first law of thermodynamic

The Forst Law of Thermodynamics is always true. But its more applicable on thermodynamic systems.

1. Free falling:
Air friction between the falling object and surrounding air produces heat dQ, which increases the internal energy of the object, dQ = dU (assume no hest lost to the surroundings)

2. Compress an elastic spring:
Rapid compression of a spring does produce heat. The heat thus be absorbed by the the spring as internal energy.

3. Hammering a metal:
Hammering produces heat. The heat produces will increase the internal energy of the metal.