Pressure

I found a very cool video on youtube

http://www.youtube.com/watch?v=BECx4E4Ug9Y

The video was a demonstration of the principle of pressure.

In mathematical terms, pressure is equal to force divided by area.

In this case, the gravitational force was large but the area is also
large.  Hundreds of nails support the weight of the person's body
instead of just one. The resulting force is distributed over all of
the nails, so the person wasn't injured. - written by Eric Hu

The Physics of Fire

Fire seems almost like magic. What is it exactly? Chemistry tells us that fire is the energy released from the chemical reaction between oxygen and organic material. This is known as combustion. This energy is released in the form of heat and light. But chemistry only goes so far in our understanding. Why does fire acts the way it does? Here's where physics comes into play.

Examples of combustion equations

Physics not only explains the shape fire takes but also why certain flames glow in different colors. Gravity makes warm air rise and this principle is what shapes the flames into their distinct shape. In fact scientists at NASA have done experiments with flames in zero gravity and the flame actually spread out in a spherical shape as seen below:

As for the light we see, physics explains this too: specifically quantum physics does. The heat from the reaction excites the molecules to emit light of a certain color, in most cases blue. Red flames are actually heated up ash and soot around the flames glowing and can sometimes cover up the blue light from the actual reaction.

The Physics of the Heart

The blood flow through the heart, or as physicists call it, hemodynamics, can be described by the equation: Flow= pressure difference on either side of the valve/the resistance to flow.  For example, when the blood is in the left ventricle, its next step is to move into the aortic arch, which then pumps the blood to the rest of the body.  However, this movement is blocked by a valve known as the aortic valve.  In order for this valve to open, the pressure in the left ventricle needs to be greater than that in the aortic arch.  Therefore, when measuring blood flow from the left ventricle to the aorta, the pressure difference would be the intraventricular pressure minus the aortic pressure.  When the left ventricle has a sufficient amount of blood in it, its pressure will surpass that of the aorta, and the valve will open.  Flow is also dependent on resistance, which refers to the relationship between the blood and its surroundings.  In the heart, resistance has to do with the size of the valve opening.  When the valve opening is small, the resistance is high, and the blood flow is slow. However, when the valve is large, the resistance is low, and the blood flow is fast.- written by Olivia McKennon

More information on this topic can be found at:

http://www.cvphysiology.com/Hemodynamics/H001.htm

 

Physics of the Eye

The eye acts as a camera, refracting light to focus an image on sensory cells to produce neural responses from which an image is constructed in our brain, thus enabling us to see. Your eyeball possesses a transparent opening on the front called the cornea. The cornea is shaped like a converging lens (meaning that it is thicker in the middle). Therefore, it converges rays of light traveling parallel to a single point, optimally on the back of the eye, or retina. The cornea also refracts light thanks to its index of refraction of 1.38, significantly greater than that of air. Light next passes through the pupil, which is basically just an opening. The pupil appears black because all of the light behind it is absorbed by the sensory cells of the retina, such that no light is reflected out of the eye. The colored part of the eye, or iris, is a muscular ring that adjusts the size of the pupil depending on how much light is available. Light then passes through the crystalline lens, a membrane capable of changing shape with the help of ciliary muscles as a mechanism to focus the image on the retina, where all the sensory cells are located. This muscle action also serves to make slight changes in the bulge-shape of the cornea changing the focal length. While the cornea does most of the refracting, the role of the lens is to make small alterations thanks to its flexibility. Generally, the focal length (or length from the lens to the point at which the light rays converge) is approximately 1.8 cm. The anatomy of the eye ensures that at this length, the image produced is focused on the retina in a reduced, inverted form (the brain reverts the image such that we perceive everything right side up). The eye adjusts its focal length via the mechanisms described earlier in a process called accommodation, which allows us to focus on images both close up and far away. Close up objects require a shorter focal length so that the image will focus on the retina. Conversely, far objects require a longer focal length. This means that for distant objects, the ciliary muscles relax, and contract for close objects. To apply this further, nearsightedness (or the inability to focus on objects far away) is caused by a bulging cornea or elongated eyeball which causes a shortening of the focal length. Corrective lenses are shaped divergently (concave in the middle) to spread out the light rays before they enter the eye, thus moving the focal point back to the retina. Farsightedness is the exact opposite; the focal length is lengthened such that convergent lenses are necessary. - written by Keith Casey

The Physics of Waterskiing

Ever wonder about the physics of water skiing?

