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.

Sometimes, objects in motion collide.
When they do, some interesting things happen to those objects, and their momentum.
In one type of collision, called an inelastic collision, objects that collide stick together. When they do, their masses combine. Their velocity does not, however increase…the object moves more slowly in the direction of the greater force.
In another type of collision, called an elastic collision, objects bounce off one another. In this case, the objects affect one another’s direction, but no momentum is lost. Some momentum may, in fact, be transferred to another object (from the object with greater momentum to lower).

The simulation below shows elastic collisions. Adjust the overall mass of one of the spheres and observe the interaction between the two.

On important concept to know here is that, regardles of the type of collision two objects may have with one another, no momentum is lost in the transaction.
The Law of Conservation of Moentum says that the amount of momentum at the beginning and end of a collision will be the same.
A useful analogy for understanding momentum conservation involves a money transaction between two people. Let’s refer to the two people as Jack and Jill. Suppose that we were to check the pockets of Jack and Jill before and after the money transaction in order to determine the amount of money which each possesses. Prior to the transaction, Jack possesses $100 and Jill possesses $100. The total amount of money of the two people before the transaction is $200. During the transaction, Jack pays Jill $50 for the given item being bought. There is a transfer of $50 from Jack’s pocket to Jill’s pocket. Jack has lost $50 and Jill has gained $50. The money lost by Jack is equal to the money gained by Jill. After the transaction, Jack now has $50 in his pocket and Jill has $150 in her pocket. Yet, the total amount of money of the two people after the transaction is $200. The total amount of money (Jack’s money plus Jill’s money) before the transaction is equal to the total amount of money after the transaction. It could be said that the total amount of money of the system (the collection of two people) is conserved. It is the same before as it is after the transaction.

A good visual example of this is a Newton’s Cradle (below). When the first sphere makes contact with the spheres at rest, its momentum is transferred through the line of spheres. While the first sphere loses all of its momentum (and stops), the last sphere takes it on, and moves in a similar fashion to the first prior to the collision.
]]> http://www.sciencewithmrmilstid.com/2009/04/momentum/feed/ http://www.sciencewithmrmilstid.com/2009/04/wind-super-quick-summary/ http://www.sciencewithmrmilstid.com/2009/04/wind-super-quick-summary/#comments Wed, 01 Apr 2009 12:01:45 +0000 admin http://www.sciencewithmrmilstid.com/?p=320 One of the most important, but often overlooked, aspects of the world around us is the weather. It has an impact on everything in our lives – from what we wear day to day, to whether or not we are able to eat!

So what is it, really?
Weather is the condition of the atmosphere at a certain time and place. It is affected by the amount of wind, moisture, pressure and temperature in the air.

In order to fully understand weather, we need to be able to understand wind.
Wind is, very simply, moving air. It is also one of the driving factors of weather – it causes and carries weather phenomena from place to place.

What causes wind?
Wind is caused by the uneven heating and cooling of the earth’s surface - both globally and locally.

For instance:
As solar energy reaches the Earth, areas around the equator heat up more than the poles. Air in this region expands as it warms, creating areas of low air pressure, which rise. The warmer the temperature of air, the lower the pressure.
Air around the poles receives less solar radiation, and are cooler than air around the poles. These regions of cooler air condense, sink and create areas of high air pressure. The cooler the temperature of air, the higher the air pressure.

Wind is created by the convection that occurs as a result: low pressure (warmer) air rises; High pressure (cooler) air sinks, and rushes in to fill the space left by the rising low pressure air.

So our definition of wind, then, can be changed to: air moving from areas of high pressure to areas of low pressure.
Incidentally, the greater the difference in pressure (and temp) between two pockets of air, the higher the force/speed of wind.

Newton’s First Law of Motion:
Objects in motion tend to stay in motion, objects in rest tend to stay at rest.
Basically, Newton claims that objects are stubborn - they like to keep doing what they’re doing. If they’re moving, they want to stay moving; if resting, they want to stay at rest.

The resistance to a change in motion is called inertia.
The greater the mass of an object, the greater the inertia; the lower the mass, the lower the inertia.
So the motion of lower mass objects will be easier to change.
Examples of inertia and Newton’s 1st Law are everywhere – consider what happens when you slam on the brakes of a car…inertia causes you to slam forward as the car stops.

Notice the motion of the crash test dummy in the video above. When the cart crashes, it stops because it reaches a barrier. Inertia causes the dummy in the cart to continue moving.
Real world connection: as you can see, inertia is the main reason we use safety belts in automobiles. In an accident without them, your inertia could cause you serious injury (e.g.: flying out of the front window!).

Newton’s Second Law of Motion:
Formally: the acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.
Otherwise: For an object with a specific mass, the more force is applied to it, the greater it will accelerate.
Also, when a set force is applied to two objects of different masses, the object with the lowest mass will accelerate the greatest.

