Rolling Along with Links

Est. Class Sessions: 3–4

Developing the Lesson

Part 2: Preparing for the Lab: Keeping Things Fair

Hold Qualifying Round. The collection of cars (those students brought from home as well as those you gathered) will need to be limited to a total number of five for the lab. The five cars will need to travel between ten and sixty links when rolled down the ramp. Hold a qualifying round to remove cars that do not travel distances within this interval or that do not roll straight. With the setup illustrated in Figure 2 prominently displayed, demonstrate the procedure for selecting the cars.

After the five cars are chosen, write a description of each one on a separate index card (e.g., red car), and post the cards on the chalkboard or a bulletin board as described in Materials Preparation.

Begin a discussion of controlled variables by asking:

Think about the things Betty did to test her car to find the best design to make it roll the farthest. What are some of the things Betty kept the same for each test? (Betty kept the ramp in the same place and used the same starting place on the ramp for each roll. She kept the ramp the same height and didn't push the car—she just let it roll. She used the same unit of measurement for distance rolled.)
Why do you think she kept these things the same for each roll? (If she kept changing things, she wouldn't be able to tell how to make the car roll the farthest. She couldn't tell which wheels were best if she changed the starting place or moved the ramp.)
Betty kept some things the same to make each test "fair" so changes didn't affect the rolling distance.

Review the Measuring Advice chart. Using good measurement techniques will also make the roll-off fair. In particular, stress keeping the link chain as straight as possible.

Discuss Controlled Variables. Explain that the class is going to determine which car is the "best roller." In planning the experiment, students should discuss how to keep everything "fair" including:

Start each car from exactly the same place.
Place the same part of the car on the starting line (back wheel, front wheel, front end, etc.).
Release the car (just let it go, instead of pushing it).
Use the same ramp and ramp height for all cars (or make the ramp and ramp height the same for all cars).
Keep the surface on which the cars roll the same for all cars.
Measure the distance the car rolled using the same procedure each time (from the bottom of the ramp to the same part of the car placed on the starting line).

Ask:

What do we need to keep the same, or "fair" when we place each car on the ramp to determine which car is the "best roller"? (Put each car in exactly the same place to start.)
Why is it important that all the cars start at the same place? Explain your thinking. (If they start at different places, it's not even because that changes the distance. If one car starts at the very top and a different car starts down the ramp, it might look like the second car went farther, but when you measure, it might be the other way around.)
How can we be sure each car starts in the same place? (Mark a starting line and put the same part of the car on the starting line.)
What part of the car do you think we should use? (Possible responses: back wheel, front wheel, front end, etc.)
What do we need to keep the same to start each roll? (Just let go, don't push.)
What do we need to keep the same with the ramp? (Use the same ramp each time and don't change how tall it is.)
How can we measure the distance fairly? (We need to stretch the chain of links straight and remember the tips on the Measuring Advice chart.)
What else do we need to keep the same when measuring? (We should always measure from the bottom of the ramp to the part of the car that was on the starting line.)

Demonstrate Lab Procedure. Demonstrate the complete lab procedure before students draw a picture of the lab.

Demonstrate how to roll the car. Have a student using tape or a small self-adhesive note mark where the car stops. Remind students about keeping each trial fair.
Tell students that if scientists and engineers were testing the cars, they would roll each car more than one time. They would conduct several trials.

Ask:

Why do you think it is a good idea to try the cars more than once? (Possible response: The scientists want to be sure they don't make any mistakes.)

Repeat the process two more times, again discussing the procedures that keep the roll-off fair. The three trials will very likely result in three distances that, while close to one another, are not exactly the same.

Instead of recording all three distances, explain that students will make a chain that is the same length as the middle distance. Students can organize their chains so that they can count by fives or tens.
Engage students in a discussion about why scientists use a single value, the middle distance (median), to represent the three trials.

Ask:

Why do you think scientists use just one measurement? (Possible response: Maybe there's too many numbers to keep track of. If they have so many cars and try each one many times, that's a lot!)
Why would it be a good idea to use the median, or middle distance, rather than the longest or shortest distance? (Possible response: You could get a really long roll or a really short one and the middle is best. It is more normal, not too long and not too short.)

While students' reasons are not likely to be precise, lead them to the idea that the middle value is a good representative of how far the car rolls since it is neither the shortest nor the longest.

Place the chain beneath the appropriate index card.
Ask students to skip count the links by fives or tens and count leftovers by ones to find how many links the car rolled. They should write the number of links on a self-adhesive note.
Place the note with the number of links below the chain, as shown in Figure 3.

Draw a Picture. Tell students they will draw a picture of the lab to plan their experiment before they begin. Students will use the Draw section in the Rolling Along with Links pages in the Student Activity Book. Help students visualize a drawing that will represent the experiment's main variables (type of car and the distance the car rolled) as well as the parts of the setup that remain the same (the supported ramp with a starting line).

Ask:

A good lab drawing shows what equipment you will use. What equipment should your drawing include? (We should show a ramp, a starting line, a car, and a chain of links to measure the distance.)
A good lab drawing also shows what you will do, or the procedure. What should be drawn to show your procedure? (We need to show the ramp setup with the car rolling or when it stops. We should show the chain stretched straight to show how to measure.)

When reviewing the drawings, look for the experiment's main variables as well as the parts of the lab that will remain the same as stated above. Note the example of a student's drawing in Figure 4.