Well, when you are water skiing physics comes into play quite a bit.  When you try to get up on a ski, it’s important to keep your ski at a fairly precise angle to the water so that as you’re pulled forward the water hitting your ski creates a downward force, enabling you to stand up.  When the upward force of the water on your ski is equal to the downward force of gravity, you can effectively stay afloat.

When you are being pulled behind the boat, the force of tension in the rope is also acting on you.  When there is constant tension in the rope, you will travel at the same speed as the boat that is towing you.  However, if you’ve ever been water skiing you know that the skier is often traveling at a speed faster than the boat.  How is this possible? Well, this is where centripetal forces come into play. The rope keeps the skier in a circular path around the boat.  Because of this circular motion, the skier experiences acceleration toward the center of the circular path, just as we saw in lab with the swinging mass.  This centripetal acceleration means that the skier can be skiing at speeds quite a bit faster than the boat is traveling-- making wipe outs that much more painful!

Finally, Bernoulli's principle factors in as well.  As the velocity of fluid flow increases, the pressure decreases.  Therefore, the speed of the boat must increase when the skis have less surface area (so, if you're doubling skiing, the boat can pull you significantly slower than if you are single skiing).  If you've ever tried to barefoot ski, this makes complete sense and explains why the boat has to pull you a such a fast speed in order to keep you going-- because after all, your feet are much, much smaller in surface area than a ski.  And this is why when you fall while barefoot skiing, it can be pretty painful!

Will It Float!

If you watched David Letterman growing up, you will remember Will It Float. Mixed in with the top 10 and the opening monologue, an entire segment was dedicated to physics. The idea of the game is simple. An object is announced and David Letterman and Paul Shaffer guess if the object will float in a bucket of water.

Whether the object floats or not is dependent on two forces. The force of gravity acts downward on the object, while an upward buoyant force is exerted by the liquid. If the force of gravity is greater than the buoyant force, the object will sink. If the buoyant force is greater the object will sink. When the forces are equal the object will hover in the water. The buoyant force is always upwards since the pressure in a fluid increases with depth. As a result the upward force on the bottom of an object is greater than the downward force on the top of an object. Another way to look at buoyancy is Archimedes' principle, which states, "the buoyant force on an object immersed in a fluid is equal to the weight of the fluid displaced by that object" (263 Giancoli).

So will the cheese log float or sink? That depends on whether or not the weight of water displaced by the cheese log is greater than the weight of the cheese log. Click on the link to find out.

http://www.youtube.com/watch?v=zm-AkvwPdZ4

Here's another link that explains buoyancy in more detail.

http://www.pbs.org/wgbh/nova/lasalle/buoybasics.html

Up!

Much like movie romance, movie physics often push the boundaries of what is real and possible. A fun example of this comes from the 2009 Pixar movie "Up". In the proud tradition of unreal movie physics, this film depicts an old man's house being torn from its foundation and lifted off the ground by an enormous bunch of helium balloons.

Now, is it possible for such a thing to occur without the magic of animation? This is the question asked by the team on National Geographic's "How Hard Can It Be?".

The answer? No, impossible.

In order for balloons to lift a house, the upward force of the balloons must exceed the force due to gravity keeping the house planted firmly on the ground. The upward motion of the balloons is due to buoyancy. The helium balloon is lighter than the air it displaces, so it moves upward. Unfortunately, the gathering the quantity of balloons needed exceed the force due to gravity is simply not feasible.

But that's no fun. So the folks over at "How Hard Can It Be?" scaled it down and did it anyway. They constructed a light weight replica home and gathered approximately 300 high capacity weather balloons. With this set-up, they were able to lift the house, with people inside, creating a real-life re-enactment of the movie. And that's still pretty cool.

http://www.youtube.com/watch?v=rV6rNqin4P8