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Mechanical energy can be converted into heat, and heat can be converted into some mechanical energy. This important physical observation is known as the mechanical equivalent of heat. This means one can change the internal energy of a system by either doing work to the system, or adding heat to the system

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Heat engines convert heat energy into mechanical energy. Thermal energy of the exhaust gases converted into thermal energy of stem through the heat exchanger. Thermal energy of steam is converted into mechanical energy in the turbine.

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What is the Mechanical equivalent of heat??
In the history of science, the mechanical equivalent of heat states that motion and heat are mutually interchangeable and that in every case, a given amount of work would generate the same amount of heat, provided the work done is totally converted to heat energy.

Who discovered it??
In June 1849, James Prescott Joule discovered the Mechanical Equivalent of Heat.

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Suppose, if we rub our palms they become warm. Due to rubbing our palms, the mechanical energy is converted into heat energy. Likewise, when two stones are rubbed sparks are produces.

When a workman rotates the sharpening wheel with his leg and places the edge of a knife or scissor on the wheel to sharpen. We see sparks which are produced due to friction.

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More examples in daily life are:

(i) When wood or coal is burnt in the hearth, heat is produced first and then light. Both heat and light energies are so closely related that the presence of one calls in the other.

(ii) When iron is heated, its colour changes from black to red and thus the heated iron beings to produce light. We know things become warm or even hot due to sunshine.