Science With Mr. Milstid

Fluids and forces can be a bit tricky at times. 
If you break their brhaviors down to a few key concepts, it’s really not all that daunting.

All fuids exert a downward pressure that increases with depth.
The reason is fairly simple - gravity pulls fluids downward. Fluids are made of molecules, which are made of matter, and therefore have mass. As a consequence of gravity, they also have weight.
As fluid molecules stack on top of one another, their incremental weights add up…the farther down a column of a fluid you travel, the greater the mass of molecules above, and the greater the pressure (weight) exerted downward.

This pattern is true of the atmosphere, where atmospheric pressure is the lower at higher elevations than at lower elevations, even though we rarely think of air having weight.
As you move upward along the mountain in the animation above, notice the number of air particles surrounding each hiker - they decrease with elevation.
This pattern is also true in regards to water. Water pressure increases significantly with depth - much more so in fact than air pressure - because of two main factors: density and, well, basic addition.
To begin with, water is much denser than air. It therefore exerts a greater downward pressure than air (there are, simply, more molecules sacked together in a similar quantity of water than in air).
The dramatic increase in water pressure is also due in part to basic addition - above water, is air. The “weight” of air is weighing down on water as well, adding to the pressure water exerts with depth.

Just as fluids exert downward force, they also exert an upward force on all objects in them - called buoyant force.
Archimedes discovered that the upward force exerted by a fluid is equal to the weight of the water that is displaced by an object in that fluid.
Objects heavier than this force will sink, objects that are lighter will float.

This concept is well related to density.
Objects that have a higher density than the fluid in which they are placed will sink, and those with a lower density will float.
Knowing this, we can use density calculations to determine the behavior of objects in a fluid - a nifty skill if you’re engineering a boat, say.

The formula for calculating density is D = M/V (or, density = mass/volume).
Because the relationship between mass and volume determines the density of objects, we can adjust either variable to affect overall density (and consequently affect the behavior of an object in a fluid).

Ways to affect density:

1. Change the shape: If you change the shape of an object without changing its mass, you effectively change the surface area and overall volume, thereby changing the density. If you increase the volume of an object, you decrease the density and vice versa.
   This is exactly how ships work - were the materials used in huge ships to be compressed into a lower volume, they would sink. But, because their overall surface area is large enough to compensate for their high masses, the float!
2. Change the mass: If you change the mass of an object without changing its volume, you can drastically affect density. Increased masses will increase density; decreased masses decrease density.
   This process is how submarines work. Subs have ballast tanks that fill with water (increasing their mass) when submarines dive. Conversely, they push water out of the vessel when rising to the surface.
   

Objects can also behave in a different way (regarding buoyancy) in fluids. When objects float, they are considered to have positive buoyancy. When they sink, negative buoyancy.
If the density of an object is the same as the fluid in which it exists, it is considered to have neutral buoyancy, and neither floats, nor sinks, but flinks.
This is how submarines “hover” in columns of water - they equalize their density to the surrounding fluid.

Matter is everything around you - air, dust, water, you, rocks. All of it.
Matter is anything made of atoms and molecules that has a mass and takes up space.

 
When we speak of mass and volume, we’re discussing very simple terms:
Mass is the amount of material in an object. Mass stays the same no matter what force is acting on the object. This makes mass different from weight, which depends on both the amount of mass and the amount of gravity. This variability means that, for instance, an elephant which has a huge amount of mass on the earth and weighs over a ton, would be incredibly light weight on the moon, even though his mass would remain constant.
(For a fun look at how your own weight might change on different planets due to variations in gravity, check out the weight on other worlds site.)

 
Volume is the amount of space an object takes up. It’s an incredibly simple concept - larger objects take up more space, and therefore have a greater volume. Small objects take up less space, and therefore have a lower volume.

 
We measure the volume of different types of objects in different ways:
The Volume of a Liquid is measured using a graduated cylinder - in most graduated cylinders, liquids form a downward curve (kind of like an inverted bubble) at the top of the liquid called the meniscus. Always read your measurements to the bottom center of the meniscus to ensure accuracy. Some plastic/polyethylene graduated cylinders will not show a meniscus. For accuracy and safety sake, still measure to the middle of the cylinder in these instances.
Liquid volume is expressed in Liters.



