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Showing posts from 2014

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…

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…

What is a gravitational wave?

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…

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 flexi…

How does GPS(Global Positioning System) work?

Q. How does GPS(Global Positioning System) work?

A. The Global Positioning System (GPS) is a network of about 30 satellites orbiting the Earth at an altitude of 20,000 km. The system was originally developed by the US government for military navigation but now anyone with a GPS device, be it a SatNav, mobile phone or handheld GPS unit, can receive the radio signals that the satellites broadcast.

Wherever you are on the planet, at least four GPS satellites are ‘visible’ at any time. Each one transmits information about its position and the current time at regular intervals. These signals, travelling at the speed of light, are intercepted by your GPS receiver, which calculates how far away each satellite is based on how long it took for the messages to arrive.

Once it has information on how far away at least three satellites are, your GPS receiver can pinpoint your location using a process called trilateration.


Imagine you are standing somewhere on Earth with three satellit…

Why Do We Always See the Same Side of the Moon?

Q: Why does the moon always present the same face to us? I find it impossible to believe that this could happen by chance.

A: Nope, not by chance — it’s pure physics.

For starters, the moon is not stuck in place with one side facing us. Our lunar companion rotates while it orbits Earth. It’s just that the amount of time it takes the moon to complete a revolution on its axis is the same it takes to circle our planet — about 27 days. As a result, the same lunar hemisphere always faces Earth.

How’d this come to be? In a word: gravity. The moon’s gravity slightly warps our planet’s shape and gives us tides. Likewise, Earth tugs at the moon, creating a rocky, high-tide “bulge” facing us. That bulge ended up working like a brake, slowing the moon’s spin down to the current rate, so the lunar high tide permanently faces us.

When that happened, about 4 billion years ago, the moon became “tidally locked,” and it has presented us the same visage ever since.

MIT Course 8.02 :Electricity and Magnetism( Study Material)

Course Highlights

This course features a complete set of videotaped lectures. The 36 video lectures on Electricity and Magnetism, by Professor Lewin, were recorded on the MIT campus during the Spring of 2002. Prof. Lewin is well-known at MIT and beyond for his dynamic and engaging lecture style.

Course Description
In addition to the basic concepts of Electromagnetism, a vast variety of interesting topics are covered in this course: Lightning, Pacemakers, Electric Shock Treatment, Electrocardiograms, Metal Detectors, Musical Instruments, Magnetic Levitation, Bullet Trains, Electric Motors, Radios, TV, Car Coils, Superconductivity, Aurora Borealis, Rainbows, Radio Telescopes, Interferometers, Particle Accelerators (a.k.a. Atom Smashers or Colliders), Mass Spectrometers, Red Sunsets, Blue Skies, Haloes around Sun and Moon, Color Perception, Doppler Effect, Big-Bang Cosmology.


What is Elastic Potential Energy?

Elastic Potential Energy

Any object than can be deformed (have its shaped changed) and then return to its original shape can store elastic potential energy. • We’re still talking about potential energy, since it is stored energy until the object is allowed to
bounce back.
• “Elastic” does not refer to just things like elastic bands…other materials that would be referred
to as elastic would be
• pole vaulter’s pole
• springs

You learned in Physics that Hooke’s Law is…
F = kx
F = force (N)
k = spring constant for that object (N/m)
x = amount of expansion or compression (m)
We can use this formula to figure out a formula for the energy stored in the spring.
• Remember that W = F d

• We might be tempted to just shove the formula for Hooke’s Law into this formula to get
W = kxd = kx2
, but this is wrong!
• You have to take into account that the force is not constant as the object returns to its original
shape… it’s at a maximum when it is deformed the most, and is zero when the object is no…

If scientists can't see dark matter, how do they know it exists?

Q.If scientists can't see dark matter, how do they know it exists?

Ans. Scientists calculate the mass of large objects in space by studying their motion. Astronomers examining spiral galaxies in the 1950s expected to see material in the center moving faster than on the outer edges. Instead, they found the stars in both locations traveled at the same velocity, indicating the galaxies contained more mass than could be seen. Studies of the gas within elliptical galaxies also indicated a need for more mass than found in visible objects. Clusters of galaxies would fly apart if the only mass they contained were visible to conventional astronomical measurements.

Albert Einstein showed that massive objects in the universe bend and distort light, allowing them to be used as lenses. By studying how light is distorted by galaxy clusters, astronomers have been able to create a map of dark matter in the universe.

Although dark matter makes up most of the matter of the universe, it only makes …

What is the Physics Behind Vacuum Cleaner?

When you sip soda through a straw, you are utilizing the simplest of all suction mechanisms. Sucking the soda up causes a pressure drop between the bottom of the straw and the top of the straw. With greater fluid pressure at the bottom than the top, the soda is pushed up to your mouth. ­
This is the same basic mechanism at work in a vacuum cleaner, though the execution is a bit more complicated. In this article, we'll look inside a vacuum cleaner to find out how it puts suction to work when cleaning up the dust and debris in your house. As we'll see, the standard vacuum cleaner design is exceedingly simple, but it relies on a host of physical principles to clean effectively.

