Imagine there are only three bodies in the universe
1> A gun
2> A bullet already fired
3> A wooden block which is target of the bullet and is stationary wrt gun before impact with the gun

All the three observers associated(stationary) with three bodies will see impact then they should have energy balance before and after impact. How?

NOTE: the wooden block will see the gun moving after impact the velocity of which depends on mass and velocity of bullet and wooden block. But the mass of gun can be varied (say in two diff problems) irrespectively hence energy balance is not possible??

You'll have to explain your question a little better.

Well, nothing happens to the gun before and after impact, so the gun is not part of the system.
The bullet and the wood when they collide obey conservation of momentum. Most of the kinetic energy of the bullet will go into the new bullet-wood object. Some energy will go into heat and vibration (sound).

I don't understand where you are having a problem. Energy is balanced in every case.

Dear Capuchin I will elaborate my problem::


While applying Newton's laws only inertial frame of references are permissible and in all these inertial (with no accelaration: def) energy balance is no problem as U have answered correctly

NOW, going beyond newtons premises
If we say 1) every motion is relative and no frame of reference is special to other
2) law of physics are same in all frame of references

Then there is no question of qualifying inertial frame of references better over other accelerating frames then can we apply energy balance there too??

Secondly what the wooden block observe:: A bullet came and hit it, there is a rise in temperature AND the gun starts moving then if statement 2 is correct what is the law of physics which makes the gun move?

You're no longer in an inertial reference frame, this means that Newton's first and second laws no longer apply.

I alredy said "going beyond newton's premises"
I am talking about relativity:cool: