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Fun Physics Projects for Tomorrow's Rocket ScientistsA Thames & Kosmos Book
By Alan Gleue
McGraw-Hill Companies, Inc.Copyright © 2013 The McGraw-Hill Companies
All right reserved.
Chapter OneCruise Control: Constant Speed
Physics is the most fundamental of all the sciences. It is the study of the basic things the universe is made of—matter, energy, motion, and force. Physics tells you how these things work together. Let's begin your study of physics by building a project that you can then use to perform EXPERIMENTS. Experiments, measurement, and analysis are the foundations of physics and of all the sciences.
In this chapter, as in all the chapters that follow, you'll put together a do-it-yourself project that teaches you something about physics. After constructing the project, you'll use what you built to perform experiments. In this first chapter, you're going to learn several ways to take measurements, as well as how to analyze the DATA you get from those measurements. Scientists make sense of the world by creating experiments, taking measurements, analyzing data, formulating equations from the data, and, finally, coming up with conclusions. Scientists discuss their conclusions and experiments with others; in this way, the ideas of science are communicated throughout the world.
For the first project, you'll build a toy car that you'll then use to look at constant speed; you'll measure it, analyze it, and see what conclusions you can draw. Motion is one of the fundamental things studied in physics.
Throughout this book, I also introduce you to people from history who made significant contributions to our understanding of physics. On the next page, you'll read about a famous scientist whose contributions I will refer to again and again.
Project: Build a Constant-Speed Vehicle
For this project, I take you through the steps used to build a model battery-powered vehicle from a kit. (See the "Resources" appendix for more information on the kit I used, along with other possibilities.) The kit I used didn't come with building instructions. To perform the experiments and measurements later in the chapter, you can build any model vehicle, as long as it moves at a constant speed. You can also build a constant-speed car from scratch rather than from a kit, but some of the parts needed may be hard to find at local hobby stores or hardware stores.
Things You'll Need
Gear wheels (rear)
3 volts (V) DC motor
DC means direct current and is the type of motor used with batteries. AC means alternating current and is the type of electricity found in most homes. For these projects, you will need to purchase DC motors.
Package of four pulleys
Package of four gears
Four rubber bands
AA battery holder (for 2 AA batteries)
Four screw eyes
Pencil or pen
Pliers or long-nose pliers
Hot glue gun with hot glue sticks
Soldering iron (optional)
Drill with small drill bits (optional)
Small nail and hammer (optional)
Hot glue guns can get very hot and cause serious burns. The electric drill can be dangerous, too. Always have adult supervision when you use any tool that can burn or cut you.
Galileo Galilei (1564–1642) was a famous physicist and astronomer who lived in Italy. Galileo's writings form the basis of KINEMATICS (kin-uh-mat-iks), which is a physics term for the study of how everyday objects move. You'll use many of Galileo's ideas throughout this book.
Galileo built telescopes, turning them to the night sky to look at the moon, planets, and the stars (astronomy). He also studied the motion of the moon and the other planets. Shown here is one of his drawings of the phases of the moon. One of Galileo's goals was to prove that all the planets, including Earth, revolve around the sun. At that time, many people believed that the sun and the planets moved around the Earth, which stood still. Galileo researched many things including constant speed and acceleration, projectile motion, and how objects move when falling toward Earth.
Assemble the Kit
These steps describe one approach to assembling the Kelvin CV model racer shown in Figure 1-1.
1. Spread the components on a clean, flat surface and put similar items together.
2. Use the eyehooks to attach the axles to the car's body. Using the ruler, first make four marks on the wood base with a pencil. Measure ½ inch from the ends along both lengths, and ¼ inch from the ends along both sides of the width of the rectangular wood base (Figure 1-2).
3. Using these four measurements, draw four lines, with darker dots at the intersections. These measurements don't have to be exactly like what's shown in Figure 1-2, but be sure to keep your lines straight.
4. Drill four small, shallow pilot holes for the eyehooks at the four dots you drew in step 2.
5. Screw the four eyehooks into the wood base. Rather than using a drill, you could also use a small nail and a hammer to create small pilot holes. You may need pliers to screw in the eyehooks securely. After the eyehooks are screwed in and aligned, make sure that your two axle rods fit into the eyes of the eyehook. (Look ahead at Figure 1-4 for a preview of the finished wheelbase.)
6. Once you have the axles lined up, remove them and insert one small front wheel on one of the axles and one of the larger wheels on the other axle. The small wheel is for the front of the car and the larger wheel is for the back of the car. The wheels should push onto the axles, although you may need to gently tap them in with a hammer.
7. Taking one of the plastic pulleys that came with the kit, insert the second-largest pulley onto the back axle so it is pushed close to the rear wheel; see Figure 1-3.