This law is expressed mathematically as F = m/a, which can be conveniently (and algebraically!) rearranged to find the acceleration of a object given force and mass (a = F/m), and the mass of an object involved in a motion (m = F/a).

Newton’s Third Law of Motion:
Every action has an equal and opposite reaction.
This statement means that in every interaction, there is a pair of forces acting on the two interacting objects.
The size of the forces on the first object equals the size of the force on the second object.
The direction of the force on the first object is opposite to the direction of the force on the second object.
In other words: all forces act in pairs.

The pennies in the video below hit the desk with a downward force. The desk also pushes back up on the pennies with equal force. Thus, the bounce.

How this works for you: every time you push on a door to open it, the door pushes back on you with equal force. The reason why this doesn’t appear to be so is because your mass is combined with the mass of the building during the push. In space, however, it’s another story (see the video).

Often, we have difficulty working with physics concepts because of the “reality” factor. On earth, many of the concepts we cover are “sticky” because forces like friction and combined masses of objects muddy concepts. In space, however, pure physics concepts are reality, as in the video above.

What exactly is gravity then? We experience it every day - take it for granted even - but most of us aren’t really all that familiar with the facts about it.

So how about a definition:
Gravity: the force that causes two objects to pull towards each other.

Something to note is that all objects have gravity. Even you attract other objects to you because of gravity. But you have too little mass for the force to be very strong.
Gravity only becomes noticeable when there is a really massive object like a moon, planet or star involved.

Also important to note is that gravity is predictable.
In small scale physics, we work off a law of gravity that predicts how it behaves.
This law, called the Law of Universal Gravitation, was created by Isaac Newton and is incredibly simple for tackling such a remarkable force as gravity.

The Law of Universal Gravitation states:

• Gravity increases as the masses of the objects involved increases
• Gravity decreases as the distance between the objects increases

Basically, this law states that larger objects have more gravity, and the further away from an object you get, the less you feel its gravity.

For instance:
The Earth has more mass than the Moon, so there is less gravity on the moon than on earth.
The gravitational force is greater on the Earth’s surface than it is in space, high above the Earth, so the Earth’s gravitational force pulls objects towards the center of the Earth.

Another important thing to note about gravity is that, it not only pulls things toward each other, it also affects their behavior and certain characteristics.

Gravity affects weight:
Weight is simply a measurement of the amount of force pulling an object downward - or gravity. Gravity affects weight in (surprise!) predictable ways. As the gravity on an object increases, its weight increases; as gravity decreases, weight decreases.

So if you move to a planet on which there is only 1/5 the gravity of earth, your weight will only be 1/5 of what it was when you left earth.

A fun example of how gravity affects weight can be found here: Weight on Other Worlds.
*Note that, if you can observe how gravity changes on another world (as in the website linked above), you can also infer characteristics of that world, such as mass and the relative amount of gravity.

One important side note to the fact above is that gravity does not affect the mass of an object. No matter where you are in the universe in any given moment, your mass remains the same.
This means that if you were able to be immediately transported to the world mentioned above (where your weight would be reduced to 1/5 of its original weight on earth), you would stay the same size. Your mass - or the amount of matter that you are composed of - would remain constant.

Another important concept to master is that gravity is constant for any object in a given environment. In other words, gravity pulls on all objects by the same amount.
So, neglecting air resistance, all objects fall toward the ground at the same rate.
A famous example of this is Galileo’s hammer and feather hypothesis: if you were to drop a hammer and feather at the same time, they would be accelerated toward the ground at the same rate.

Proof!


Clearly this doesn’t work on earth, where air resistance opposes the motion of falling objects, but it does prove that gravity acts on all objects in a similar fashion.

So what is this constant rate?
All objects are accelerated downward due to gravity at a rate of 9.8 m/s². So then, for every second that an object falls, its downward velocity increases by 9.8 m/s.

This rate is shown in the table below:

We can use this constant rate to determine how fast, and how far a falling object moves.

To determine the velocity of a falling object, we simply multiply the constant for gravity (9.8 m/s²) by time(s).
Or: v = g*t

To determine the distance a falling object travels, we multiply 1/2 the constant for gravity (.5*9.8 m/s², or 4.9) by the time of the object’s fall squared (t²).
Or: d = .5g*t²

An interactive example of calculating the distance of falling objects is shown below:
]]> http://www.sciencewithmrmilstid.com/2009/03/gravity-an-introduction/feed/ http://www.sciencewithmrmilstid.com/2009/03/forces/ http://www.sciencewithmrmilstid.com/2009/03/forces/#comments Thu, 12 Mar 2009 01:51:25 +0000 admin http://www.sciencewithmrmilstid.com/?p=252 We’ve been discussing motion in terms of speed, velocity and acceleration. But what is it that allows an object to be in motion in the first place? Force!
In order for an object to move or stop, an external force must first act upon it. Without some force to get an object moving, the object would just sit in place, forever.

