# Doubt from WEP

why is option D incorrect please explain .

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It is right. But itâ€™s not a major factor. People will elongate the barrel not solely on this idea. Thatâ€™s just my opinion.

I mean to really see a difference in range ( considering that velocity of launch is same) changing the barrel length by a little amount wonâ€™t be much feasible.

And this velocity of launch will be changed by force due to expansion of gases. When we elongate the barrel.

So you can see actually what causes the difference

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Uhh is it B?

D seems to be irrelevant to the overall scenario here. I am confused about how to explain why it is irrelevant but I will try:

The position of the gun barrel and the exit point of the shell fired, has a negligible effect on the trajectory in comparison to various other factors such as the angle of projection, and the velocity of the shell. For practical purposes, you can go into ground frame and you will see that the initial kinetic energy dwarfs the initial potential energy.

A longer gun barrel allows the expanding gases to push outwards on the shell for a longer period of time and thus the kinetic energy imparted to it increases with the length of the barrel. @Adarsh

C option is confusing, again. What say @Rudra_d

The force should be comparable naa? Only the distance across which the force is applied will change

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The total KE provided to bullet should depend on the amount of expansion. So the force wonâ€™t increase on elongating the barrel. Instead the total energy delivered over time will increase on elongating the barrel.

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Yes that is exactly what I had in mind too although I framed it a bit differently.

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https://webpath.med.utah.edu/TUTORIAL/GUNS/GUNBLST.html
Might be of some help the force increases up to a point and then decreases as explained through a graph on this gun site

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That makes perfect sense too. Fuel gotta burn up completely before fumes exert max pressure. Nice

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Should the answer be B? I know C is wrong, D seems right but I donâ€™t know why they are considering the ratio of KE and PE. A is justâ€¦unrelated.

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yes answer should be only B

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the only conversion of potential to kinetic energy here is burning of fuel and the kinetic energy gained by the liberated gas. Its not depended on barrel at all. only the kinetic energy of gas molecule is being transferred to the bullet here.

Also, option B is correct cuz it gives the reason WHY this happens, where as option D doesnâ€™t states the reasoning, which will again lead to the same question.

We have to choose the best answer in mcq, even if option D were to be correct , it would not be the most accurate answer the way it is.

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