Hands On Physics

The Great Bungee Jump
Core Project
Step 4: Getting Data
The Bungee Experiment

Now we time accurately how long it takes the mass to fall through different distances. You should now have a working bungee drop with an accurate timer that can do this. Now you are ready to start getting data for the model bungee jump.

1. Set up the jump. Tie one end of about 1.5 meters of bungee elastic cord to your lead sinker mass. Be sure that it is firmly tied and will not slip while you experiment. You may want to tie two knots to make sure the cord will not slip.

Figure M1
Bungee Setup

The purpose of this investigation is to see if a full-sized bungee jumper could escape from a burning skyscraper. To make your model represent this, you need to adjust the length of the bungee cord so that your "jumper" nearly touches the ground. Experiment with the length of the elastic until your model works right, then tie the cord firmly to the top of your tower. Duct tape wrapped around the knot helps avoid slipage.

2. Measure the height, mass, and spring constant. Later, you will need to know some numbers for your particular model. These include its height, how much mass you have and how much "spring" your bungee cord has. Record these values:

  1. The height, in meters, above the ground of the mass when it is released. Measure the height to the bottom of the mass.
  2. The height, in meters, above the ground of the mass when the bungee just becomes taught and starts to pull.
  3. The height, in meters, above the ground of the mass when it is just hanging from the bungee cord and is at rest.
  4. The mass, m, of the lead sinker, in kilograms.

3. Predict your results. Before taking data, try to predict what your results will look like. Drop the mass several times and watch carefully what happens. Make three sketches using vectors to show the forces acting on the mass:
  • Immediately after it is released
  • About half way down
  • At the bottom of its fall.
Sketch a graph of what you think the velocity of the mass will be at different times. Remember that the mass is not moving when you release it. It is also not moving at the very bottom of its drop. Discuss your graph with your group and your teacher. Use this velocity graph to sketch a graph of the height of the mass as a function of time. Use the velocity graph to sketch a graph of the acceleration of the mass as a function of time.

4. Collect data. Measure the time it takes the mass to fall through ten evenly-spaced heights. For each height, make several voltage measurements and average their values. Convert your averages to elapsed time. Make a table of time and height values. Summarize your data as a graph of height as a function of time. Compare your results with your predicted graph.

5. Compute the forces (optional). Your data can tell you the approximate acceleration acting on the mass at each instant in time. You can do this through the following steps:
  • Graph velocity against time. The velocity graph is just the slope of the height graph. See the math section to learn how to graph the slope of a graph.
  • Graph acceleration against time. The acceleration graph is just the slope of the velocity graph. Just repeat the process of graphing a slope graph.
This procedure gives very "noisy" data. You will not get smooth lines because slight errors in your data get magnified by computing the slope twice. Your graph may look like little more than random dots. If so, don't be discouraged. Still, see whether you can see any pattern in your data. Does it look at all like your predictions?

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