Gear up for science fun
Leyden, Michael B
As students prepare to jump on their bicycles and ride off into the summer sun, take the opportunity to explore the inherent science, math and technology involved in these pragmatic, pollution-free status symbols.
EXPLORATION. Send a student outside to walk his or her bike into the classroom. Measure the circumference of the bicycle wheel by putting a piece of masking tape on the bottom of the tire and rotating the wheel one time. Measure this distance on the floor.
Cut a piece of string this length and with a pen, masking tape, or twist ties (for plastic bags), divide it into tenths. Tape the marked string to the bottom of the tire. These marks will help determine what action of a rotation the wheel spins when the pedal moves (see Figure 1). (Figure 1 omitted).
Younger students can also use bikes to understand actions. From an arbitrary starting point, wrap part of the wheel in colored paper and label it “a little bit more.” Wrap the other section in a different color and label it “a lot more.” The kids’ analysis of the effect of gears can be stated using these phrases. For example, if the wheel truly spins 1.08 times, the child can state that as “one time and a little bit more.”
Put the bicycle upside down in the center of the room. Ask its owner to slowly spin the pedals one complete turn while putting pressure on the back wheel with one hand to prevent it from “coasting.” Does the wheel spin more than one revolution, less than one or exactly one revolution?
Focus the students’ attention on the number of teeth on the pedal sprocket and the number on the back wheel sprocket. With a 10-speed bike, there are two rows of teeth on the pedal sprocket and five rows on the back wheel sprocket. This is how “10 speeds” are obtained: The chain can be on either one of the two rows of teeth on the pedal sprocket in combination with the five possibilities on the back wheel sprocket. Five times two is ten.
Make a chart like that in Figure 2 and have students take turns counting the teeth on the sprockets. (Figure 2 omitted).
CONCEPT INTRODUCTION. When the teeth on both wheels are compared, a quantity called gear ratio is derived. The gear ratio tells how many times the back wheel turns when the pedals turn 360deg. Another term for gear ratio is mechanical advantage.
When the gear ratio is a low number, the bike is in low gear. High gear means the bike is moving faster because the back wheel may be rotating more than two times for every revolution of the pedals. However, there is some overlap in the gear ratio between high and low gear. Interpret the data collected. Note that three pairs of gear ratios are essentially the same: 1.30 & 1.36; 1.86 & 1.90; 1.50 & 1.53. So, a 10-speed bike really has only seven speeds!
CONCEPT APPLICATION. List the advantages and disadvantages of low and high gears. Gears are mechanical wheels with notches in them. Upon interlocking with other gears, a change in speed, direction and/or force may occur. When two gears interlock, one spins clockwise and the other spins counterclockwise.
On a bicycle, a chain connects the pedals to the back wheel, so the direction of spin is the same for both. However, the spin is clockwise when viewed from one side of the bicycle and counterclockwise when seen from the other side.
Using the lowest gear makes hill climbing seem easier, but while the force needed to make the bike move is less, the bike also moves less distance per pedal rotation. The pedals turn 360deg with less effort, but the wheel rotates less than one turn. There’s a trade-off–less effort through a complete rotation of the pedals results in a greater force over less than one rotation of the tire.
In high gear, the pedal sprocket is large and the chain spins around a small sprocket in the back. This means one rotation of the pedals may cause two or more rotations of the back wheel. However, a large force is needed to cause the bicycle to move “that” far. The speed is present if the large effort is there, too.
Riders are well aware of these force requirements. If a spring scale is available, this can be demonstrated with the upside-down bicycle. Tie a string onto the hook at the bottom of the spring scale and pull in a direction tangent to the circle of motion. The numbers on the scale will verify what bike riders’ muscles have been telling them for years. Gears make things happen, and these “things” are a change in speed, direction and/or force.
In a standard transmission vehicle, shifting “down” to a lower gear slows the vehicle and gives the driver greater control as he or she moves through a curve or prepares to stop. In high gear, the vehicle moves with greater mechanical and fuel efficiency.
Actively “pedal” the idea that science has to become an integral part of students’ daily lives as they ride through summer vacation.
Michael Leyden is Science Education Consultant, Eastern Illinois University, Charleston, IL, and a Teaching Editor of Teaching K-8.
Copyright Early Years, Inc. May 1994
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