Why Does a Nail Get Hot When I Hammer It?
When we are in the sunshine, or standing before a fire, we feel hot; when we take snow or ice in our hands, they feel cold. The thing which produces these sensations is called heat. When we feel heat, it is because heat is absorbed by our bodies, and when we feel cold, it is being thrown off by them.
To answer this question, we must see how heat may be produced. If we draw a cord rapidly through our fingers, they feel hot, and if we rub a coin briskly with a cloth or our hands, it becomes warm; if we take a nail and hammer it on a hard substance, it becomes too warm for us to hold. In these instances heat is produced by retarding or checking the motion of a body. When we draw a cord through our fingers, it moves less easily; we retard its motion by gripping it and this is what makes the heat we feel. When we strike the nail with a hammer, the motion of the hammer is checked by the nail, and the faster we pound with the hammer, the hotter the nail becomes. From these experiments we learn that whenever the motion of a substance is checked, or retarded, heat is generated, and the substance made hot.
In explaining this method of producing heat, it was at one time thought that all bodies contained a substance which produced the heat and that, when rubbed or hammered, this substance was thrown off. About the end of the 18th century, however, it was shown by Benjamin Thompson (Count Rumford), that substances when rubbed give off heat. From this we learned that heat is not a substance, because the quantity of any substance, present in a body, cannot be limitless. If it were a substance which produced the heat, the supply would sooner or later be exhausted, and rubbing could no longer produce heat.
Heat produced by rubbing, or by striking substances together, is caused as follows: If two substances are struck upon each other, the whole of those substances are checked, but the molecules of the substances are made to vibrate very rapidly, and these vibrations produce the heat we feel.