# Hands On Physics

### The Story: Introduction

After watching "Towering Inferno", some Hands on Physics students realized that there was a big need for a sure way to escape quickly from a tall building. During a fire, you cannot use elevators and internal stairs might be impassible. A personal, quick and direct means of escape would seem to be best. People can jump from the windows into nets held by firemen, but this only works for a few stories. Collapsable or rolled up escape ladders might work, but a 20-story escape ladder would be very large, heavy, and expensive. It might take too long to climb down and might burn up while people are on it!

The students hit on a novel idea. They thought trapped people might make a bungee jump out a window. If the right length cord was used, the jumper would stop just at the ground, get off, and walk safely away from the building. A bungee cord is relatively small, easy to store, inexpensive, and a quick way to get out!

The students described their idea in a letter to a large insurance company. The company was thrilled by the possibility of saving lives (and money) and eagerly asked the students to answer to the following questions:

1) Will there be too much force on a jumper?

2) Would it be possible to "get off" safely at the bottom of the drop?

3) Would a jumper crash into the side of the building?

4) What might be some of the problems that would limit the use of this escape system?

The students decided to study a small-scale model of the bungee jump. They figured that if they could understand this model, then they could later scale it up to a full-sized test. This raised some more questions:

5) Can you understand the behavior of the small bungee drop mathematically?

6) How can you scale up the small model to figure out how much cord would be needed for a person of given weight to use to escape from an eight story building?

Figure I1
Bungee Questions

Your job in this unit is to see whether you could save lives
with a bungee cord escape system.

Do your work carefully, because lives might depend on getting the right aswers. Would you stake your life on your results by jumping out of a building attached to a bungee cord?

### Goals

The bungee jump is a mechanics problem that is so complex that the best way to understand it is to model it. We will use both a physical scale model and a mathematical model. In order to understand the scale model, we will have to take split-second measurements that rely on electronics.

It is remarkable how much you can learn when you investigate something carefully. By concentrating on the bungee jump, you can learn about how anything moves and how to use technology to make careful measurements. These concepts and skills are important and widely used in science and technology.

By the end of this unit, you should be able to:

• Explain the relations between the position, velocity, and acceleration of an object in one and two dimensions.
• Describe the forces acting on a bungee jumper at all parts of a jump.
• Account for all the forces acting in other one- and two-dimensional motion problems.
• Describe the relation between the forces on an object and its acceleration.
• Describe how to account for the effect of air friction on the motion of the bungee jumper.
• Apply your results from the scale model to a full-sized jump.
• Build and use a timer circuit using a capacitor.
• Explain the function of each componenet in your circuit.
• Characterize the errors in your timing measurements
• Determine best estimates from data that contain errors.
• (Optional) Obtain motion data from a video using appropriate software.

### Learning Strategies

This section describes how best to learn from this unit.

#### Learn by Doing

Most people learn concepts by making things and then thinking about them. Too often students try to jump ahead and memorize the equations and definitions without giving themselves time to think. This is why Hands On Physics units emphasize "hands on" building.

#### Using the HOP Structure

There are three major sections to all HOP units: "messing around," "core project," and "extensions." The "messing around" part is a chance to learn the big physics concepts without worrying about a lot of details and computations. The "core project" is an extended construction project that everyone does. Then you choose one of a number of "extensions" to work on. Try "Messing Around" to get a feeling for what happens during a bungee jump. After you learn how to measure the short times involved, work through the core project and then choose an extension.

#### Think in the Lab

It is important that you use your mind while you are in the lab doing these various projects. You cannot just follow the directions and fill in the blanks. We don't tell you every little step because you should be learning how to do things yourself. Eventually, we want you to be able to undertake an entire project. To get to this level, you have to make larger and larger steps without help.

#### Fill in the Gaps

You may find this frustrating. You may get mad at the instructions that seem vague and you may wish your teacher could help you all the time. But before asking for help, talk it out in your group; try to invent a way out of your problem. If you are not sure whether you are doing the "right thing", write down what your problem was and what you decided.