1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Help a dumb weightlifter out

  1. May 11, 2008 #1
    I train in the sport of weightlifting (i.e., the same event that you see in the Olympics that involves two lifts, the snatch and the clean and jerk). I am planning on using the diagnostic tool of filming my lifts with a camera positioned facing me directly from the side. I then plan to advance the frame of the lift 1 frame at a time (each frame will be 1/30th of a second) and plot the path of the bar (using the center of the bar as the point to track). If you would like to see an example of what I am talking about, please look at this website: bennsweightliftingvideos dot info (this site won't let me post the URL) (click on the link along the side that says "2004 Olympic Trials" and you can play any of the video clips and see the sort of plotting I am talking about).

    Now, I haven't taken a physics course in something like 20 years, but I would like to be able to calculate (and graph) each moment of force acting on the bar throughout the movement. I will use this information, along with visual cues from watching the lift and the shape of the curve, to help correct flaws in my form, and to train using the correct exercises to perfect where I am weak in the movement.

    The problem is, I simply cannot remember how to do this. Here is what I will know:

    *The distance between each data point from frame to frame (I will be able to take take a known size, in this case, the diameter of the bar, and determine the what each pixel equates to).
    *The time between each date point (1/30th of a second).
    *The mass of the bar.

    So can you help a dumb weightlifter figure out how I can graph how much force is acting on the bar from one data point to the next given that we will have a bar moving (from the perspective of the camera) in two dimensions?

    Thanks for your help.
  2. jcsd
  3. May 11, 2008 #2
    I should also add that in weightlifting (for those that care, not sure that this really matters as far as figuring out the how I do what I want to do). The first pull is from the ground to just above the knee, where the weightlifter will have generated enough momentum on the bar to readjust his knees under the bar (the so-called double knee bend) and execute the second pull, which is from just above the knee to full extension of the legs, hips, and traps. As the bar continues to travel up, the weightlifter then executes the third pull, in which he is pulling himself down, underneath the bar into the catch position while the bar is still moving up (at least for part of the movement).
  4. May 11, 2008 #3
    You came to the right place!

    What you need to calculate is acceleration. You say you have 2D data points seperated by 1/30 of a second. Is it by any chance is it in a spreadsheet like this:

    X Y
    3.24 4.56
    3.13 4.59
    ... ...


    Where X and Y are horizontal and vertical coordinates at each moment in time. It is best to do the measurements in metric units, so the distance is in meters and the mass is in kilograms and the force is in Newtons.

    You time step is 1/30 = 0.0333

    Next you need to to calculate the velocity in both the horizontal and vertical directions.

    velocity = (change in distance)/(change in time)

    This means (in spreadsheet terms) to make a new column called VX and start with (X2 - X1)/(0.0333) and drag that down, and then do the same thing with a new column VY starting with (Y2 - Y1)(0.0333).

    acceleration = (change in velocity)/(change in time)

    To get columns of horizontal and vertical accelerations make AX and start with (VX2 -VX1)(0.0333) and for AY do (VY2 - VY1)(0.0333).

    Force = (mass of the entire bar + weights) * (acceleration)

    To find the force in the horizontal direction, make a new column FX and start with (mass of the bar + weights) * AX1 and drag that down. In the vertical direction you do the same thing but also add on 10*(entire mass of bar + weights) to account for gravity.

    If you want to find the total force it is Sqrt(FX^2 + FY^2). Good luck!
  5. May 11, 2008 #4
    Thank you very much! That actually makes sense to me, assuming I understood it correctly. Let me see if I get this right:

    Step 1 = Plot data points both horizontally and vertically.

    Step 2 = Find the velocity of the bar both horizontally and vertically from one data point to the next by simply finding the change in x and y respectively divided by 1/30th of a second.

    Step 3 = Take the numbers I measured in Step 2, and find the change between them from one data point to the next both in the x and y axes and divide each by 1/30th of a second to find the acceleration.

    Step 4 = Measure the force horizontally at each data point by multiplying the measurement in step 3 for the x axis times the mass of the bar + weights.

    Step 5 = Measure the force vertically at each data point by doing the same thing as in step 3, but multiply that number by 10 (9.8, actually, right?) to account for gravity.

    Step 6 = Find the total resulting force at each data point by using Pythagorean's Theorem to find the hypotenuse (which would be the vector representing the total force, correct?)

    Did I understand that correctly?

    This is all vaguely ringing a bell in the far reaches of my mind.
  6. May 12, 2008 #5
    Yes, it sounds like you understood the instructions well.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook