## Hands On Physics

### The Great Bungee Jump Core Project Step 2: Building the Electronics

Overview. This page details one way to make the electronic measurements needed for the Great Bungee Jump.

1. Make the circuit.

Figure C5
Timing Circuit

Assemble the circuit shown below. This is identical to the circuit in Messing Around Investiation 3, except without the jumpers you used for switches there. This time, use R1 = 1 Mohm (brown, black, green). C1 = 1 mF (microfarad.)

Figure M9
Timing Circuit

You will recall that when switch 1 is closed, the meter will read zero. This is because this switch (S1) makes a direct connection between the voltmeter leads, so there can be no voltage difference between these leads.

The capacitor will start charging through the resistor when switch one (S1) opens, and stop charging when switch two (S2) opens. When both switches are open, the capacitor and voltmeter are isolated from the power supply. Any charge in the capacitor is trapped there, because there is no easy way for it to get out except by leaking through the capacitor or voltmeter.

2. Make S2. S2 is the trip wire at the bottom of the drop. When the trip wire is in place, it contucts electricity, and S2 is closed. When the falling mass hits the trip wire, it knocks out the wire and breaks the circuit, opening the switch.

Bend the two metal pipe hangers as shown below. Attach 1.8 m of green hook-up wire to each hanger. The wire can be slipped under the screw used to tighten the hanger to the pipe. Slip the hangers onto the vertical pipe so that the flat straps are near, but do not touch each other. Adjust the screw on each hanger so you can slide them on the pipe, but they stay firmly in place. You can now stand the tower in the base and tape it into position. Arrange the hangers as shown in the photo and lay a wire with bare ends across them. Test the switch with a falling mass using the continuity test found in most multimeters.

Figure C2
Stopping Switch

3. Make S1. On the bungee jump scale model, S1 is made from contacts on a clothespin at the top of the tower. The mass that drops is part of this switch. As soon as you release the mass, the switch opens.

Cut two lengths of red hook-up wire about 1.8 m long. Make one contact of S1 by stripping about 5 cm on one end of one wire and wrap it around the narrow part of one jaw of the spring clothespin. Repeat with the other wire on the other jaw of the clothespin. Be sure that these wires do not touch when the clothespin "jaws" are closed.

Figure C1
Starting Switch

Using a hose clamp, fasten one of the long arms of the clothespin to the side of the 30 cm pipe section. Attach this to the top of the tower with a 90° PVC elbow. Tape these parts together so that he mass will drop cleanly from the jaws of the clothespin and hits the trip wire. Test the switch with the mass but do not let it drop yet.

4. Complete the circuit. Position your tower so you can safely reach the top. Place the circuit on a nearby table. Connect S1 to the circuit board using the red wires and S2 using the green wires.

Where the mass will hit the floor, place foam or something soft to protect the floor.

Test allignment by clipping the mass into the clothespin at the top and positioning the trip wire across the hangers. You need to adjust the position of switch 2 so that the wire is knocked off of the hangers by the falling weight. Caution! Do not drop the weight on your partner!

5. Test the circuit.
Apply power to the circuit. With the mass clipped in S1 your voltmeter should read zero. If it does not, check the connections on the clothespin and the way it is attached to the circuit. When the meter reads zero, drop the mass. After the drop, the meter should give a steady reading of a few hundred mV. When your tower and circuit are working, try several drops. Is the voltage almost the same for each drop? How much does it vary? What percentage is this variation?

Congratulations! You now have a working bungee jump model and are close to being able to investigate the physics of the Great Bungee Jump! But before you do, you need to ensure that your timer is accurate.