Final answer:
Rubbing hands together creates heat by transforming mechanical energy from the motion into thermal energy through friction. This is a quick but relatively inefficient way to generate a small amount of heat. Shivering also generates heat but is not very efficient compared to a resting state.
Step-by-step explanation:
When people are cold, they often rub their hands together to warm up. This action converts mechanical energy from the motion of rubbing into thermal energy (heat) via friction. The heat comes from the work done on your hands when you overcome the frictional force between them. As you rub your hands, the force of friction resists the motion, and this resistance causes the mechanical energy to be transformed into heat energy, increasing the temperature of your hands.
Consider a woman who rubs her hands back and forth for a total of 20 rubs, with each rub covering a distance of 7.50 cm and an average frictional force of 40.0 N. The work done can be calculated using the formula Work = Force × Distance. The energy from this work is then transformed into heat, leading to an increase in temperature of the hands.
In the context of efficiency, rubbing hands together for warmth is not particularly efficient in terms of energy expenditure versus heat produced, especially compared to other methods like using clothing or a heater. However, it is a quick and convenient way to generate a small amount of heat. Likewise, shivering is an involuntary response to lowered body temperature. The efficiency of the body when shivering is not as high as the body at rest, but it is a quick method for generating heat when the body is cold.