# Internal Kinetic Energy

by tonyjk
Tags: energy, internal, kinetic
 P: 95 Hello, Please i have a question about the internal energy: The Internal Kinetic energy is the motion of molecules and it's a random motion. But at macroscopic level the whole object formed by those molecules move in a same direction? how it can be? In a flowing fluid not static? thank you
Thanks
P: 2,951
 Quote by tonyjk Hello, The Internal Kinetic energy is the motion of molecules and it's a random motion. But at macroscopic level the whole object formed by those molecules move in a same direction? how it can be? In a flowing fluid not static? thank you
In a solid object, the molecular movement isn't completely random; it's more as if the atoms are vibrating back and forth while keeping more or less the same general position relative to one another. If you want an analogy, imagine a bus full of unruly small children - the children may be in continuous random motion, but the ensemble of bus and brats is still moving steadily down the road.

Even in a gas or a liquid, it is possible that all the molecules have the same average velocity over time, even though at any given moment many of them will have small random deviations from that average.
 P: 95 Thank you. So when we say that the internal energy is related to the microscopic kinetic energy what is the velocity taken here?
Thanks
P: 2,951

## Internal Kinetic Energy

 Quote by tonyjk Thank you. So when we say that the internal energy is related to the microscopic kinetic energy what is the velocity taken here?
Imagine an observer moving at the same speed as the object; as far as that observer is concerned, the object is at rest, the time-averaged speed of each molecule in the object is zero, and the object has zero kinetic energy. Now the random molecular movement is the only movement, so all of the kinetic energy from all of that movement is counted as internal energy.
 Mentor P: 11,230 Another way to put it is to imagine an observer from whose point of view the total momentum of the molecules is zero. That is, the molecules are flying around in random directions so that their momenta cancel out completely when you add them together. According to that observer, the total KE is the "internal KE."