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## Mousetrap

A simple snap-back mousetrap is a clever machine. With just a few parts (a wooden base, a spring, a metal bar, and a trigger mechanism) it can do its job quickly and efficiently.  When a mousetrap is set, the spring in the center is compressed, becoming a source full of potential energy. This energy is being stored, not used, but as soon as the trap is released, it is converted to kinetic energy (the energy of motion) that propels the snapper arm forward.  This is a perfect example of conservation of energy.  It takes an amount of force to set the mousetrap and when the trap is triggered, it creates a force onto the mouse that triggered it.

## Speed of Sound

The speed of any wave/the speed of sound depends upon the properties of the medium through which the wave is traveling.  But first if there is no medium for the wave or sound to go through, then there will be no sound.  For example, there is no medium in space so there is no waves/sounds travelling in space.  There are two factors that effect speed of sound.  One of them is the elastic properties of the medium/material.  Elastic properties of an object is how easily the object is able to bend or deform when a force is acted upon it.  So the phase of matter effects the elastic properties of the medium.  For example, longitudinal sound waves travel faster in solids than liquids and gases due to their elasticity properties.  Another factor that affect the speed of light is the density of a medium.The greater the density of individual particles of the medium the slower that the wave will be.  A sound wave will travel faster in a less dense material than a more dense material.

## Ferris Wheel

A couple of summers ago, my family and I went to Hersey Park for vacation.  I'm afraid of heights but I love to go on roller coasters and I remember that there was a Ferris wheel that my sisters persuaded me to go on.  It was scary because you could see how high you were from the ground.  But it was also cool because you could see everything.  Anyways, a Ferris wheel can be related to physics because of its shape.  It is related to centripetal force and torque.  So basically, I could calculate the torque that a rider feels on the Ferris Wheel.  All I need is the radius, the mass, and the linear acceleration.  To find the linear acceleration, I would calculate the centripetal acceleration of the Ferris wheel.  So I would need the velocity and radius of the rider.  Then I could convert the centripetal acceleration to angular acceleration.  Then I would calculate the moment of inertia by doing mass times the radius squared.  Finally to find the torque I would do the moment of inertia times the angular acceleration.

## Yo-Yos

When I was a kid, my parents bought me a yo-yo. At first I was puzzled and wondered how to play with it. I spent a good amount of time practicing and I could finally make it roll and then come back to me. I thought that was a huge accomplishment, but then I saw on TV a yo-yo contest with these people doing insane tricks with their yo-yo. I never knew how they could do it. So I decided to see the physics behind a yo-yo.  I found that when people do string tricks that makes the yo-yo roll on the string is due to friction.  There is friction between the string and the axle that prevents the yo-yo from spinning, allowing the yo-yo to roll on its own string and not giving out more string.  This may seem simple but it applies to an yo-yo. Newtons first law that says that an object will stay in motion until an outside force is acted upon it. This is why a yo-yo comes back to your hand. You can flick your wrist and cause a force that push the yo-yo from its string and then you flick your wrist back to get the yo-yo back to its original state.

## Opticall Ilusions

Our brain are set up to receive and interpret messages from the eye. Optics, a branch of physics, studies the interaction of the light and the eye. This interaction plays an important role in optical illusions. Optical illusions use light, colors and other features to trick the mind into thinking of things that are or aren't there. For example the Lilac Chaser Illusion. In this optical illusion, the viewer sees purple blurry dots arranged in a circle around a focal point. As you stare at the plus sign, it will appear as if a space is running around the circle of lilac discs. But after the viewer continues on staring, they will eventually see a green disc moving around the circle instead of the space. Then if the viewer continues on staring, they will then see the disappearance of the blurry dots and only see the green dot moving. We perceive movement and when we see something at one point and then at another, we believe that it is in motion. Also, when blurry objects are located in the periphery of our visual field, eventually they disappear when we have our eyes fixated on a certain spot.