8. Push the axles through the eyehooks, and place the second front wheel onto the front axle and the second rear wheel onto the rear axle (Figure 1-4).
9. Once you finish the wheel assembly, push the car around to make sure it rolls in a straight line and with minor friction. Now you're ready to work on the motor assembly.
10. Find the battery holder, switch, and motor. By making a complete circuit, these three devices make up the electrical and power-generation parts of your car.
11. There are four wires: two coming from the battery pack and two coming from the switch. Before you connect any of these wires, strip more from the ends using a wire- stripper so you have about ½ inch of stripped wire from each end.
12. Put two fresh AA batteries into the battery holder, and make sure the batteries are aligned correctly: positive (+) to + and minus (-) to -.
13. Take the red wire from the battery holder and twist it with one of the wires from the switch (see Figure 1-5). One option is to use the solder and a solder gun to make your connections more permanent and durable. I found that by twisting my wires together tightly, soldering wasn't necessary.
Burns from a solder gun can be very painful and severe. If soldering, do it under adult supervision.
14. Notice that the motor has two connections (A). Take the other wire from the switch and run it through one of these connections (B). Make sure to run the wire through the connection tightly and then twist the wire around the connection to secure it.
15. Now you're ready to make the final electrical connection. Thread the other wire from the battery pack through the other motor connection (C). Again, tightly twist it so you have an electrically sound connection (D).
16. Turn on the switch and see the motor's axle rotate. You have created a pathway for the electrical energy to run from the batteries through the switch to the motor. If the motor doesn't work, first check that your connections are making good electrical contact, and then make sure your batteries are properly and securely placed in the battery pack.
17. Put the small pulley on the motor's axle shaft.
18. You are now ready to add the rubber band to the pulleys on the motor shaft and the rear axle. Put the battery pack, switch, and motor on top of the car's wood body toward the middle and front of the car (see Figure 1-6).
19. Put one of the rubber bands around the motor shaft pulley and then around the rear wheel and around the pulley on the rear axle. There should be some tension or tightness in the rubber band, like Figure 1-7, but not excessive tightness. If the rubber band is too loose, it will fall off the pulleys too easily. If the rubber band is too tight, there will be excessive pull against the motor.
20. Once you have the battery pack and motor in position, hot glue the motor, battery pack, and switch to the wood top of the car.
Remember, hot glue guns can cause serious burns.
21. Retest your car's switch and electrical connections. Make sure pushing the switch activates the motor and spins the motor shaft, turning the rubber band and the back axle. You may need to make some minor adjustments.
22. Put the car on a smooth floor and turn on the switch. The car should roll forward in a straight line. You may want to test the car on different surfaces. You might find your car moves better on carpet than on a smooth floor. If your car is not moving in a straight line, check the straightness of your axles; you might need to make minor adjustments to the eyehooks. Make sure the wheels have adequate spacing and are not rubbing up against the side of the car.
Figure 1-8 shows two views of your new car.
Determine the Speed of Your Vehicle with a Stopwatch and Meter Stick
For your first experiment you need
Your constant-velocity vehicle
Software to graph your data
Physics is a science, and science involves learning things through experiments. In an experiment, you test a question or a HYPOTHESIS. Will your little vehicle move at a constant speed? That's what you want to determine. In this experiment, you
1. Acquire data.
2. Graph the data to construct a distance-time graph for the vehicle.
3. Determine the speed of the vehicle by taking the slope of the line formed.
4. Draw CONCLUSIONS concerning your question or hypothesis.
You can complete this experiment in several ways. Probably the easiest way is to run your car in a long hallway or room—like you might find at school—or a flat, smooth surface like a driveway, and use a stopwatch to measure your time. Either way, your testing area should be around 4 or 5 meters (around 5 yards) long. You need to measure the course and have a stopwatch handy.
This book uses the metric system (centimeters, meters, kilometers) rather than the United States customary units (inches, feet, miles) because the metric system is more commonly used in scientific studies. See the "Resources" appendix for some tips on conversions.
Using a meter stick, create a 4-meter (m) grid marked off in 1-meter units (at 0 m, 1m, 2m, 3m, and 4m), as shown in Figure 1-9. Start the car a short DISTANCE (say, half a meter) before the starting line to allow the car to get "up to speed." (Even though this is a constant-speed vehicle, the car needs some time to get up to its cruising speed.)
Start the stopwatch when the car gets to the beginning of the track and then time how long it takes to go 1 meter. After this, return the car to the starting position and time how long it takes the car to go 2 meters, then 3 meters, and finally 4 meters.
I timed each distance for two TRIALS and then took an AVERAGE time for each distance.
It's important to repeat things several times in experiments—that's the definition of a "trial." Repetition lessens the chance of any errors and makes for a better set of data.
Your data will most likely differ from mine because your car may move at a different speed.
