News flash: You can now keep track of the calories you burn with our running journal. As long as you have a Google ID, you don't even need to create an account with us. Just log in using your Google ID and create your journal. And if you add the Calorie Burn Calculator to your iGoogle page you can return to add new entries easily.
A calorie measures ability to do work. The number of calories burned while exercising is, for the most part, determined by weight, resistance, and distance. Although there are quite a few factors that affect your calorie burn rate, speed plays only a marginal role, so sprinting a mile won't burn significantly more calories than jogging, or even walking that same mile.
From classical physics, work is measured by force times distance. The heavier an object, the more force required to move it.
But weight and distance are only part of the equation. Clearly, riding a bicycle on a flat surface requires less energy than running over the same flat surface even though you're carrying more weight across the same distance. This is where efficiency and resistance factor in. It's far more efficient for a wheel to roll across a surface than for a person's legs to traverse it. Consider also that when running or walking, a lot of energy is used in the up and down bobbing motion with every footstep. This "wasted motion" accounts for a large number of calories burned.
Since the classical physics equation doesn't account well for inefficiencies, we rely on studies of the efficiencies of various types of exercise to apply a coefficient to the force times distance equation. The coefficient represents an average from person to person. For burning calories, the most important rule of thumb to remember is to exercise at a pace that allows you to cover the greatest amount of distance, since distance is the greatest determining factor for the number of calories burned during your workout.