6
$\begingroup$

What causes two objects sliding against each other to produce heat? Why don't they generate visible light or something else?

$\endgroup$
6
$\begingroup$

They produce heat because the surfaces on small scales are rough like canyons rather than flat like the ocean. As these rough surfaces come into contact with each other they repel. When two atoms are brought very close together they store potential energy. When they move apart that energy becomes kinetic. However, this kinetic energy generally isn't enough to escape the object they are attached to so the energy becomes randomly distributed as kinetic energy exchanged between the atoms of the object also known as thermal energy. In some cases the energy is enough to break bonds and indeed friction can cause objects to deteriorate and fall apart as in a meteor falling through the sky being torn apart by atmospheric friction.

As for visible light, if you've ever seen a meteor shower then you know that they do produce a lot of visible light due to friction with the atmosphere.

$\endgroup$
  • 2
    $\begingroup$ I feel it may be nice to elaborate a bit on the last paragraph, adding some info about energy scales associated to certain frequency ranges of the EM spectrum, and why this explains the comet thing differing from your typical friction. $\endgroup$ – Danu Aug 14 '15 at 22:45
0
$\begingroup$

Molecules are undergoing elastic collisions. For any given substance, the faster its molecules are colliding, the hotter it is. When an object slides along another object, the total kinetic energy plus gravitational potential of its molecules plus gravitational potential energy must be conserved. It can be proven that in Newtonian physics, for any system whose momentum doesn't change, its kinetic energy increases by the same amount in all frames of reference though not necessarily the kinetic energy of each individual object in the system. That means the total kinetic energy of each sliding object can be espressed as the sum of its kinetic energy of movement and the kinetic energy of its molecules. When they slide across each other, kinetic energy of movement is converted to kinetic energy of molecules. Since an object requires such a high speed of molecules like 300 m/s to have room temperature, not very much heat gets created for the amount of kinetic energy lost to friction. That's why it's easy to not notice that friction can produce any heat.

$\endgroup$

protected by Qmechanic May 2 '16 at 20:56

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.