Question: According to a simple physiological model, an athletic adult male needs 20 calories per day per pound of body weight to maintain his weight. If he consumes more or fewer calories than those required to maintain his weight, his weight changes at a rate proportional to the difference between the number of calories consumed and the number needed to maintain his current weight; the constant of proportionality is 1/3500 pounds per calorie. Suppose that a particular person has a constant caloric intake of I calories per day. Let W(t) be the person's weight in pounds at time t (measured in days).
(a) What differential equation has solution W(t)?
(b) Find the equilibrium solution of the differential equation. Based on the context, do you expect the equilibrium to be stable or unstable?
(c) Solve this differential equation.
(d) Graph W(t) if the person starts out weighing 160 pounds and consumes 3000 calories a day.