## Jenga

Jenga, it's the classic block-stacking, stack-crashing game that everyone played as a kid. You and the person you played with, stacked up pieces of block into a sturdy structure and then you remove these blocks from the bottom or middle and placed them on the top. As you removed a block from the structure you had to be careful of how you removed it because one wrong twist or turn, you could collapse the structure and lose. The reason why it's so hard to remove the block from the structure because there is a friction on the block that resists you from pulling the block fast and smoothly. If the block isn't removed smoothly then the structure will collapse. The reason why this game works and why the structure stays in place when you remove blocks is because of the center of mass of the structure.  Even if you take a block from the middle of the structure, this doesn't affect the structure to fall down immediately because the center of mass doesn't move when move the block. It stays constant and keeps the structure from falling. Since the center of mass of the structure doesn't move, the only time it falls is when a block is removed that makes the structure unbalance and fall over.

## Riding a Bike

Did you know that you transfer about ninety percent of your force upon a pedal of a bike into kinetic energy? Riding a bike is so simple but there is so much physics behind it. As you ride a bike there are multiple forces on you. There is a force of gravity downwards on you, so as you slow down, the force of gravity will push you and the bike down. There is also a drag force and frictional force acting on you and the bike. The drag force is the air resistance you feel when you go downhill. If you're going at a high speed with your bike then you can feel the air resisting you from going down the hill.  Also you can't forget the frictional force between the tires and the road. Then there is a force pushing you forward which is caused by the work the person does by pedaling. As the person decreases its work then, the frictional force will be greater causing the bike to slow down. But if the person increases its work, the force going forward will be greater than the frictional force causing the bike to speed up.

## Metamaterials

Could you create an invisibility cloak?  I mean if it was possible it would be insane.  But what if there exists a material that scientist created that allows it to bend light or an electromagnetic radiation of an object, giving the appearance that it isn’t there at all.  Light is electromagnetic radiation, made up of vibrations of electric and magnetic fields. Natural materials usually only affect the electric component.  However metamaterials can affect both the electric and magnetic field.   Metamaterial is a material engineered to have a property that is not found in nature. They are made from multiple elements composite from materials such as metals or plastics.  Physicists from the UK and Germany made one small device that made small objects invisible to near-infrared radiation and worked in three dimensions.

## Hovercraft

The hovercraft hovers by creating a cushion of air with enough pressure to the weight of the craft and the passenger.  The fan constantly blows air molecules into the cushion.  The cushion inflates, and there are a couple of holes in the cushion that allows some air molecules to escape so the cushion doesn't explode from the pressure.  The trick to get the craft to hover is to have the air molecules exert greater pressure or force than the weight of the craft.  The air pressure needed to lift the hovercraft equals the weight of the craft and the passengers divided by the area. Pressure = Force / Area.  It seems like pressure and area are inversely proportional, however, the larger the surface area, the greater the weight of the craft and therefore more pressure would be needed.  Also the larger the area, the greater drag or resistance on the craft is created.

## Time Travel

Is time travelling even possible? Maybe, but to time travel a person has to be faster then light which is impossible because no one has enough energy to move faster than that speed.  However, Einstein’s special theory of relativity, developed in 1905, shows that time passes at different rates for people who are moving relative to one another - although the effect only becomes large when you get close to the speed of light.  If anyone has seen the TV show, the "Flash," the Flash is able to run at incredible speed.  He is relatively faster when a person watches him.  But in his own body he is running normally while everything else in the background is a blur.  Relative to him running at a speed close to light, his perception of fast is different to a person that is much slower than the speed of light.  But there may be an out to be found in general relativity, there are a possibility of wormholes – a kind of tunnel through space-time connecting otherwise very distant parts of the universe.f the “mouths” of the wormhole are moving relative to one another, then traversing the bridge between different points in space would also take a traveler to a different point in time to that in which she started.  The Flash is fast enough that he creates his own wormhole where he is able to travel between worlds and time itself.

