Internal Energy

Today, we investigated internal energy. Internal energy is the sum of the kinetic and potential energy of particles in a substance. In an ideal gas, the particles move in a random, constant and straight motion. They can also move in translational, rotational and vibrational mode. As they are moving, each particle contains kinetic energy. Potential energy is a force that is like string between particles. It stores when 2 particles have been pulled apart or pushed together.

It is interesting to know how a substance does not contain heat but internal energy. However, what is the difference between internal energy and mechanical energy? Don't they both represent the sum of energies in a substance? How does temperature affect internal energy in a system?