# Hands On Physics

## The Great Bungee Jump Messing Around

### More of the Story: Initial Investigations

Our intrepid Hands on Physics students realized early in the investigation that it would not be practical to experiment by dropping people out of the window and decided to build a small scale model instead. After some initial experimentation, they decided that they needed a way to accurately measure how long it takes for the jumper to fall different distances. The timer must be quick, accurate, and triggered automatically by the jumper.

### Background

Time is one of the most important quantities to measure in physics. Unlike a lot of things, time can't be held in order to be measured. No matter what you do time continues to pass. People have devised many ways to measure time. A lot of these methods rely on either accumulating something, like water in a water clock, or draining something like sand in an hourglass. Water clocks and hourglasses are fine for things which happen at a leisurely pace, but are far too slow to measure something like a falling bungee jumper. In this section you will experiment with a time which accumulates electrons! Electrons are small and quick, and like the water dripping into a water clock the amount of electrons can be used to measure time. We will use a capacitor to accumulate electrons. A capacitor is like a container for electrons. It can be filled from a source of electrons like your low voltage power supply, and it can be emptied by draining the electrons through an electrical circuit. For our purposes, a voltage meter will give us an indication of the number of electrons in the capacitor.

### Questions

1) What can you learn about a bungee jump escape without instruments just by observing a scale model? 2) Why do you need a timer? 3) Can you build a working timer based on a capacitor? 4) What is the role of each component in the timer circuit? 5) How accurate is this timer? 6) What is the range of times the timer can measure accurately? 7) Can you change the range of the timer? 8) Does the capacitor voltage depend linearly on time?

### Tools & Materials

#### Activity 1: A First Model

• About 1 m of eleastic to model the bungee cord. This can be a chain of rubber bands or a single rubber strip. (Hobby stores sell these for wind-up toys.)
• A mass. This can be a bolt, rock, a mass hanger, or a sinker used in fishing. It should be heavy enough to stretch your eleastic to about 1.8 m.

#### Activity 2: Capacitors

• Some very large capacitors. We recommend using four 1.0 F, 5.5 V electrolytic capacitors.
• Two light emitting diodes (LEDs). Any LED is fine. (Radio Shack cat. # 276-214 is a suitable red jumbo LED)
• A hand electric generator. We recommend the Genecon manual electric generator by Nakamura Scientific Co., LTD. (available form Science Kit & Boreal Laboratories, catalog # 64811-00. ph. 800-828-7777)

#### Activity 3: The Capacitor Timer

In this investigation you will need a low voltage power supply (LVPS). Instructions for making this circuit are in the HOP unit "Introductory Electronics: Security Systems." If you kept the LVPS from then, use it here. If not, you have three options:

• Build the LVPS described in Introductory Electronics: Security Systems.
• Use a battery. Any small battery will work: best would be a 1.5 V "A", "C" or "D" cell.
• Use any small lab power supply.

You will also need electronic breadboarding parts:

• A breadboard (a means of making reliable solderless connection.) If you use the LVPS, there should be room on its breadboard.
• Wire. 22 gauge solid (not multi-strand) hook-up wire. A selection of colors is helpful.
• Resistors: 10 Mohm, 4.7 Mohm, 1 Mohm. This circuit does not generate much heat, so any power rating is fine, even as small as.1 W.
• Voltmeter. We suggest using a digital multimeter with high impedence (>100 Mohm) on its low voltage range. The lowest-cost Radio Shack meter has this property. If you are not sure, test your capacitor and meter.

### Activities

• Activity 1. A First Model Here you simply attach a mass to a rubber band to make a scale model of the bungee jump. You can learn a lot just by observing carefully how this acts.
• Activity 2. Capacitors To collect accurate data on the bungee jump you need an electronic timer. You will use a clever, low-cost time circuit based on a capacitor. Before building the timing circuit, take some time to understand how a capacitor works. In this activity you will investigate how capacitors store electric charge. You will use a small, hand powered generator to make electricity which can be held in a large capacitor. Light emitti ng diodes (LEDs) will be used to show the direction of current.
• Activity 3. The Capacitor Timer Here you build and test a timer capable of measuring accurately to .001 second. This is essential for studying the bungee jump scale model but can be used in may other studies as well.

### Reporting

Keep a journal as scientists do, even when messing around. Record all your questions and observations in a bound notebook. Don't erase ­p; just cross out errors. Write enough so someone else could reproduce your experiments. Diagrams can save lots of words and help make your ideas clear. If you make measurements, record your data, with units. Tables are useful for repeated similar measurements. To make the logic of any calcuations clear, first record the formula you used, then show the numbers used to evaluate the formula, and finally your answer.