## Airplanes

The last time I was on a airplane was when I was traveling to Florida for vacation.  I wondered how almost 200 people and the mass of the plane didn't weigh the plane down.  The forces on the airplane is at equilibrium when the airplane reaches at a certain altitude.  Additional when the airplane reaches at a constant velocity therefore the forces on it all must be balanced.  This means that the lift force (L) generated by the airplane wings must equal the airplane weight (W), and the thrust force (T) generated by the airplane engines must equal the drag force (D) caused by air resistance.  The airplanes wings and the fins in the back of the air plane, cuts down on drag force and increases the lift of force when the plane is increases its altitude.  The wings and fins makes the airplane aerodynamic letting the airplane go faster when its flying in the air.

## Dancing

Both of my sisters used to dance and when I was younger I went to their dance recitals.  Every year I went, there was always that one dancer who would spin on her toe for the longest time.  I always notice that when ever the dancer slowed down while spinning, she whipped her legs around in a circle again and then she started to spin faster.  I have always wondered how one leg motion could keep you spinning for the longest time.  Well this simple spinning of a dance can be explained through angular momentum.  When the dancer starts she extends her legs out to have a larger radius.  With this larger radius, her angular speed is small, but when she whips her legs around and tightened up her leg to her body, the radius is smaller.  The angular momentum is the angular speed times moment of inertia initial equals the final.  The dancer with the large radius has a large moment of inertia and low angular speed.  When she brings her legs together, the moment inertia becomes less and the angular speed increases.  Therefore she can spin faster when she puts her legs out and then whips it back into her body.

## Music

Music, who doesn't like music?  Music is an universal langue. Music can be heard in any mood or activity.  It's just a thing that everyone does.  Yet how can we listen to music in general? Well sound is produced when a medium is being vibrated.  A medium could be air, water, etc... Vibrations in air are called traveling longitudinal waves, which we can hear.  The reason why sounds can't be heard in space because there is no medium where sounds can vibrate in.  Since sound can't vibrate in a medium, sound can't be heard in space.  Sound waves consist of areas of high and low pressure called compression and rarefaction. Additionally the wavelength and the speed of the wave determine the pitch, or frequency of the sound. Wavelength, frequency, and speed are related by the equation speed = frequency * wavelength. Since sound travels at 343 meters per second at standard temperature and pressure (STP), speed is a constant. Thus, frequency is determined by speed / wavelength. The longer the wavelength, the lower the pitch. Lastly the height of the wave is its amplitude. The amplitude determines how loud a sound will be. Greater amplitude means the sound will be louder.

## Eyes

I have horrible eye sight, so that's why I need glasses to see better.  And yet I never really understood how a mirror could make you see better.  Until last year, when we learned in physics of optical glasses.  Our eyes are concave, and the distance between our cornea to our retina is the focal length.  So the light that hits our eyes allows us to see images.  People who have twenty/twenty visions have eyes that aren't small or elongated and the focus is at the retina.  This allows people to see images clearly.  However people who have bad eye sight can either be far-sided or near-sided.  The people who are far-sided, have their focus behind the retina because the eyes are short, therefore convex lenses can fix this.  Convex lenses has a positive focal length which moves the image of a person far-sided forward.  People who are near-sided, have their focus in front of the retina because the eye is elongated. Concave lenses can fix that because tit has negative focal length so it moves the image back to focus.

## High Jumping

Watching the summer Olympics last year was really intriguing because of the High Jump.  I have always wonder how someone with just a pole could jump so high above a bar.  Now I know there's physics behind it.  First of all, before the person jumps above the bar, the person with pole has  to generate speed to the high bar.  This speed generate before the person jump increases the kinetic energy of the person.  Then the person plants the pole down at an angle and jumps, the person is then able to be at the highest trajectory of their motion.  Additionally, the kinetic energy of the person is then transferred into potential energy causing the person's height to increase.  This allows the person to jump at a high height and be able to be at the highest point of its trajectory.  The hang time of the person in the high is due to the pole's elasticity.  The pole bends then whips the person to the bar, causing the person to have a longer time in the air.  Then the person has to arc their backs to get over the high bar.  Then they land on the soft pads to help their landing because it increases the time of the force impacting the body causing no harm to the body.

