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?

You first need to do the following conversions:

1 pound force = 4.44822162 newtons

1 foot = 0.3048 meters

I C = 1 kcal = 0.252 kcal = 1055 J

So,

13 lb * (.4482N/ 1 lb) = 57.8266 N

1.5ft * (0.3048m/1ft) = .4572 m

100 Cal = 100 kcal, 100 kcal * (1055J/0.252 kcal) = 418650.79 J

So then you need to use the equation of Heat, Q

Q = mgH, where m = mass, g = gravity and H is the Total Height.

SI units: Joules

The m*g = W, which also is the Newton value in this case.

You need to solve for H. Once you find it you need to divide by the distance (in meters) to get your reps.