You might not realize it, but every time you weight lift you are actually performing work on the barbell.
By definition Work is the "transfer of energy by mechanical means". Work can be calculated by multiplying the amount of force (N) applied to the object by the distance (m) that the object moved.
W = F x d
Where work is measured in the Joule(J), force in Newtons(N), and distance in meters(m)
The unit of work is a Joule(J) which is named in honor of the physicist James Prescott Joule (1818 - 1889) who discovered that there was a link between heat and other forms of energy. One Joule is equal to one Newton-meter.
For example, if you are performing a hang clean, every time you push the bar upwards, it is accelerating against the force of gravity (Fg = 9.8m/s^2). Lets say that you started with the weight resting on the floor and raised it above your head to a final height of 6m above the ground - the distance the bar traveled is 6m up. If you pushed upwards on the weight with enough force to accelerate the bar 14.8m/s^2 upwards, the total acceleration of the bar would be (14.8m/s^2 up - 9.8m/s^2 down) 5m/s^2 in the upwards direction. If the bar had a mass of 10kg then the total amount of force that you applied to the bar would be 50N (Force is measured by the unit Newton(N), 1N = 1kg m/s^2). Now you can substitute those values into the equation W = F x d. Your final answer should be W = 300J (W = 50N x 6m).
In order to calculate the work that has gone into lifting your weights, you must first know how to calculate force. The force applied to an object is equal to the mass of the object times the objects acceleration.
Fa = m x a
You might be thinking 'well that sounds simple, all I do to calculate work is multiply the distance I lifted the weight by the force that I had to apply to it'. There is however one very important condition.
For work to be done on an object, the force has to be in the same direction as the object's movement!
Weights are not always lifted in perpendicular or parallel lines to the floor, sometimes weightlifting involves tricky things called angles. If your weightlifting exercise involves angles, such as the 45 degree leg press, in order to properly calculate the amount of work you do on the weight, you have to visualize a triangle and split up the distance into both vertical and horizontal parts by using the following set of equations:
Tan (angle theta) = opposite/adjacent
Cos (angle theta) = adjacent/hypotenuse
Sin (angle theta) = opposite/hypotenuse
If you were lying on your back and raised a 45 pound weight (20.45 kg) 1 meter at a 45 degree angle how much work would you have performed?
In order to calculate the above problem you have to remember that to calculate work the force applied to the object has to be in the same direction as the distance the object is moved. Although you are pushing the weight over and up a total of 1 meter you are only raising the weight .707m (Sin45 = y/1m y = .707m vertically). Now that you have the distance the object is moved you have to calculate the amount of force acting on the object. Since F = ma the total force acting on the weight is 200.41N (F = 20.45 x 9.8m/s^2). You now have all of the necessary components to calculate work. W = (200.41)(.707m) = 141.69J