### Welcome to Talha's Physics Academy

To Help Teachers and Students.

Talha's Physics Academy is an exploration environment for concepts in physics which employs free Physics Books and other linking strategies to facilitate smooth navigationThe entire environment is interconnected with thousands of links, reminiscent of a neural network.

New content for Talha's Physics Academy will be posted as it is developed,It is my intent to keep this material continuously available except for brief maintenance times.

All the Branches of Physics are covered.

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### Free Kick and Physics

One footballing situation particularly stands out as relevant to physicists: the free kick.

Since the goal is typically defended with a wall of players, scoring a goal means that the attacking player must bend the ball around that wall. Doing so takes advantage of a phenomenon known as the Magnus Effect, after Gustav Magnus who investigated it in 1852.

Striking the ball off-centre gives it a spin, which changes the airflow around the ball and creates a turbulent wake. The airflow is deflected in the direction of spin, giving the ball a horizontal force and resultant motion.

The amount of curvature in the ball’s path can also increase mid-flight. This happens when the ball slows enough that the airflow around it instantaneously changes from chaotic flow  to laminar flow. The air pressure on the ball, and therefore the drag it experiences, increases, slowing it down further and heightening the influence of the Magnus Effect. (In the absence of gravity, the ball would eventually produce a spiral flightpath.)

It’s a tough skill to get right – only about a tenth of direct free kicks in the English top flight find the back of the net. The structure of the ball and atmospheric disturbances within the stadium can have an effect.

But ultimately the amount of curvature produced mainly depends on the coefficient of friction between ball and boot, how far off-centre it’s struck, and its speed. So to bend it like Beckham, kick a dry ball at an angle – and belt it hard.

### Weird Rainbows

Upside down rainbows

Upside down rainbows, or ‘circumzenithal arcs’, to give them the proper name, are not caused by rain. Normal rainbows form when light refracts through raindrops, mist, or sometimes even sea spray. The upside down kind however, are caused by ice crystals in the air. They are more common in cold climates, but still fairly rare.

Double rainbows

Double rainbows occur when the sunlight is reflected twice inside the raindrops. The second rainbow usually sits outside the first, and looks dimmer and more blurry than the original. Because of the angle of reflection, the second rainbow appears with the opposite colour scheme to the first.

Supernumerary rainbows

It sounds complicated, but really a supernumerary rainbow is one with smaller repeating rainbows inside it. The smaller rainbows tend not to have the same colour patterns as a normal rainbow, and the colours are lighter.

### How does an ice pyramid work?

Ice Spikes
If you go out one morning, just as the temperature has really started to drop, you might be forgiven for thinking that tiny aliens landed in your birdbath during the night. Birdbaths, as still pools of water that are left out in freezing conditions, are the most likely place to see inverted ice pyramids.

Regular ice spikes form because, at just the right temperature, the sides and top of a body of water - usually water in an ice cube tray - freeze first. As they freeze they expand, putting pressure on the water in the middle. If there is a tiny hole in the ice forming at the surface of the still-hardening cube, the liquid water is pushed upwards. The water pushed up through the hole forms a little frozen mound on the top of the ice cube. This little mound also has a hole in its center, through which more water is pushed, and the whole thing builds up into a spike.

Ice pyramids form through a variation on the process.The water doesn't freeze continuously, moving from the sides of the container to the middle. It freezes in what can best be described as "sheets." These sheets hang down vertically from the surface of the water. Sometimes they can be parallel to each other, as if someone were taking orderly slices from the water. Other times they can form at all angles to each other.

Inverted pyramids are formed when these sheets are at just the right angle to each other. Essentially, they form the shape, or the mold, for the pyramid under the water. In the meantime, the surface of the water freezes in roughly the shape of the pyramid base. At that point, the only way for the pyramid to go is up and out of the water. The only important part is that the "tip" of the pyramid doesn't freeze over, so more water can be forced into the pyramid "mold." As they ice sheets keep freezing and expanding, the pyramid is pushed up and up. Because the sheets are freezing, the underwater mold is getting smaller, the pyramid eventually tapers off to a point.

Eventually, the entire thing is frozen in place, waiting for people to be astonished by it the next morning. If you have a pond, a bird bath, or any small body of water that can freeze over, keep an eye out for an ice spike. Under the right conditions, the "spikes" can be pyramids, "cubes," or even vases.

## What is a gravitational wave?

A gravitational wave* is a concept predicted by Einstein's theory of general relativity. General relativity states that mass distorts both space and time in the same way a heavy bowling ball will distort a trampoline.

When an object accelerates, it creates ripples in space-time, just like a boat causes ripples in a pond (and also similarly an accelerating electrical charge produces an electromagnetic wave). These space-time ripples are gravitational waves. They are extremely weak so are very difficult to detect. Missions like LISA or LIGO hope to spot gravitation waves detecting small changes in the distances between objects at set distances; satellites for LISA and mirrors for LIGO. As the strength of the wave depends on the mass of the object our best hope of detecting gravitational waves comes from detecting two black holes or pulsars collapsing into each other.

Gravitational waves have been inferred from watching two pulsars spinning and noticing they are slowing down, due to losing energy from emitting gravitational waves.

Gravitational waves are important in telling us about the early universe. The cosmic microwave background gives us a snapshot of the universe about 380,000 years after the start of the universe. Looking very closely at the cosmic microwave background there are patterns seen which can are also be measured in the large scale structure of the universe (so galaxies and clusters) today. These patterns in the cosmic microwave background were caused by very tiny random perturbations from the time when the universe expanded rapidly, known as inflation.

Inflation should also generate gravitational waves. These waves affect the polarization (the way the wave oscillates) of the cosmic microwave background. Measuring the strength of the polarization due to gravitational waves gives us a ballpark figure of the amount of energy involved at the time of inflation and helps pin down when inflation occurred.

*Not to be confused with a gravity wave (which is a wave driven by the force of gravity).

### How do speakers work?

Q.How do speakers work?

A. Speakers come in all shapes and sizes, enabling you to listen to music on your iPod, enjoy a film at the cinema or hear a friend’s voice over the phone.

In order to translate an electrical signal into an audible sound, speakers contain an electromagnet: a metal coil which creates a magnetic field when an electric current flows through it. This coil behaves much like a normal (permanent) magnet, with one particularly handy property: reversing the direction of the current in the coil flips the poles of the magnet.

Inside a speaker, an electromagnet is placed in front of a permanent magnet. The permanent magnet is fixed firmly into position whereas the electromagnet is mobile. As pulses of electricity pass through the coil of the electromagnet, the direction of its magnetic field is rapidly changed. This means that it is in turn attracted to and repelled from the permanent magnet, vibrating back and forth.

The electromagnet is attached to a cone made of a flexible material such as paper or plastic which amplifies these vibrations, pumping sound waves into the surrounding air and towards your ears.

Inside a speaker:
1. Cone
2. Electromagnet (coil)
3. Permanent magnet

The frequency of the vibrations governs the pitch of the sound produced, and their amplitude affects the volume – turn your stereo up high enough and you might even be able to see the diaphragm covering the cone move.

To reproduce all the different frequencies of sound in a piece of music faithfully, top quality speakers typically use different sized cones dedicated to high, medium and low frequencies.

A microphone uses the same mechanism as a speaker in reverse to convert sound into an electrical signal. In fact, you can even use a pair of headphones as a microphone!