Imagine that you drop a brick on the ground and it lands with a thud. Apparently the energy of this system tends to spontaneously decrease. Explain why.

A brick is dropped to the ground and it lands with a thud.

Energy of this system tends to spontaneously decrease,

Helmholtz free energy can be determined by
F=U-T S
Where, F= Helmholtz free energy, U=Internal energy,T=absolute temperature of the system and S=entropy of the system.

The total energy of the brick = kinetic energy + potential energy

From the law of energy conservation this is always constant.

When the brick hits the ground, its kinetic energy is zero, but the potential energy remains the same. The kinetic energy is redistributed into the thermal energy of the molecules of the brick and to the ground where the brick hits.

So a part of the energy is transferred from the brick to the ground.

Therefore the energy of the system tends to spontaneously decrease.