Question

Physics: Choose the correct option “?”

Assertion: A shell at rest explodes. The centre of mass of fragments moves randomly in space
Reason: In explosion, total kinetic energy of system remains same

1. Both Assertion and Reason is true and reason is correct explanation of assertion
2. Both Assertion and Reason is true but reason is not the correct explanation of assertion
3. Assertion is true but Reason is false
4. Both Assertion and Reason are false

Key says 4 is the correct option. How? I think 3 should be the correct option.

Also, will the centre of mass of shell moves in parabola?

Answer 1

I agree with option 4.

Explanation:

A certain part of the question has a potential to be interpreted 2 ways. That part is "The centre of mass of fragments moves randomly ...". I think it was meant to be interpreted this way: "The centre of mass of the group of fragments moves randomly ...".

And that assertion is false. Due to the principle of Conservation of Momentum, the centre of mass of the group of fragments will possess the same momentum that the shell had before the explosion. So number 4 is the right answer.

You also asked if the centre of mass of shell moves in parabola. The question says that the shell was at rest. But if it had been shot up as a projectile, then yes -- both before and after the explosion. The centre of mass of the group of fragments will follow the same path that the shell would have followed if it had not exploded. (Ignoring the possibility that the aerodynamics might be different between the shell and its fragments.)

I hope this helps,
Steve