Study Guide

Fluids Terms

Fluids Terms


Anything that has mass and volume.

States of matter

Matter takes on the forms of solids, liquids, and gases. Water is still water whether ywe're putting ice in a drink, boiling it to cook pasta, or drinking some after beating our brother at tennis.

Heat of Vaporization

How much heat is needed to vaporize a quantity of a liquid? That's the heat of vaporization.


A number; used in scalar multiplication.


A mathematical structure that has both magnitude and direction, represented by an ordered pair of components.


We think of a force as a great influence in our lives. Forces decide what happens to what, where, when, and how and does it all from various distances including but not limited to gravity, friction, and magnetism. Force is an influential friend with telekinesis powers.

Newton's First Law

Objects are like us. Once they get comfy, they don't want to move. This is the law of inertia, or the couch potato law as we like to call it. Except for when we think about how hard it is to stop an asteroid: "An object in motion tends to stay in motion" is just as true as "an object at rest tends to stay at rest."

Newton's Second Law

In math terms, F = ma. The force applied on an object is equal to its mass times the acceleration it experiences. If the object isn't moving at all, then F = 0. If it's moving but going at a constant speed, then F is also zero. This is known as equilibrium, a kind of boring state where not much happens. Gravity is a force, though, that causes acceleration on falling objects. Look out below!

Newton's Third Law

To every action, there's an equal and opposite reaction. Even when gravity hurtles us to the earth, we're pulling back on the earth with our own gravitational force but our mass and gravity are negligible compared to the Earth, so it wins. A bulldozer pushing dirt is opposed by the dirt pushing back on the bulldozer, even when the dirt is moved.


Extreme importance or seriousness


Weight is a force, not a mass. In this case, W = mg, where g is the acceleration due to gravity of the Earth. Picture walking on the moon with Neil Armstrong. Weight is a lot less over there, though mass remains the same. This is because acceleration due to gravity on the moon is a lot weaker than on Earth.

Normal Force

In vector land, the normal force is the contact force perpendicular from the surface on which an object lies at rest. For surfaces that lay flat horizontally, the normal force points straight up, the opposite direction of the force of gravity. It's what keeps objects from falling through whatever matter lies between it and the center of the earth. For a hill or other inclined surface, the normal force points diagonally up, perpendicular to the surface.


The force opposing a motion. It's air resistance, surfaces rubbing each other and slowing down a motion, or the force creating the great grip of rubber on the road. Friction affects everything. Ice on ice has low friction, and rubber on rubber high friction.


If you ask us, equilibrium should've been spelled EQUALibrium. In equilibrium, every force that acts on an object is canceled out. An object in equilibrium will either stand still or move at constant speed (without acceleration) unless a force changes and puts it out of equilibrium, causing acceleration.


Substances (gases and liquids) that flow freely.

Macroscopic Scale

The real-world scale, as seen by our eyes. And telescopes. But not microscopes.


Density is a measure of an object's mass in comparison to its volume (density= mass/volume). Density can increase either by increased mass or by decreased volume. Picture a jar of raisins and a jar of peanut butter. If both jars are the same size, the peanut butter would have a higher mass and consequently a higher density. To increase the density of the raisins you would have to either increase the mass of the raisins (squash more and more raisins into the original jar) or decrease the volume (take the same amount of raisins from the original jar and squash them into a smaller jar).

Specific Gravity

Nothing to do with gravity, sorry. It's the ratio of the density of a fluid to the density of water. Think of it as specific density (specific to water, that is.)

Specific Volume

Physicists are always trying to impress us by coming up with new terms. They could've very well called this one the opposite of density. We like to call specific volume anti-density, since it's simply volume divided by mass, the reciprocal of density.


Force per unit area.

Atmospheric Pressure

As it name implies, this is the pressure of the Earth's atmosphere, or the amount of force the atmosphere applies on the Earth's surface. At sea level, the atmospheric pressure is defined as 1 atmosphere (atm).


A measure of the volume change of a fluid or solid as a response to a pressure.

Hydrostatic Equilibrium

The equilibrium of one or more fluids in contact with each other. Think of balanced forces.

Pascal's Law

Changing the pressure at one point in a fluid changes it everywhere in the fluid. This is similar to Dante's description of his epic journey to the Inferno. When we sit in a dentist's chair, we anticipate the terrors that might await us. Incidentally, the chair we are sitting in works with Pascal's Law.

Enclosed Fluid

A fluid that is enclosed in a sealed container. Who woulda thunk?

Archimedes' Principle

The volume of an object is equal to the volume of water it displaces. If someone steps into a bath, he'd displace an amount of water equal to his body's volume. Crying "Eureka" is optional.

Buoyant Force

The force acting against water displacement, equal to the weight of the water. How much does the water spilled on the bathroom floor weigh? The water weight is equivalent to the volume that was spilled. So buoyant force = weight of spilled water = Mom gets mad. (Try calming her down by explaining it's all in the name of learning physics).


The ability to stay afloat, and it is all a matter of density. In fluids like water, objects with higher densities (like rocks) sink while those with lower densities (such as inner tubes) float.

Neutral buoyancy

Neutral buoyancy is when an object has the exact same density as the fluid it is located in. Neutrally buoyant objects neither float nor sink.

Surface Tension

The force per unit length used to overcome the microscopic forces between molecules at the liquid-air interface.

Ideal Fluid

In an ideal world, we couldn't compress a fluid and it would have zero viscosity.


Thickness of fluid; water has less viscosity than syrup


Move steadily and continuously in a current

Flow Line

Fluids are composed of particles. These particles flow in one direction. Guess what: that's a flow line.

Flow Tube

The popular hang-out place of flow lines. Any flow line tha''s anybody is seen there. Water molecules love to hang out in pipelines, which are flow tubes by another name.

Conservation Law of Mass

Mass is always conserved. If some water molecules freeze or evaporate or are moved somewhere else, their mass is found in the new location. It's neither created nor destroyed (except in specific conditions involving quantum physics, but we don't need to go there right now).'

Volume Flow Rate

A rate refers to how fast something moves. Logically, volume flow rate refers to how fast a volume flows. So it's determined by the volume of a fluid divided by time.

Continuity Equation

The volume throw rate is constant in any flow tube. In a pipeline that narrows and later expands, this equation says that the volume flow rate of a fluid is the same everywhere. If the flow rate was different, then the flow wouldn't be smooth, or continuous.

Bernoulli's Equation

The equation to relating pressure, height, and flow rate of fluids based upon the conservation of energy. This equation is named after a scientist. We guessed it: his name is Bernoulli.

Law of Conservation of Energy

In Einstein's words, "Energy is neither created nor destroyed." In Shmoopy words, "Nothing can't just poop something."