How Does Friction Generate Heat?

[jon c]

Hi

My son (and now me too) want to know how friction develops heat.

The only explanation I can think of (and that makes sense to me) is that during the rubbing molecular bonds are broken completely, or then remade at lower force levels, then the surplus energy is then transferred into heat and so causes changes of temperature.
Is this the current accepted theory in Physics? I want no maths in any answers only the underlying physical concepts etc. The maths we can do but please focus only on the physical conceptualizations of what is going.

thanks

(He then asked me if different materials develop different amounts of heat for the same force required to rub them together. I will post this as a separate thread)

 

[zhermes]

The only explanation I can think of (and that makes sense to me) is that during the rubbing molecular bonds are broken completely, or then remade at lower force levels, then the surplus energy is then transferred into heat and so causes changes of temperature.

Some molecular bonds (etc) are likely broken in the process, but this isn't especially important for the change in temperature. Friction occurs when there is a difference in velocity of two contacting surfaces. When this occurs, collisions occur between the constituent particles transferring energy from the overall motion to thermal energy of the particles.

Think of a truck driving through the air (not the same process, but similar ideas). As the truck collides with the air, air-molecules gain energy from the collisions, increasing their average thermal velocity---thereby increasing their temperature. This decreases the energy of the truck, observed as a net friction (or drag) force. It is a similar process for friction, just a different geometrical setup.

(He then asked me if different materials develop different amounts of heat for the same force required to rub them together. I will post this as a separate thread)

If by 'different amounts of heat' you mean, they 'heat-up' faster/slower etc. then definitely. This just depends on the heat capacity of the material: how much heat (energy transfer) is required to affect a given change in temperature.