Graph Your Data
You can turn your raw data into a distance-time graph using a ruler and graph paper. I created a graph using a spreadsheet program called Microsoft Excel, which can be faster and easier. You probably have access to this software on your school computers or at home. If you're not sure, ask a teacher or parent to show you how to use the program. Other spreadsheet programs work just as well (see the Resources appendix for more information), and many of them are free and available online.
Spreadsheet programs are a popular tool for scientists because they allow you to do multiple calculations at once and to substitute new data for old easily. A calculator serves the same purpose, but with a calculator, the operations must be entered by hand each time. A spreadsheet is more efficient!
If you have compiled a grid of your own data from the trials you ran, you can transfer it to a spreadsheet program and make your own graph.
To find an average, sum, or add, the data and then divide by the number of trials performed.
You can use a data table like the one I used here for your experiment:
Using Excel, I created a DISTANCE-TIME GRAPH by combining both columns, averaging time and distance, and instructing Excel to place the average time on the x axis and the distance on the y axis. You can see my graph in Figure 1-10.
This graph makes a diagonal line called a TRENDLINE. A diagonal line on the distance- time graph means my car is traveling at a constant speed. (In the next chapter, you'll find out that other shapes on a distance-time graph mean other types of motion.) This is important as it tells me the type of motion.
If you're using Excel, once you've created a graph, you can right-click on one of your data points. Then choose Add Trendline from the menu.
Now you see a screen with Trendline options. Select Linear. Click to check the two boxes toward the bottom: Set Intercept = 0 and Display Equation on chart. For 0 distance, your time is 0 seconds. This is your y intercept, and it should be 0. Consequently, your graph should go through the ORIGIN, and checking the Set intercept = 0 box forces the graph through the origin. Close the window.
The origin of a graph is typically the location on a graph where the two graphing axes meet and is labeled (0,0). For your purposes, this signifies that at zero time your car has moved zero distance.
Excel shows your trendline and EQUATION, so you can see your SLOPE. This slope represents the SPEED of the car (Figure 1–11). The y represents the distance, x is the time, and the slope, represented by m, is the speed of your car.
The slope for my vehicle is 0.7614 meters per second. Speed is a distance measurement divided by a time measurement. If you measure your grid in meters and time the car's motion in seconds, your speed is shown in "meters per second."
My car's slope or speed is slightly greater than 0.75 meters per second or 75 centimeters per second. Each second the car moves approximately 75 centimeters. Your car will probably have a different speed depending upon its motor, batteries, and weight. Regardless of your car's speed, its graph should be diagonal and represent constant- speed motion.
This equation, known as a kinematics equation, is called a constant-speed (or velocity) equation.
It works for any object traveling at a constant speed. The object could be a car, plane, train, planet, satellite, or galaxy. In physics, we say that distance traveled equals speed multiplied by traveling time. In concept form, you would write:
Distance traveled equals speed multiplied by time traveled
Or you can identify speed as distance traveled divided by time. Here is how to look at it in the form of an equation:
Speed = Distance/Time (speed is defined as distance per time)
This equation is powerful. By knowing and using this equation, you can make PREDICTIONS. How far would the car have moved in 10 seconds? Take your speed and multiply it by 10 seconds. Using my data, 0.7614 multiplied by 10 seconds gives a distance of 7.6 meters. The car would move a little more than 7.5 meters if you let it travel for 10 seconds.
You can use this equation for a real car, too. If you set the cruise control so the car travels along the highway at a constant speed of 60 miles per hour (96.5 km/hr), then you can use the constant-speed equation to predict how far the car will have traveled in three hours.
Cruise control is an instrument in a real car that forces the car to remain at a constant speed. Ask an adult to show you how this instrument works.
Physicists like to find equations because equations help make predictions. And we often discover an equation by graphing data and looking at the shape of the graph. The data graphed comes from the experiments we perform. Although you can certainly graph the data with graph paper and pencil, computer software tools make it easier and faster to analyze the data and see the trendlines.
Use a Video to Determine the Speed of Your Vehicle
Now I'll show you a second technique for taking measurements—one that is very popular in physics classrooms and involves using computer software.
For this, you need
Digital video camera (or a phone or camera that takes video)
Tracker software (see "Resources" appendix)
With a stopwatch and a meter stick, you can obtain time and distance data that you can then use to graph the data and find the slope or speed of your object. In some situations, however, you might have trouble using a stopwatch and a meter stick. For instance, the object may be traveling too fast to measure easily with a meter stick. Another method of taking measurements involves taking a short video of the object in motion. You then analyze this video using motion analysis software.
Excerpted from Fun Physics Projects for Tomorrow's Rocket Scientists by Alan Gleue Copyright © 2013 by The McGraw-Hill Companies. Excerpted by permission of McGraw-Hill Companies, Inc.. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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