The reason why we put the forces into two different categories will be obvious when we start solving physics problems, but for now, you can think of it like this: when total amount of work is done on an object is done by only internal forces, energy will change forms (like going from kinetic to potential energy), and the total amount of mechanical energy is conserved, and the forces are also conserved. When the total amount of work done is done by an external force, the forces are not conserved and the object with either gain or lose energy.
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The temperature change of the air, ball and string are sooo very slight that for everyday usefulness, we tend to neglect it. It’s important to know about, otherwise it wouldn’t make sense why the pendulum slows and eventually stops.
Karis asks: If a pendulum is swinging back and forth and the friction increases the temperature of the string and ball, why does the air temperature around it get lower?