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Reaction problem

Asked Alvy
4mg as there are invited4 coins above the 5th coin from bottom.

Newton’s Third Law of Motion and some doubts

Newton’s Third Law of Motion states that

“Every action has equal and opposite reaction”

In other words,

“Whenever a body exerts a force on another body, the second body exerts an equal and opposite force on the first body”

 Osho Garg asks:

“if a person is sitting on a chair both the chair and person exert force against it so we called that ” there is always an equal and opposite reaction ” but for example a elephant of mass 584 kg sit on a chair, the chair will broke.  So what we called for this . Please explain.



When an elephant “sits” on a chair, it breaks because it cannot give the “equal reaction” and  it “yields” to it.

Therefore, here the force is doing “work”.

More discussions are welcome

Usage of energy in nuclear fission and fusion

I read in a biographic book about Albert Einstein that the energy produced from burning materials is much less than the energy stored in them (the authored suggested the book itself as an example – he wrote that it can power a ship for about 100 years).
I would like to know how much of the energy of a material is used in nuclear fission and fusion.

Asked Avishag


Nuclear explosion
Nuclear Explosion (Source: WIkiPedia)

Energy obtained by burning a substance is meagre compared to the energy contained in it. According to Einstein’s mass energy relation (E=mc2), the energy released by converting 1 gram of any substance completely into energy = 0.001 (mass) x 300,000,000 x 3,00,000,000 (square of velocity of light).

But, in nuclear reactions, the entire mass is not converted into energy. The tremendous energy liberated during a nuclear explosion is the result of a small portion of the mass of nucleus undergoing the reaction getting converted into energy.

For example, when a uranium 235 atom undergoes nuclear fission the enrgy liberated is 200 MeV = 200 x 1.6 x 10 -19 J

In 235 g of U-235 there are 6 x 1023 atoms.

So if that much of U-235 atoms undergo fission, the energy liberated will be about 1,00,00,000 J.

Hope that you might have had an idea of it.

Another problem from Kinematics

Shibu posted:

“a car is running at a speed “u”. Seeing a child on
the road, the driver applies the break so as to
bring the car to a halt within a distance S. Show
that hte reaction time of the driver is 2S/u”

How to tackle problems with constraint relations?

“Can you suggest a way to tackle problems on constraint relations easily??” – Aritra posted this question.

Answer: The common way to solve problems involving constraints is to replace the constraints with their reaction forces. As the question is not specific, I cannot tell much now. If you have further doubts regarding this issue, please post as comments to this post.

Atom Bomb, Hydrogen Bomb, Nuclear Bomb

Charan asked:

what is the difference between a atomic and a nuclear bomb?


image Any bomb depending on nuclear reactions for its operation is a nuclear bomb.

Atom Bomb depends on Nuclear fusion reactions (Uncontrolled Chain reaction)

Hydrogen Bomb is a nuclear fusion Bomb, and requires an atom bomb explosion first to generate the necessary temperature for it to start working.

Both Atom Bomb and Hydrogen Bomb are nuclear bombs.

Formation of Heavier Elements

H. Manishankar (KV Pattom) Asked

Question: Iron is the heaviest element that can be formed by nuclear reactions in a star.So how the heavier elements after iron were formed?And could there be further heavier elements out there in the cosmos?


Nuclear fusion inside a star is not the only process my which new elements are created. There are many others too.

During a supernova explosion many unpredictable nuclear transmutations occur, fusing elements and forming heavier elements.

But however, the amount of elements heavier than iron is less.

It’s a good idea to think back (say from the time of BigBang)

Every explosion has a consequent implosion resulting in creation of heavier elements

Remember that birth and death of stars are taking place everyday in some part or other of the universe. The earlier stars had relatively lighter elements in their core. As they die, the remains form part of other stars and other celestial bodies thereby forming heavier elements in the universe.

(I don’t claim that this is a complete answer. Visitors are requested to suggest amendments to this Answer via comments)

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