Q.Explain Joule's experiment of the mechanical equivalent of heat.

Step 1. In Joule's classic experiment (Figure 8.19), two masses $m$ are attached by a rope, run over pulleys, to a paddle wheel submerged in an insulated container of water; a thermometer measures the water's temperature.

Step 2. As the two masses fall through a measured height $h$ under gravity, they together lose gravitational potential energy $2mgh$; this falling motion turns the paddle wheel inside the water.

Step 3. Friction between the rotating paddle wheel and the surrounding water converts the wheel's rotational kinetic energy into the water's internal energy, which is observed as a measurable RISE in the water's temperature, read off the thermometer. …

Trace the energy chain: gravitational PE → rotational KE of the paddle wheel → internal energy of …

• Calling the result 'the mechanical equivalent of heat' as though heat were literally being produced as a storable substance -- what is really mea …