It may look like a complicated machine, but the conventional vacuum cleaner is actually made up of only six essential components:

An intake port, which may include a variety of cleaning accessories
An exhaust port
An electric motor
A fan
A porous bag
A housing that contains all the other components

When y…

Kaniz e zehra asked What is Heat?

11/13/2014 10:06:13 Kaniz e zehra asked What is Heat?

Consider a very hot mug of coffee on the countertop of your kitchen. For discussion purposes, we will say that the cup of coffee has a temperature of 80°C and that the surroundings (countertop, air in the kitchen, etc.) has a temperature of 26°C. What do you suppose will happen in this situation? I suspect that you know that the cup of coffee will gradually cool down over time. At 80°C, you wouldn't dare drink the coffee. Even the coffee mug will likely be too hot to touch. But over time, both the coffee mug and the coffee will cool down. Soon it will be at a drinkable temperature. And if you resist the temptation to drink the coffee, it will eventually reach room temperature. The coffee cools from 80°C to about 26°C. So what is happening over the course of time to cause the coffee to cool down? The answer to this question can be both macroscopic and particulate in nature.

On the macroscopic level, we would say that the coffee…

Kaniz e zehra asked Two unlike capacitor is charged to a certain potential difference it is then immersed in oil what happen to it s capacitance ,charge and potential?

11/13/2014 9:49:27Kaniz e zehra asked Two unlike capacitor is charged to a certain potential difference it is then immersed in oil what happen to it s capacitance ,charge and potential?

Ans.If you mean that an air capacitor is charged to a certain potential difference it is then immersed in oil what happen to it’s a)charge b)Potential and c) capacitance
Then the answer is :The dielectric constant Єr of the oil is grater than that of air.Whan an air capacitor is immersed in oil then
a)Its charge remain constant b)PD b/w the plates decreases c)the Capacitance increase.

But if the questions inlove two capacitors then the question will be same what like :Two unlike capacitors of different potentials and charges are joined in parallel.what happens to their pD?How are their charges distributed ?Is the energy of system affected? then answer will be :i)The potential difference will remain same.(ii) The charge is distributed (iii)The energy of the system decrease

Plz, I need the solution of the problem (14.3.9 part b) Arfken, Mathematical Methods for Physicists as soon as possible. I need the solution with details, because i tried hardly to solve it,but i could NOT. regards

What is Voltage?

Voltage is electric potential energy per unit charge, measured in joules per coulomb ( = volts). It is often referred to as "electric potential", which then must be distinguished from electric potential energy by noting that the "potential" is a "per-unit-charge" quantity. Like mechanical potential energy, the zero of potential can be chosen at any point, so the difference in voltage is the quantity which is physically meaningful. The difference in voltage measured when moving from point A to point B is equal to the work which would have to be done, per unit charge, against the electric field to move the charge from A to B.

What is the Physics Behind Walking on Broken Glass?

For this demonstration the glass bottles should first be soaked in water to remove any paper labels. An alternative is to use Mason jars. It is best to use fairly large bottles so that the pieces formed will have only a gentle curve to them. When breaking the bottles I place them in a canvas sack and use a hammer, being sure to wear gloves and eye protection. The glass should be broken into fairly small pieces. The bed for the glass may be made from half-inch-thick plywood framed by pieces of 2"34” wood. Once the glass has been poured into the bed it should be spread out to a uniform depth. Any piece that has a right-angled bend in it, where the sidewall of the bottle meets the base, is moved to the edges of the bed so that only relatively flat pieces of glass are included in the center of the bed where the walking takes place. As an extra precaution, I cover the glass with a cloth and then use a large cast iron skillet to pound the surface firmly. This ensures no points of glas…

What is proof of Kepler law of planetary motion that all planet has elliptical orbit? And why they have elliptical orbit instead of a circular orbit?

The orbit of a planet is an ellipse where one focus of the ellipse is the sun.
An ellipse is defined by two focii and all points for which the sum of the distances are the same. The semimajor axis (a) is the long distance from the center to edge of the ellipse. If r1 and r2 are the distances from the focii to any point on the ellipse then r1 + r2 = 2a. The short axis is called the semiminor axis.

How “elliptical” an orbit is can be described by the eccentricity(e). The eccentricity is equal to the distance between a focus and the center (c) of the ellipses divided by the semimajor axis (a). That is, e = c/a. 
See the elementary proof: View

What multiple of distance between the centre of the earth and that of the moon is the distance between the centre of the earth and the geostationary satelite which always stays above the fixed location on the equator ? take the cycle of revolution of the moomn around the earth to be 27 days ?

According to Kepler’s third law of planetary motion T2=R3 ( For geo stationary satellite)-------1 T’2=R’3( For Moon)----------------------------2 Dividing eq(2) by eq(1), we get T’2 / T2=R’3/ R3 Since T’=27 T Then we can write as 27 T2 /T2= R’3/ R3 Or R’3=27 R3 Or
R’=3 R or R= 1/3 R' so the distance between the centre of the earth and the geostationary satellite which always stays above the fixed location on the equator is 1/3 times distance between the centre of the earth and that of the moon .

Where can I see the aurora?

