Friction is an important concept
-less friction = more slippery
-To slip on a banana peel, the force of linear velocity must overcome downward static friction
-Once you start to slip, it is difficult to stop because kinetic friction is less than static
Newton's 1st Law of Motion at work here
-Definition: an object in motion will stay in motion unless acted on by an outside force
We also see Newton's 3rd Law of Motion
-Definition: for every action there is an equal and opposite reaction
-When you slip, there is an accelerated force forward
-In response, there is a backwards force on your body which can cause you to fall
Mythbusters conducted an experiment examining this concept.
Hypothesis #1: A banana peel on the ground is guaranteed to cause the person who steps on it to slip and fall. BUSTED. In their first test, a blindfolded Jamie (wearing body protection) stepped on a banana peel while walking, but did not slip. In the second test, multiple banana peels were laid down in Jamie’s path, but he did not slip. He then tried running through the banana peels but still failed to slip. They performed further tests by measuring a banana peel’s static friction and kinetic friction and comparing it to lubricant, with the lubricant having far less kinetic friction but the banana peel having somewhat less static friction. In their full scale test, the MythBusters built a race course that had the entire ground covered with banana peels, and later lubricant. They compared how quickly and easily they could negotiate the course with banana peels and the lubricant, doing comically poorly in both cases. In the end, they found that banana peels would not guarantee a fall but could still prove to be very slippery on a smooth enough surface.
Their conclusion?
-Bananas are slippery, but slipping on a banana peel is unlikely due to the strong static friction between a shoe and the tough outside skin of the peel
-In their experiments, the individual only fell when multiple bananas and lubricant were on the ground
Here is the experiment:
—http://www.youtube.com/watch?v=YZRq3XxCZXo
Friday, December 2, 2011
Thursday, December 1, 2011
The Physics of Sailing
Effective Sailing is dependent on balancing 3 basic forces:
1. the driving force of the wind, caused by wind flowing across the sail
2. the sideways force
3. the heeling force
The Velocity of the wind relative to the boat results in a force that acts on the sails. This force has two components, one perpendicular and one parallel. "Lift" is the force component that acts perpendicular to the wind direction, while "Drag" acts parallel to the wind direction.
The sideways and heeling forces are a result of the force exerted by the water on the hull and the keel of the boat (a structure that provides support and resembles a wing of an airplane). This force of water also has both perpendicular and parallel components, and the keel offers a counter force to minimize the sideways movement of the boat.
Sailboat racers rely on physics principles to properly adjust their sail and boat angle to the wind to maximize their speed. The optimal wind angle for achieving good boat speed is when the wind blows directly over the side of the boat. At this angle, the lift force points in the forward direction, and the forward push force of the wind on the sails remains constant, and is independent of the speed of the boat. Therefore, a sailboat is able to reach speeds that are faster than the wind itself!
Here's an interesting video that gives some context into sail trim, and the speed that boats can achieve!
http://www.youtube.com/watch?v=FWINygISxDE
Tuesday, November 29, 2011
Can Kobe Bryant jump over a moving car?
In 2008, a commercial promoting the new "Hyperdunk" line of basketball shoes went viral on the internet. The commercial involved Los Angeles Laker, Kobe Bryant, jumping over a moving Aston martin.
"If 'Rambo Part XX' can be a one-man militia, I can jump over an Aston Martin" he told the Los Angeles Times.
Understandably, there was a lot of speculation as to whether or not the stunt was real. Can we use physics to determine the validity of the stunt?
The "Dot Physics" blog of Wired website used physics to examine whether or not the jump was fake. They used Tracker Video Analysis to get the x,y positions of each body part in each gram and plotted center of mass values against time. They then fit a quadratic function to the center of mass and related it to the kinematics equation and found that the data fit the expected parabola very well.
Next, they examined Kobe's horizontal position during the jump using the same method. They found that the horizontal motion of his center of mass was fairly constant, which is what one would expect for a jump. There is no force in the horizontal direction while in the air, so there should be no horizontal acceleration.
Thus, by examining his vertical and horizontal positions, it seems as though the jump itself was real. However, an image overlapping two frames shows that Kobe was actually standing very near to the front right tire of the car, not in the middle of the car. This suggest that a degree of deception was involved.
"Sports Science," which is an ESPN television series that explores the science and engineering underlying athletic endeavors, also examined whether the stunt was real or fake. They, however, took a different approach: they tried to recreate the stunt that Kobe apparently performed.
The stuntman must perform the very difficult task of perfectly synchronizing the peak of his jump with the top of the car passing underneath him. In order to clear the car, a jump of at least 48 inches off the ground is required. The stuntman himself can jump 52 inches off the ground. If the Aston Martin is travelling at 55 mph it moves 1 foot every 0.01 seconds, meaning the stuntman must leave the ground when the car is exactly 24 feet away. However, it takes the brain 0.25 seconds to react to visual stimuli, meaning the stuntman must decide to jump when the car is actually 44 feet away.
After many many trials, they were ultimately unable to recreate the stunt. The stuntman was able to jump a high enough height to clear the car but the timing was impossible - 0.01s accuracy was required. The only time they were able to recreate the stunt was when the stuntman wore a harness connected to a 40 foot crane in order to give him an extra boost and to keep him safe.
