I have a difficulty in understanding this 'mass never changes'. Say, i punch/hit a ball until it deflates, or bursting a balloon with a needle – doesn't the mass of the ball/balloon change? the air already diffuses out from the ball.
[Physics] Can force change the mass of an object
forcesmassnewtonian-mechanics
Best Answer
Let's look at your system: a needle, a balloon skin, and a gas inside the skin. There is some total mass in this system.
Force of the needle pressed against the skin causes a stress which exceeds the strength of the skin material, making a hole. The air escapes through the hole after the force is gone, but the mass of the air doesn't change. The mass of the needle and the mass of the balloon skin don't change.
Force didn't make the mass of the system change. The mass of the system is simply redistributed. This would be the same as pulling apart a lump of clay and making two smaller lumps: the total mass is the same, but has been redistributed.