answer 2011

Hello to all, unraveling the tribological Christmas Riddle.


This year it was a very difficult riddle.

Why?

Unfortunately we humans do not have good sensors for heat radiation. And all, that we do not see, feel, taste or smell, does not exist, so we partially are pretty simple.

The snakes and vipers would have smirked about our puzzle.

They see their mostly warm prey animals glaring in the pitch-black night.

So, what happens, if we push a plate out of window glass on to the rotating plastic ball ?

Friction is generated.

From the friction and the sliding velocity the friction power arises. And out of this and the thermal conductivity the friction temperature of the sliding parts results.

For glass and KBr, the friction is similar.

Glass, however, is completely opaque in the near infrared.

A glass plate in front of a candle flame is as opaque as a sheet of aluminum.

Hard to believe, but a physical fact.

First conclusion: We do not see infrared radiation through the glass plate. We therefore do not see a thermally visible friction trace (ring) on the ball.

But the infrared camera sees a circular color spot, hot inside, becoming colder outwards.

This is the temperature gradient, extending from a point-shaped heat source.

The point-shaped heat source is the friction point, but on the other side of the glass plate.

That what we see is the "warm front", which penetrates through the glass and heats up the rear.

Taking away the glass plate, it carries with it the warm dot, which then slowly disappears.

 

Quite logically? Or what?

 

By the way: Even a 500 °C hot plate out of KBr is thermally invisible.

However, the fingers trying to grab it, are sensing a very sharp pain.

What do we learn from this ?

Never believe what you see !

Or:

What you do not see may even so be there.

 

Thank you for your participation !