Thus, the stunt was fake.
Furthermore, the earlier mentioned Dot Physics blog found that the Aston Martin that Kobe apparently jumped over was travelling at a constant velocity of about 22 mph. If Sports Science could not perform the stunt when the car was travelling at 55 mph, it is even less likely that Kobe was able to perform the stunt when the car was travelling at 22 mph since the slower velocity of the car means that Kobe had to be in the air for a longer amount of time.
In conclusion although the jump itself was real, the stunt was faked in some way.
The Physics of X-Rays
What is an X-Ray?
X-Rays are electromagnetic waves with a wavelength of .01 to 10 nanometers. They are generated in an X-Ray tube. X–Ray tubes work by shooting a stream of electrons at a piece of metal, usually tungsten. There is a magnetic field around the tungsten, which drastically slows down the electrons. Because energy is conserved, the kinetic energy of the electrons must be converted into another form when they are slowed down. The kinetic energy is converted into X-Rays, which are a form of radiant energy (the energy of electromagnetic waves). 
How X-Ray Images are Generated
An X-Ray sensitive film is placed below a designated area of the body, and X-rays are shot at this area. Dense materials in the body, like bone, absorb or scatter the X-rays and do not let them reach the film in that area. This causes a white shadow to appear on the film that shows the shape of the bone. Less dense materials like muscle and fat allow most of the x-ray particles to pass through and reach the film, making the film dark in these areas. - Post written by Sammy Kay-Green.Monday, November 28, 2011
Popping Popcorn with Cell Phones
Have you ever thought about popping popcorn with your cellphone??
http://www.youtube.com/watch?v=V94shlqPlSI
Well, sorry, you can't.
An electromagnetic wave is made up of alternating electric fields, a charge exposed to it will experience forces regularly changing in direction. For water molecules, which are dipoles, the net effect would force the molecules into rotation.. These agitated water molecules then posess heat energy to transfer into the food, thus cooking it. The key note here is that the microwave acts as a protective box, directing the waves directly at your food. Microwaves operate at a frequency of up to 300 GHz.
Cell phones cannot direct all that energy straight into a tiny popcorn kernel since it is not in a closed box like a microwave. Additionally, they do not emit enough energy (they operate at a frequency of roughly 2 gHZ) to heat up the water in the kernels to make them pop, and if they did, they would probably boil the water in your hand while you were on the phone and eventually make your hand explode!
Sunday, November 27, 2011
The Physics of Draining a Container
If you have ever bought a 2.5 gallon Poland Spring container you know that they don’t work very well as purchased. The water will flow out of the nozzle at a slow pace for a few cups of water and then come to a halt. In order to get the water to flow at an acceptable rate you must punch a hole in the top of the container. This is because when there is no hole at the top of the container a vacuum builds in the interior of the top of the container and a pressure gradient force caused by the difference between the low pressure at the top of the container and atmospheric pressure outside of the container causes air to move in through the same hole that is letting out water. This causes the water to drain from the container in a discontinuous way and at a slow pace.
Putting an additional hole at the top of the container allows the air to enter the container though a hole that is separate from the hole through which water is exiting. This allows the water to exit in a continuous stream at a much faster pace.
- Post written by John Mahon
Wednesday, November 23, 2011
The Physics of a Water Polo Shot
In water polo a player can shoot the ball as fast as 50 miles per hour without contact with a surface to provide force and stabilization. I want to explain how a water polo shot works and consider a few physics concepts that are at work in scoring a goal.
Procedure:
1) Player lifts the ball out of water with his/her shooting arm raised above the shoulder and trunk rotated away from the goal
a. Non-throwing arm is outstretched in the direction of the goal for aim
2) The player raises their body as high out of the water as possible to minimize FD
3) Shot is initiated by rotation of front arm away from goal and trunk rotation toward goal
a. As the trunk is rotated forward, the throwing arm is left behind for greater potential energy due to FT in triceps and shoulder muscles
4) The shoulder is rotated and elbow extended to release the ball
a. Trunk lean is away from shooting arm to improve position relative to the axis of rotation (spine) and maximize velocity
5) The wrist snaps for follow-through and fingers can be used to provide spin on the ball
Many of these procedures are similar to throwing a baseball with a few special considerations due to the fact that the player is never in contact with the ground and does not have a stabilizing force. In water polo, the action of the legs is key for support, balance, and production of force behind the shot. During the shot there are several key actions of the legs to aid in balance and force production. The player starts with their non-shooting-side foot pointed to ward to goal for aim. Force is produced during the shot by “snapping” in the shooting-side foot to initiate trunk rotation. During the forward swing, the non-shooting-side leg is forcefully extended to achieve force balance from the upper body rotation.
The key physics concepts used in scoring a goal in water polo are buoyancy and drag force, tension of muscles, angular momentum, and distribution of forces.
Videos:
Some great water polo shots: (showed shot at 4:39 in class)
A clip from the movie “Children of Glory” about the "Blood in the Water" match between Hungary and the USSR at the 1956 Melbourne Olympics during the Hungarian Revolution. It is a fun video to watch, but we only saw a clip of the penalty shot at 3:34.
Post written by Kelsie Anson
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