What is force?
Force: A push or pull on an object.
All forces have both size and direction.
Forces are measured in a unit called the Newton.

Forces are what cause changes in acceleration, motion, direction, etc. All movement is caused by force.

Forces are not always easy to detect, however.
We will discuss two main categories of forces:

Balance of Force
In order for a force acting on an object to cause a change in motion, that force must be greater than 0N.

When the total force acting on an object is not 0N, it is an unbalanced force. In these cases the total motion of an object will change.

When the total force acting on an object is 0N, it is a balanced force. In these cases, the overall motion of an object will not change.

For example:
In the next image, a book is at rest on a table.
All forces are balanced, and no change in motion occurs.

If I shove the book right, forces will be unbalanced, and the book will change motion.
If no other force acted on the book, it would continue to move forever to the right.
Friction applies a leftward, unbalanced force to the book, which eventually changes its motion and stops the book.

Net Force
More than one force may (will, if you are on earth) act on an object at any give time.
This makes it useful to understand the effect of net force on an object.
Net Force: the vector sum of all forces which are acting on an object.

Rules for calculating net force in 1-dimension are fairly simple:
Add forces in the same direction.

Subtract forces in opposite directions.

The final change in motion always occurs in the direction of the largest vector.

Unfortunately (at times) for us humans, changes can be caused to the atmosphere, and the general conditions of the world as a consequence, by pollution.

Air pollution is: the contamination of the atmosphere by the introduction of foreign substances by human and natural sources.

There are two main types of air pollution:

1. Primary Pollutants
2. Secondary Pollutants

Primary Pollutants
These are pollutants that are directly put into the air by human or natural sources.
They can include gases like carbon dioxide and carbon monoxide, dust, sea salt, smoke from forest fires, volcanic ash and gas, and chemicals from human activity.

Secondary Pollutants
Secondary Pollutants occur when primary pollutants react with atmospheric gases to make new, hazardous substances.

For example – when the car exhaust produced by vehicles reacts with sunlight in the atmosphere, it produces smog, which can be hazardous to breath, etc.

Effects of Pollution
There are myriad effects of air pollution.
Just a few of them are:
Acid Precipitation
Air Pollution can mix with water in the air to create acid rain.
Acid rain can damage plants, animals and ecosystems around the world.

Damage to the Ozone Layer
As air pollutants (such as CFCs) are added to the atmosphere, they may damage the ozone layer. As the ozone layer is damaged, and begins to be perforated, it can let in excess UV radiation, which itself can be damaging to life on earth.

We will discuss the effects of air pollution on human health, and global climate change in future classes.

Or, view the whole class set at Flickr

Or, view the whole class set at Flickr

Atmospheric Composition
The atmosphere is made up of just a few main molecules:

The rest of it is made of things called trace elements like water vapor, ozone, and other particles and molecules floating around.

Layers of the Atmosphere
There are 5 layers in the atmosphere.
From the earth up, they are:

1. Troposphere
2. Stratosphere
3. Mesosphere
4. Thermosphere
5. Exosphere

Almost all weather occurs in the Troposphere, the lowest layer of the atmosphere, which extends from the surface up to 8 to 16 kilometers above Earth’s surface (lowest toward the poles, highest in the tropics). Earth’s surface captures solar radiation and warms the troposphere from below, allowing it to be habitable.

Temperatures in this layer decrease with elevation/altitude (by about 6.5°C with each kilometer of altitude).

At the top of the troposphere is the Tropopause, a layer of cold air (about -60°C), which forms the top of the troposphere and creates a “cold trap” that causes atmospheric water vapor to condense.

The next atmospheric layer, the Stratosphere, extends upward from the Tropopause to 50 kilometers. In the Stratosphere temperatures increase with altitude because of absorption of sunlight by stratospheric ozone.
(About 90 percent of the ozone in the atmosphere is found in the stratosphere in a section called the Ozone Layer. The Ozone Layer is important because it absorbs harmful UV radiation, preventing it from reaching the surface of the earth.)

At the top of this layer is the Stratopause, where temperatures peak at about -3°C.

In the third atmospheric layer, the Mesosphere, temperatures once again fall with increasing altitude, to a low of about -93°C.

Above this layer is the Thermosphere, where temperatures again warm with altitude, rising higher than 1700°C.

The highest layer of the atmosphere is the Exosphere. This layer is not well defined: it blends into space.

Figure 1 below shows the pattern of temperature fluctuations across layers of the atmosphere.

Figure 1. Structure of the atmosphere

See larger image

Source: © 2006. Steven C. Wofsy, Abbott Lawrence Rotch Professor of Atmospheric and Environmental Science, lecture notes.

Atmospheric pressure in the atmosphere changes with elevation consistently across all layers of the atmosphere: as elevation increases, pressure decreases because the total weight of air above you lowers.
Figure 1 illustrates this trend as well. This fact is what causes pressure changes in your ears while flying in an airplane.