Auroras usually occur in ring-shaped areas centered around the magnetic poles of Earth. The complete rings, called the auroral ovals, can only be seen from space. False color picture of the auroral oval in ultraviolet light. The brighter the color, the more intense the aurora.  The crescent of color on the left is from sunlight
scattered over the upper atmosphere. The best places to see the aurora are in Alaska, Canada, and Scandinavia, during the late evening hours.  Resident of the northernmost United States – near the Canadian border – typically see auroras several times a year.  On rare occasions – perhaps once per decade – auroras are visible as far south as Florida or Japan.

Why does aurora take different shapes?

Scientists are still trying to answer this question.  The shape of the aurora depends on where in the magnetosphere the electrons came from and on what caused them to precipitate into the atmosphere. Dramatically different auroral shapes can be seen in a single night.

Why the different colors formed in aurora?

The color of the aurora depends on which gas is being excited by the electrons and on how much energy is being exchanged. Oxygen emits either a greenish-yellow light  (the most familiar color of the aurora) or a red light; nitrogen generally gives off a blue light.  The oxygen and nitrogen molecules also emit ultraviolet light, which can only be detected by
special cameras on satellites

What causes the aurora?

The “northern lights” are caused by collisions between fast-moving particles (electrons) from space and the oxygen and nitrogen gas in our atmosphere.These electrons originate in the magnetosphere, the region of space controlled by Earth’s magnetic field.As they rain into the atmosphere, the electrons impart energy to oxygen and nitrogen molecules, making them excited. When the molecules return to their normal state, they release photons, small bursts of energy in the form of light.
When billions of these collisions occur and enough photons are released, the oxygen and nitrogen in the atmosphere emit enough light for the eye to detect them.  This ghostly glow can light up the night sky in a dance of colors.  But since the aurora is much dimmer than sunlight, it cannot be seen from the ground in the daytime.

Young's Double Slit Experiment

This is a classic example of interference effects in light waves. Two light rays pass through two slits, separated by a distance d and strike a screen a distance, L , from the slits, as in Fig. 22.10.

Figure 22.10: Double slit diffraction

If d < < L then the difference in path length r1 - r2 travelled by the two rays is approximately:

r1 - r2 dsin

where is approximately equal to the angle that the rays make relative to a perpendicular line joining the slits to the screen.

If the rays were in phase when they passed through the slits, then the condition for constructive interference at the screen is:

dsin = m ,m = 1, 2,...

whereas the condition for destructive interference at the screen is:

dsin = (m + ) ,m = 1, 2,...

The points of constructive interference will appear as bright bands on the screen and the points of destructive interference will appear as dark bands. These dark and bright spots are called interference fringes. Note:
In the case that y , the distance from the inte…

Common Emitter Transistor Amplifier

The larger collector current IC is proportional to the base current IB according to the relationship IC =βIB , or more precisely it is proportional to the base-emitter voltage VBE . The smaller base current controls the larger collector current, achieving current amplification.
The analogy to a valve is sometimes helpful. The smaller current in the base acts as a "valve", controlling the larger current from collector to emitter. A "signal" in the form of a variation in the base current is reproduced as a larger variation in the collector-to-emitter current, achieving an amplification of that signal.

The larger collector current IC is proportional to the base current IB according to the relationship IC =βIB , or more precisely it is proportional to the base-emitter voltage VBE . The smaller base current controls the larger collector current, achieving current amplification.

What is a mirage? How is it formed?

A traveller has lost his way in the desert. Enduring thirst and hunger, he suddenly saw an oasis, so the overjoyed man quickly ran towards it. To his great disappointment, it was just an illusion produced by a mirage. Such an episode was often pictured in movies, yet the optical magic that the nature plays with us - mirage - really exists in reality. Its formation is a result of the refraction and the total internal reflection of light in the air.

To investigate the formation of a mirage, we firstly need to understand why light is refracted in the air. Regions of air at different temperatures have different refractive indexes, just like many different mediums. The closer the air is to the ground, the hotter it will be, and its refractive index will be smaller. We could imagine the air as many layers of medium with a particular refractive index for every layer, and the refractive index is smaller for those that are closer to the ground. Thus when light travels in air, its path is as …

Electromagnetic Induction:C.R.Q's /Questions

15.1 Does the induced emf in a circuit depend on the resistance of the circuit?does the induced current depend on the resistance of the circuit?
Ans. The Induced emf in a coil depends upon the rate of change of flux through it (E=-Nt) .Hence its value does not depend upon the resistance of the coil.But the induced current that flows through a coil is equal to I=E/R and it’s value depends on the resistance of the coil.If , resistance increases then the current flowing through the coil decreases.Because the product of I and R must remains constant.i.e. I x R = Constant.
15.2 A square loop of wire i moving through a uniform magnetic field.The normal to the loop is oriented parallel to the magnetic field.Is emf induced in th loop?Give a reason for your answer.
Ans.No, induced emf will not be produced because there is no change of flux linking to the loop.i.e.t=0 .So according to the relation (E=-Nt) ,E=0.If the square loop is being rotated in magnetic field in such a way that the loop is cut…