THE WEIGHT EQUATION

What exactly is weight? Physics defines weight as a force measured in Newtons. Force is calculated by multiplying the mass of an object in kilograms by acceleration in meters per second squared (m/sec2). For our purposes on earth, acceleration is the effect of gravity, or 9.8 m/s2.
Weight or Force (Newtons) = mass (kg) x acceleration (m/sec2).
The mass of an object is more difficult to define but in general terms it is a fundamental measure of the “amount of matter” present in an object. In lay language, we tend to use weight and mass interchangeably but in physics they are distinctly different entities. If we lived on the moon or in outer space where gravitational forces are lower, then theoretically our weight would be lower (but our mass would remain constant). I guess one way to lose weight quickly would be to orbit the earth in space for a few days where gravity (acceleration) is low, but you would gain it back just as quickly when you return. Short of toying with gravitational acceleration, we have to be content with decreasing our body mass in order to lose “weight”.
In regular verbiage, we tend to say that we have “gained or lost weight” to mean that we have “gained or lost mass.” Therefore short of traveling through space, the fundamental question then becomes why do we gain or lose weight over time. The best way of looking at weight gain or loss is to relate it to the amount of energy taken in or put out of our body over time. One of the fundamental laws of physics is that energy is not destroyed; it just changes form over time. For example, when we turn on the gas oven to cook food, we are converting chemical energy (natural gas) mostly into heat energy (to cook), and light energy (the blue flame we see). Similarly, long term fluctuation in body weight is a reflection of the input of energy into the body versus the output of energy from the body over time. I’m not talking about the one or twp pounds you “gain or lose” over a few days. Rather concerned with the steady weight gain that occurs over months and years that sticks with you.
If energy input exceeds energy output over a period of time, weight gain occurs. If the reverse is true and energy output exceeds energy input, then weight loss occurs. If energy input and energy output are equal, then weight does not change. These relationships are summarized below.
ENERGY INPUT > ENERGY OUTPUT INCREASE IN WEIGHT
ENERGY INPUT< ENERGY OUTPUT DECREASE IN WEIGHT
ENERGY INPUT = ENERGY OUTPUT NO CHANGES IN WEIGHT
Sounds simple? It would be if energy input and energy output factors did not change over one’s lifetime. When we talk about energy input and energy output in terms of the body, we are essentially talking about calorie input and output. Human input and output of calories is highly dependent on the interaction of numerous societal, psychological, physiological and time dependent factors. But before we get further into the reason why weight gain occurs, we have to define some of the terms that we use frequently when we talk about weight gain or loss, diets, and energy.