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Here is a system which is travelling left to right at the speed of 0.5c (half of the speed of light). The system is made of two parts. Both the parts are made of a light source and a light receiver or detector; but both are placed opposite to each other (that means the receivers are at the ends of the system).

Then the both sources emit light at the same time and the receivers receive the pulse. According to Maxwell’s equations, the speed of the pulse of light will not be affected by the speed of its source (and the system). We know that the speed of light is same for all observers. So, the receivers will receive the light at the same time. But here rises a problem: how relativity will explain this phenomenon (that the speed of light is constant here too). Here, we should remember that the system is travelling at 0.5c and we can’t use the length contraction and time delay. Why? It is because:
1.      If the system is affected with length contraction and time delay, both the parts will be affected not a single part. So, if we try to explain how the speed of light become constant in the right part with length contraction and time delay, we shall be unable to explain how the speed of light become constant in the left part with the same length contraction and time delay!
2.      If we suppose that the receivers received those pulses at different time, it will become clear that the speed of light is not same and constant for all observers and the theory of relativity will break down!
Am I right? Please reply.

Ritu Lahkar

Adieu !! Stephen Hawking …..

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Stephen Hawking, the world renowned Physicist, the inspiration to millions, who outlived doctors’ predictions after suffering from progressive motor neuron disease  and rised to the most coveted chair in Physics…The author of “A Brief History of Time” and many such inspiring books …. finally passed away at the age of 76.


AskPhysics family expresses its deepest condolences

You could barely move your limbs and body, but you moved our hearts and minds and compelled us to think deep and wide pondering the mysteries in connection with the evolution of universe and the future.

Adieu !! Hawking… Your discoveries, theories and books will continue to inspire us!

Randomness and The Universe

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Is anything truly random? Is random a human made idea to describe something that is hard to predict? Or is it possible for something to actually be random?

If nothing is truly random then if the universe was created again, would everything turn out the same? I don’t see why it wouldn’t but im not finding much about this online and i want to know what you think.

Thank You


Bravais Lattices/Particle Physiscs

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I have a few exercices (two) on Bravais lattices and I can’t figure it out about the best way to approach them. A few tips on the steps, or theories that I should base my resolution on would be helpful.

I also have another 2 exercices that approach particles collision times and the kinetic theory of gases.

By the way, the exercises I mention are attached.

If there is any helpull suggestion that allows me to solve this problems, I would be very happy.




Download (PDF, 125KB)

How was CBSE Physics class XII exam?

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When asked students there was mixed reactions on this years CBSE Physics exam for class 12. What is your opinion?

How was the CBSE Physics exam ?

Gravitation – Why acceleration due to gravity increases at pole instead of decreasing?

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Acceleration due to gravity is found to decrease with increase in depth and vice versa. But, on the poles its value is more than as compared to that on the equator even though the depth is increasing.Why is this so? I know about the relation g is inversely proportional to R squared, but without this relation I can’t seem to b able to answer it with the depth relation.

Asked Thakrei Ruivah


The force of gravity is inversely proportional to the the distance from the centre of earth and hence it is evident that at poles the acceleration due to gravity should be more since the polar radius is less that the equatorial radius.
Putting it simply, the entire mass of earth is attracting the object kept at the pole towards it centre and the distance from the centre is less. Therefore the force and hence the acceleration due to gravity will be maximum.

The decrease in g with depth is due to the fact that:
At any depth, the mass of earth coming within the sphere with radius equal to the distance from the centre of earth to the object under consideration will be responsible for the force of gravity and hence the value of acceleration due to gravity decreases.

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Inelastic vs Elastic collisions

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In class we we’re working on a lab, We had to shoot a stationary toy truck with a dart pulled back and giving it’s energy through a spring. We are given the mass of the dart and the truck, and found the amount of time the truck travelled for and its distance. I’m unsure which equation to start with in order to find the darts initial velocity.




This is based on Law of conservation of energy


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