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askphysicslogoAccording to the theory of relativity, the length will decrease to zero.

Please refer to ther following links for more information on the question.

  1. http://en.wikipedia.org/wiki/Length_contraction
  2. http://io9.com/5527521/what-happens-if-youre-traveling-at-the-speed-of-light-and-turn-on-your-headlights
  3. http://wiki.answers.com/Q/What_happens_when_you_travel_at_the_speed_of_light
  4. http://www.physics.org/interact/time-twins/special-relativity.html
  5. http://www.physlink.com/education/askexperts/ae169.cfm
Time dilation in transversal motion. The requi...
Time dilation in transversal motion. The requirement that the speed of light is constant in every inertial reference frame leads to the theory of relativity (Photo credit: Wikipedia)

Zeno’s paradox of Achilles and the tortoise

Can you please explain Zeno’s paradox Achilles and the tortoise, well i have read about it in many websites and books but i don’t completely understand it.

Asked Sudha Parimala


Achilles and the tortoise paradox is one of the four paradoxes collectively known as

Zeno’s paradoxes. 

The statement of  Achilles and the tortoise paradox goes like this:

“A fleet-of-foot Achilles is unable to catch a plodding tortoise which has been given a head start, since during the time it takes Achilles to catch up to a given position, the tortoise has moved forward some distance.”

But the above paradoxical statement is not real. It will of course make us think and many may tend to agree with that.

A detailed explanation was found at the link below


The story tells that the tortoise argued with Achilles that, if given a head start, He will never be able to overtake him. He argued on the lines and convinced the Achilles and He  agreed to the argument.

The above link also presents Thomson’s paradox (http://platonicrealms.com/encyclopedia/Zenos-Paradox-of-the-Tortoise-and-Achilles)

Consider a lamp, with a switch. Hit the switch once, it turns it on. Hit it again, it turns it off. Let us imagine there is a being with supernatural powers who likes to play with this lamp as follows. First, he turns it on. At the end of one minute, he turns it off. At the end of half a minute, he turns it on again. At the end of a quarter of a minute, he turns it off. In one eighth of a minute, he turns it on again. And so on, hitting the switch each time after waiting exactly one-half the time he waited before hitting it the last time. Applying the above discussion, it is easy to see that all these infinitely many time intervals add up to exactly two minutes.

QUESTION: At the end of two minutes, is the lamp on, or off?

ANOTHER QUESTION: Here the lamp started out being off. Would it have made any difference if it had started out being on?

Think it over!!!

What is charge basically?

It may be rather silly but still I’m really curious to know about this.  What is charge basically?  I’m tired of reading and hearing charge is an inherent property of matter,  it is that which causes electric field etc.


Asked Sudha




First of all, I would like to tell that I am interested mainly in such questions which are branded silly. But many of those silly branded questions are not so silly.


Charge is defined in intermediate Physics as below


“Electric charge is the physical property of matter that causes it to experience a force when close to other electrically charged matter. “


The concept of electric field was introduced by Faraday to explain how two charges exert force on each other without physical contact. A charged particle (and also a time varying magnetic field) produces an electric field.


English: Field of a positive point charge. Thu...



Doubt on Alpha Rays

Alpha rays are helium nuclei since they have two protons and no electrons. They are He 2+ ions. However we have learnt that helium has highest ionization energy  and an extremely large amount of energy is needed for it to form an ion. According to this, it is not possible for a dipositive cation of helium to exist, however it does . What could be the reason?



First of all Alpha particle is not a doubly ionized Helium Atom. It is emitted from the nucleus of a discintegrating atom.

The "curve of binding energy": A gra...


The energy required to peel off the two electrons is not too much as compared to the energy  involved in the alpha particle emission which is a nuclear reaction. Alpha particles are normally emitted as a part of radio active disintegrations, emitted by naturally unstable nuclei or artificially instabilized nuclei.


The ionization energy of helium is discussed in the purview  of the energy required to remove the electrons. Helium is having a stable duplet electronic configuration which makes it a stable atom. But the energy considerations in nuclear reactions are very much different and higher.


The first ionization energy of helium  is 2372.3 kJ/mol and the second ionization energy is 5250.5 kJ/mol


16 MeV = 16×1.6×10-13 J per nucleus 2.56 x 10-12 J per nucleus


which makes of the order of 1010 kJ per mol of Helium. This is evidently a huge amount compared to that of the ionization energy.


Basically there are a number of differences between chemical reactions and nuclear reactions.


1. In chemical reactions the electrons are taking part in reactions whereas in nuclear reactions the nucleons (protons and neutrons) are involved


2. In nuclear reactions the energy involved is very much large as compared to that in chemical reactions


3. The identity of the element dioes not change in chemical reactions whereas the identity is changed in most of the nuclear reactions.



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