Food, heat, and work. A runner uses about 80 kcal of...

Food, heat, and work. A runner uses about 80 kcal of internal energy to run each kilometer. (a) If the runner loses $120 \mathrm{~g}$ of perspiration while running $1 \mathrm{~km},$ find the heat lost in the process. (b) How much work does the runner do in running that kilometer?

Key Concepts

Energy Conservation

This principle states that within a closed system, energy can neither be created nor destroyed, only transformed from one form to another. In problems involving food, heat, and work, the chemical energy stored in food is converted into mechanical work and thermal energy, illustrating how energy is transferred and conserved in different forms.

Internal Energy

Internal energy refers to the energy contained within a system, such as the chemical energy stored in the muscles of a runner. It represents the total of all microscopic forms of energy, including chemical bonds and thermal motion, which can be transformed into other energy forms like work or heat during physical activities.

Work

Work is defined as the energy transferred when a force acts upon an object to move it over a distance. In the context of physical activities, it quantifies the mechanical energy output achieved by converting internal chemical energy into motion and overcoming resistive forces.

Heat

Heat is the form of energy that is transferred between systems or objects due to a temperature difference. When chemical energy is not fully converted into work, it is often lost as thermal energy, which can be dissipated through processes such as perspiration, thereby cooling the body.

Latent Heat of Vaporization

Latent heat of vaporization is the amount of energy required to convert a unit mass of a substance from a liquid to a vapor without a change in temperature. This concept is crucial for calculating the heat lost through the evaporation of perspiration, as it directly relates the mass of water evaporated to the thermal energy expended.

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