You have just completed building GEARBOT (click here for building instructions) — a machine with 2 gears. You will use this vehicle to check whether or not the arrangement of gears of different sizes makes a difference in how a vehicle moves.
The gear connected to the motor is called the drive gear or the input gear. In some machines, a small gear drives a larger gear (that is, a small gear is connected to the motor, moving a larger gear); in other machines, a large gear drives a smaller gear.
The following two tests will help you figure out what these two different gear arrangements are good for.
What to do: One of you will operate GEARBOT and the other will measure time using a stopwatch. You will then record the time in the chart on page 2. Together you will find out how long it takes GEARBOT to move from the start line to the finish line. You can think about it as if it’s on a racetrack. To do this investigation, you need a simple program that turns the motor on. The firmware installed when your RCX was set up should have such a program installed in program slot 1 in your RCX.
· Have the person operating GEARBOT place it with its front wheels lined up on the tape marking the start line.
· Have the person holding the stopwatch stand by the finish line, with the stopwatch set to 00.
· GEARBOT operator will call “ready, set, go,” pressing the Run button on the RCX as she or he says “go.”
· The timekeeper will start the stopwatch as she or he hears the word “go” and stop it when GEARBOT’s front wheels cross the finish line.
Start with a large gear driving a small gear as indicated in Step 10 of the GEARBOT building directions.
· Practice 3-4 times and when you feel you’ve got the hang of it, run 4 trials and take measurements each time.
· Write each measurement in the left column.
· Calculate the average time it took GEARBOT to travel from the start line to the finish line.
A large gear driving a small gear
A small gear driving a large gear
When you have finished taking the first set of 4 measurements, switch the gear arrangement so that the small gear drives the large gear. Follow Steps 11 through 14 in the building instructions to do this.
· Take 4 additional time measurements with the new gear arrangement.
· Write each measurement in the right column above.
· Now calculate the average time it took GEARBOT to travel from the start line to the finish line for each data set.
Which gear arrangement makes GEARBOT go faster?
Look carefully at the gears: you have one gear attached to the motor. This is called the drive gear or the input gear. This gear is driving another gear attached to the wheel axle; this gear is called the output gear. The input gear drives the output gear and the output gear causes the axle to turn. So how fast the wheel axle turns depends on how fast the output gear turns.
Does the output gear turn faster when it is smaller than the input gear or when it is bigger? How come?
It will be easier to answer this question if you compare two GEARBOTS side by side.
· Find another team with a GEARBOT and set up one GEARBOT so the larger gear is attached to the motor and is driving the smaller gear.
· Make sure the RCX units are turned off.
· Set up the other GEARBOT so the small gear is attached to the motor and is driving the larger gear. Leave the wheel off the axle so you can see the gears easily.
· Have two people hold the GEARBOTS.
· Each person should turn the driving gear on their GEARBOT with their finger. They should try to turn them with about the same force.
· Observe how fast the output gears on the two GEARBOTS are turning.
Which one turns faster?
Look at how each output gear connects to the axle and causes the wheels to turn. Which GEARBOT’S wheels are turning faster?
Now you are going to test your GEARBOT in a new situation. You already know that when a small gear drives a larger gear GEARBOT doesn’t move as fast. Why would anyone want a gear arrangement that slows a vehicle down? The following test is going to help you answer this question.
Be sure to have the larger gear attached to the motor so it drives the smaller gear.
What to do: Set GEARBOT 2 feet in front of a box with weights in it (ask for it, if you can’t see one). Set the RCX to Program 1 and run it.
What happens when GEARBOT touches the box?
Run it again paying close attention to how fast GEARBOT goes before it reaches the box. In a minute you’re going to be switching the gears and running this test again. So notice and try to remember how fast it goes.
Switch the gears so that the smaller gear will be the drive gear (on the motor).
Set GEARBOT 2 feet in front of the same loaded box. Set the RCX on Program 1 and run it.
What happens when it touches the box?
Run it again. Notice how fast it is moving before it reaches the box. Is it moving faster or slower than with the opposite gear arrangement?
What is the advantage of having a small gear driving a large one?
Which gear arrangement would you choose if you wanted to race your GEARBOT?
Which gear arrangement would you choose if you wanted your GEARBOT to push or haul a heavy load?
When GEARBOT pushes a heavier load
In this activity you will have GEARBOT push increasingly heavier loads for 3 seconds. You will measure the distance traveled for each load. As you take these measurements, keep in mind that GEARBOT operates with the same motor (power). You will then try to figure out what else changes as the load gets heavier.
What you will need
· GEARBOT geared for force (with small gear driving large gear)
· program EX-Gearbot Load (go forward for 3 seconds then stop)
· GEARBOT Pushing Different Loads Data Sheet and Graph Sheet (attached)
· measuring tape or yard stick
· a small box or container that can hold various loads
· loads of equal units
· at least one partner
· masking tape to mark a start line
What to do: Find a clear space where you can operate GEARBOT. Depending on your working environment, your teacher may have a designated area for this activity.
1. Download the EX-Gearbot Load program into your RCX program slot 5.
2. Have GEARBOT geared for force (smaller gear on motor).
3. Mark a start line with a piece of masking tape and set the front of GEARBOT at the edge of the line.
4. Place one load unit in the box.
5. Set the box with its long side along the edge of the start line. Make sure GEARBOT touches the box. Remember this setup for the rest of the measurements you will be taking in this activity.
6. Turn GEARBOT on and run the EX-Gearbot Load program (Prgm 5).
7. Wait until GEARBOT and the box have come to a complete stop. Do not move the box. Then remove GEARBOT and press the run button to stop the program, being careful not to move the box. BE SURE TO LEAVE THE BOX IN POSITION UNTIL YOU TAKE YOUR MEASUREMENT.
8. Measure the distance between the start line and the edge of the box that is facing the start line. Take your time and be precise. Measure to 1/8” accuracy.
9. Write the distance in the area marked as 1 unit in the Load column on your Data Sheet.
10. Repeat the measurement for 1 unit two more times (a total of 3 measurements).
11. Calculate the average distance traveled.
12. Increase the load by one unit and repeat Steps 4 through 11 for each load. Be sure to take 3 measurements for each load and to calculate the average.
13. Plot the averages of the measured distances for each of the loads on the Graph Sheet. Notice that you will need to add labels for the values to the Y-axis, based on your distance measurements. Decide before you start how you are going to number the Y-axis.
14. Study your graph and answer questions 1 and 2 on your Data Sheet.
Click Here for Data Sheet
Click Here for Graph Sheet