Question

For each of the scenarios below, determine how the total mass of the described object or system is changing, and say whether the scenario Increases the Mass, Decreases the Mass, or that the Mass Remains Unchanged:

A hot stone cooling down

A closed, thermally isolated box containing a controlled exothermic nuclearreaction

A closed, thermally isolated box containing a controlled exothermic chemicalreaction

A glass full of ice cubes melting

A compressed spring slowly relaxing

A car batter being charged

Answer 1

1) hot stone cooling down.

Density = Mass / volume.

While cooling down it's volume decreases so that density increases and mass kept constant. So while cooling down mass is not changed in hot stone.

In nuclear exothermic reaction (loss of mass) energy is released in the form of heat and radiation. The starting mass will not be equal to the fission products i.e one atom is converted into another.

Main thing in chemical reaction is mass conservation. No atoms are created and destroyed in chemical reactions like nuclear reaction only reshuffling of atoms takes place in the reaction.

While a glass of full ice cubes melts, phase change takes place only. Again the mass of the system is not changed.

In newtonian mechanics the energy of the system doesn't change the mass. So there is a change in energy of the spring while compressing and expanding but the mass is constant.

In a car battery chemical reactions are takes place while charging. No mass is inserted into the system. so the mass of the system is not changed.