## Explosions

It's hard to think about a how an explosion has momentum conserved because an explosions blows up everything in little pieces.  However, those infinite amount of pieces all contribute to conserve the momentum of the explosion before it exploded.  The law of conservation has a simple equation of mass times velocity initially equals the mass times velocity final.  An explosion before it happens is equal to zero because it has no velocity at all.  After the explosions, the pieces broken off in the explosion will go everywhere.  The direction of the pieces matter because after the explosions, the direction of the pieces will have equal magnitude but just in the opposite direction.  So when you had up all of the individual pieces of its direction, mass and velocity, the pieces will even each other out.  Therefore the sum of all of the infinite pieces after the explosion will have a momentum equaling zero.  Therefore the momentum of the explosions is conserved.  It's crazy to think that the aftermath of an explosion has zero momentum.

## Roller Coaster

Roller Coasters, how do they move so fast without any motor pushing them? Energy!  More specifically kinetic and potential energy.  In the beginning of a roller coaster ride, there is an ascension to the top of hill.  The purpose of this is that as the roller coaster gets higher and higher up, it gains potential energy.  You could calculate this by doing mass times gravity times height.  After the the roller coaster reaches to the top and starts fall down, the potential energy is then transferred in kinetic energy, which moves the roller coaster.  Kinetic energy can be calculated by doing one-half times mass times velocity squared.  Energy can never be destroyed or created so the the roller coaster has a constant energy total.  Therefore kinetic energy can be also transferred to potential energy.  This is why roller coasters can move without any motor pushing it.

## Sledding

Sledding, a fun activity to do over the winter, applies to Newtons laws of motion.  Newton's First Law of Motion, the Law of Inertia, states that an object will not change unless it is acted on by an outside force. This means that an object at rest will stay at rest until a force causes it to move. Additionally, an object in motion will stay in motion until a force stops it.  When you are on top of a hill, there is no force pushing on you therefore you don't move. When you go down the hill, the force of gravity is on you therefore you have some velocity.  When get to the bottom, the force of friction slows you down until you stop.  Additionally, the acceleration of the sled is affected by the frictional force of the snow to the sled.

## Wi-fi

What is wi-fi? I used it all the time, but never really knew what it was. Wi-fi uses radio waves and sends out these waves to different devices to give them the power of the internet.  They send out around 2.2GHz - 2.4 GHz, These waves have similar frequencies as mircowaves.  These waves travel through the air, undetectable by the blind eye and they send out "information" to other devices.  The reason why people lose connection to their wi-fi after traveling some distance is that waves can only travel a certain distance.  Also the reason why wi-fi can be slow is because they can be blocked by interference by other waves crossing them. Additionally, there are certain amount of waves available, so the more devices connected on the wi-fi, the slow the connection will be.

## Washing Machine

Ten months from now, I have to do my own wash, so my parents are making me practice now.  Anyways, while I was doing my wash, I realized there is physics behind the washing machine rotating the the clothes around.  I realized that someone could calculate the centripetal force of the machine.  A person would have to mass the washing machine, then a person would have to measure the radius of the machine.  Then to find the velocity of you would use kinematics.  First you know the amount of time of the washer machine because you can set it up.  Then a person could calculate the rotational distance.  Measure how long it takes for one revolution.  Then with the rotational distance, it could be translate it to translation distance, with the radius known.  Then the person could find the velocity of the machine.  With that, the centripetal force could be found with mass times velocity^2 divided by the radius.

