If there is no change in mass while doing work or heat transfer, the energy involved is typically accounted for by changes in other forms of energy, such as potential energy, kinetic energy, or internal energy.
Let's break it down:
Work: Work is the transfer of energy that occurs when a force is applied to an object, causing it to move. If there is no change in mass during the work process, the energy transferred is usually manifested as a change in kinetic energy or potential energy. For example, if you push a stationary object horizontally and there is no change in mass, the energy you exert is converted into the kinetic energy of the object as it starts moving.
Heat transfer: Heat is a form of energy that is transferred between objects due to a temperature difference. If there is no change in mass during heat transfer, the energy involved can lead to a change in the internal energy of the system. Internal energy includes various forms such as the kinetic energy of molecules (associated with their motion) and potential energy (associated with intermolecular forces). The energy transferred as heat can cause an increase or decrease in the internal energy of the system.
In both cases, the conservation of energy principle applies. Energy is neither created nor destroyed, but it can change form or be transferred from one object or system to another. So, if there is no change in mass while doing work or heat transfer, the energy involved will be accounted for as a change in another form of energy within the system.