky, or gives them viscosity.

Imagine you are very small (as small as a molecule) and wading through water. As you move, you have to fight these other molecules that are blocking your movement. If you were Gulliver, you could easily move through a crowd of Lilliputians, but if you were a Lilliputian yourself, then it would take you much longer to get through the crowd.

Also, think of the play pits that are filled with plastic balls. It is hard to move though this pit of balls, because the balls you are moving through are nearly the same size as you. Now imagine walking through air. Air molecules are a lot smaller than you, so you barely notice them.

At low Reynolds numbers, or in extra sticky fluids, skin drag dominates. The fluid sticks to the surfaces of objects, or their skin, thus the term skin drag. This is just like the friction between an object sliding on a surface, it is the contact between the fluid and the object which creates friction and slows something down. The total exposed surface area is all that matters- a tiny long string has a lot more drag than a big fat cylinder of equal volume.

Pressure drag dominates at large Reynolds numbers. Pressure is basically a description of how “annoyed” a fluid is with its current situation, and how much it would rather be somewhere else. Fluids only get annoyed at big Reynolds number, at lower Reynolds numbers, the fluid sticks together and can’t really move around much at all, therefore its ability to bounce around and get annoyed is reduced.

Pressure drag results from a fluid’s annoyance at having to go around an obstacle. A tiny string, no matter how long, is no problem to go around, but a big fat cylinder pushes a lot of fluid to the side and is quite a pain. A high pressure zone builds up in front of the cylinder, where everything is really annoyed at having to move around this object. And a low pressure zone builds up behind the object, which is where everything wants to be.

The bigger the object, the bigger the pressure drag because the fluid has to move even farther around the object and gets even more annoyed. Think of driving in a traffic jam, with an accident ahead. If the accident is only in one lane, traffic slows down and you are slightly annoyed, but everyone moves out of that one lane and keeps going. On the other hand, if you have a three lane accident on a four lane highway, traffic has to really get redirected, resulting in a lot of annoyance and a buildup of pressure.