There are so many types of foods out there, that it really depends on what the food is.
Yes, calories from different foods are metabolized differently. The source of the calories will effect how they are metabolized.
Calories are not a form of weight. It could be different amounts for different foods no-matter what the weight is.
About 10% of the total calories intake.
There is no such thing as "common foods". Foods are specific sometimes ot certain regions or areas, and there are so many foods out there. Please be more specific in your question, so WikiAnswers can provide you with the best possible answer.
All foods have calories.
Use the equation E=mc2 . where c is the speed of light and multiply that by how many million milligrams you've got. Different foods have different calories not because they have different milligrams or weights, but because they are different foods altogether. A milligram of chocolate would have more calories than a milligram of water because you get more energy from eating a milligram of chocolate than drinking a milligram of water.
foods free of calories
all foods have calories in, some just have more than others!
There is a few foods that equals to 3,500 calories. You can get foods that are fresh like veggies and fruits.
The United States Department of Agriculture has a "National Nutrient Database" which contains details of calories and nutrients in many different foods. http://www.nal.usda.gov/fnic/foodcomp/search/
calories.
It depends on what the calories are from. Some calories are from foods rich in proteins, while other are foods that have little to no protein in them at all.