## Cello

Cello, the best string instrument, creates beautiful music.  But how does a cello create sound?  Well, sound is produced by the vibrations of the string, and these sounds resonate inside the cello.  Cello strings are fifths (five notes apart) and each string creates their own frequency when you place your finger down on different parts of the string.  As you place your finger down the fingerboard to the bridge of the cello, it creates a higher frequency.  As you place your fingers up the fingerboard, it creates a lower frequency.  Each string has different frequencies because of the thickness/wavelength of the string.  The more thick/smaller wavelength the string is, the lower frequency the cello creates.  Additionally, cello's have harmonics on each of the four strings.  Each at a difference frequency and when you play the harmonic, it produces a loud clear sound.  When you play a harmonic, this creates a shorter wavelength which in turn produces a higher frequency sound.

## Laser Surgery

Apparently, I have a mole that has a small chance to become cancerous, so I have to undergo laser surgery to get rid of the mole.  That lead to me to think, how is it possible that a laser could remove skin off a person's body, without hurting the surrounding skin.  So I learned that LASER is an acronym the represents Light Amplification by the Stimulated Emission of Radiation. So basically, lasers emit monochromatic (single color/wavelength) light that are designed to send ultra-pulses of light energy which is used to remove moles from the skin.  The heat/energy of the laser is absorbed by the pigment of the mole, it heats the mole up, and then he mole burns away.  Lasers are produced to have specific wavelengths of light that are being absorbed by certain pigment of the mole and not affect the surround skin cells.  When the laser energy is applied for the right length of time, at the right level of energy, and in the proper wavelength, the mole on the skin is selectively targeted.  This is why laser surgery is harmless to the surround skin of the mole.

## Breaking a Board

I used to practice martial arts and one day in our dojo, my sensei decided that we should have fun by breaking boards.  Although breaking a board seems impossible for a person who doesn't do martial arts, it is quite simple if we look at the physics of breaking a board.  For example, a person needs to generate a certain velocity to impact the board at a certain position.  A person should make the impact/position of where your hands end, up past the board.  Otherwise your hand will have a tendency to slow down when it reaches the actual position of the board.  Additionally, a person should position their strike on the board where the board it weakest.  For example if you punch the center of the board, it is harder to break it because that's where it is the strongest.  On the other hand, if you punch a board at the near an end of the board, it will break more easily.  It can still be very hard to break some boards because of how the thickness of the board.  As the board increases in thickness, you velocity of the strike has to be fast and more precise on the board.

## Hockey Shots

Shooting a puck, wrist or slap shot, requires a player, using their stick to apply a force greater than the frictional forces(very little, due to ice being relatively smooth) resisting the puck's movement.  Players have the ability to generate lift because all stick blades have a certain "tilt" angle.(the face of the blade is turned slightly upwards).  During the shot, the puck slides along the face of the blade and it is the tilt which allows it to be lifted off the ice surface.  Players who generate high speed velocity of their slap shot, has a large force and time of impact of the blade and hockey puck.  Players wind up for a slap shot, to generate a large force and then hit the hockey puck in a certain time.  Force * time = impulse = change of mass * velocity.  Therefore a large impulse equals a pretty large velocity.  Players who attempt wrist shots, have a less wind up because they want more accuracy then power.  The less force the player has on the hockey puck from the blade of the stick, means less impulse.  Therefore a lower velocity of the hockey puck relative to the hockey puck hit by a slap shot.

## Firing a Gun

Recently I was watching Point of Interest, a TV show, and I was thinking about what kind of physics are behind firing a gun.  I concluded that when the shooter shoots a gun, the force on the bullet is equal to that on the gun-shooter. This is due to Newton's third law of motion (for every action, there is an equal and opposite reaction).  The force of the bullet is equal to the gun-shooter due to the law of conservation of momentum.  A person with a gun have a combined mass M and the bullet has a mass m. When the gun is fired, the two systems move away from one another with new velocities V and v respectively.  Also, the person with a gun moves in the opposite direction of the bullet.  Therefore, the initial momentum is equal to the final momentum due to the law of conservation of momentum.  Since the net force is equal to the change of momentum, the initial change of momentum of the person and gun is equal to the final bullet's momentum.  Therefore, the person with a gun has a equal force and a opposite direction of the bullet.