During a workout, a person repeatedly lifts a 13 lb barbell through a distance of 1.5 ft. How many "reps" of this lift are required to burn off 100 Cal?

The efficiency for converting energy to work in muscles is not 100%. It is typically in a 18% to 26% range and varies with the individiual and the muscle.

See
http://en.wikipedia.org/wiki/Food_energy

They may expect you to assume 100% efficiency to answer your question. If so, "they" (whoever prepared your course material) are very wrong.

Since the body consumes food energy at a "1 MET" level (which also varies with the individual) even when NOT doing work, if you wait long enough, zero reps are necessary to burn 100 Calories

To determine the number of reps required to burn off 100 calories during a workout, we need to know the calorie burn rate for the specific exercise. The calorie burn rate for lifting weights can vary depending on factors like the intensity of the workout and the individual's weight.

However, let's assume a standard calorie burn rate for weightlifting as approximately 207 calories per hour for a person weighing around 155 lbs. Keep in mind that this is an estimate and can vary for different individuals.

Now that we have the calorie burn rate, we can calculate the reps required to burn off 100 calories:

1. Convert the barbell's weight from pounds (lb) to calories: Since the weight is 13 lb, we can assume that lifting the barbell burns 13 calories. (Note: This is not an exact conversion but an approximation)

2. Divide the total amount of calories needed to be burned (100 calories) by the number of calories burned per rep (13 calories):

100 calories / 13 calories per rep ≈ 7.69 reps

Hence, approximately 7.69 reps of lifting a 13 lb barbell through a distance of 1.5 ft are required to burn off 100 calories. Keep in mind that this is only an estimate and may vary depending on individual factors.