Conservation of Mechanical Energy
Introduction:
In this experiment a pendulum bob is pulled to one side and held in place. In this position there is only potential energy. When the bob is released, the potential energy begins to be converted to kinetic energy as the bob descends. The total energy of the system (ignoring air resistance and friction) remains constant. KE +PE=0. This conservation law allows us to predict the speed of the pendulum at the bottom of its swing. To do this you must measure the initial and final heights of the pendulum (or change in height). Your prediction for speed is compared to results found experimentally using the photogate.
You will set up the photogate and pasco glx to measure the time it takes for the pendulum bob to cross the photogate at the bottom of it’s swing. You should use a long piece of string—perhaps with a ring stand on the table and photogate on the floor.
Procedures:
- Decide on an initial height above the table from which you will release the pendulum bob. You may use a ringstand on the table to tie string to, and a second ringstand on the floor to mount the photogate. This allows you to use a long piece of string. The pendulum should be lifted by anywhere from about 30cm to 100cm above the low point. It's up to you to select the height.
- Make and record measurements that allow you to use conservation of energy to predict the speed at the bottom of the swing.
- Make your prediction.
- Now that you have your prediction for the speed of the pendulum at the bottom of the swing, release the pendulum from the appropriate height, and use the photogate set-up to determine the speed.
- Repeat the measurement several times to obtain a good average.
Analysis:
- Determine the percent difference between your prediction and your measurement (as we did in the last lab).
- Calculate an uncertainty (as in the last lab) in the predicted speed of the bob at the bottom of the swing---this is your energy measurement and is based upon the distance the pendulum mass falls.