 
The Volume of a Regular Solid is measured by using the standard mathematical formula for volume: length x width x height.
The Volume of a regular solid should be expressed in cubic centimeters (where 1 cm³ = 1 mL).

 
The Volume of an Irregular Solid is measured by submerging the solid (like a marble, or something with hard to measure sides) into water and measuring the amount of water that is displaced. For instance, if a marble is placed into a graduated cylinder that contained 10 mL of water, and the level of water rose to 20 mL, we know that the volume of the marble is also 10 mL (cm³).

 
Directly related to the amount of matter in an object is a property called inertia.
Inertia is the tendency of an object at rest to remain at rest, and of an object in motion to remain in motion. Generally, objects with higher masses have greater inertia, and objects with lower masses have lower inertia. This means that objects with high masses (and high inertia) will be less willing to start moving if they are sitting still, and harder to stop once they get going. The opposite is true for lower mass objects.
A simple illustration of this can be found by thinking about baseballs vs boulders. The lower mass object (baseball) is much easier to get moving - you simply add a bit of force and overcome its inertia, causing it to fly through the air - whereas a boulder is next to impossible to budge (its inertia won’t be overcome without a huge! amount of force). Once both objects are actually in motion their relative inertias dictate how easily it will be to stop them. You can easily catch a flying baseball and stop it, but step in front of a boulder and you might find yourself in an uncomfortable situation.

 
Inertia is wonderfully illustrated in the videos below. The first shows several simple demonstrations of inertia, while the second is a little more removed - this video shows flatland bmx bicycle tricks, which rely on momentum and inertia (the tendency of their bikes to remain in motion once started) to perform tricks.


 

 
All matter has distinguishable characteristics that can be used to describe them called properties.
There are two kinds of properties: physical properties & chemical properties.
A physical property of matter is a descriptor of that object that can be observed or measured without changing the matter’s identity. For example, color (a physical property) does not change when observed or described.

 
Examples of physical properties are:

1. Conductivity: the rate at which a substance transfers heat.
   The higher the conductivity, the faster an object transfers heat.
2. Malleability: the ability of a substance to flex, or be rolled or pounded into thin sheets.
   The higher the malleability, the easier to bend an object.
3. Ductility: the ability of a substance to be pulled into a wire.
   The greater the ductility, the easier to pull into a wire.
4. Solubility: the ability of a substance to dissolve into another substance.
   The higher the solubility of a substance, the easier it is to dissolve into a fluid.
5. Density: the mass per unit volume of a substance. (D = m/v)
   Basically, density is equal to the amount of matter of a substance crammed into a certain size.
   An easy way to visualize this is to think about baseballs and tennis balls – they’re about the same size as each other, but one has much more mass (we feel this as weight) because there is more tightly packed matter in it.
   Another example might be sponges and bricks. Kitchen sponges are incredibly airy and feel light. This is because they don’t have much matter crammed into the space they take up. They aren’t dense. Bricks, on the other hand, are very dense and thus have a high mass (and feel heavy).

 
Density is an incredibly useful property for scientists - it can tell us a number of things including: how a substance will behave in a fluid, and even its identity.

 
Density in liquids
If an object is placed in a liquid its density will cause it to either float or sink.
If the object’s density is higher than the fluid in which it is placed, it will sink. If lower, it will float.
When fluids of different densities are placed together, they will separate and form liquid layers where the lowest density rises to the top, and the highest sinks.
This is what causes the oil in salad dressing to rise above the vinegar.



Here’s an experiment you can try on your own to illustrate the liquid layers concept.

Density is also useful for identifying objects - such as minerals - because it is an intensive property (it doesn’t matter how much of a substance you have, it will always be the same.
Properties of matter can be sorted into either intensive or extensive categories (where extensive properties matter how much of the substance you have).