# Archives

## Modeling Tectonic Plates

Project-based learning is a core part of the curriculum at Bullis Charter School. In seventh grade social studies, students are posed this driving question: “How can we represent tectonic plate movement in 3D with earthquakes and geologic features?” The project begins with an introductory field trip to the San Andreas fault in which students collect physical evidence of the 1906 earthquake. Next, students work in groups of 2-3 to research plate tectonics and write a short paper. They then build a three-dimensional model of a particular fault, and design mechanisms to demonstrate how tectonic plates move relative to one another. Then they animate that model using the Hummingbird with Scratch or Snap!.

Differentiation: Tectonic models can vary in complexity to make the project accessible to all academic levels. Programming complexity can also vary from basic movement to adding sensors and variables to calculate movement based on the Richter scale level of the earthquake.

## Get a Grip!

#### Materials

• computer with Birdbrain Robot Server and Scratch
• plastic bottle
• Hummingbird board
• Vernier BTA Sensor Adapter for Hummingbird
• 1-hole stopper with tapered valve
• Vernier Gas Pressure Sensor
• heavy-wall plastic tubing

#### Procedure

1. Connect the plastic bottle to the pressure sensor as shown below.
2. Connect the Vernier BTA sensor adapter to the Hummingbird. Then connect the pressure sensor to the adapter. The jumper on the adapter should be set to 0-5 V.
3. To read a measurement from the pressure sensor, you will use the HB voltage block in Scratch. This block will read a voltage between 0 and 5 V.
4. The Vernier manual for the pressure sensor provides a linear equation that you can use to convert voltage to pressure: pressure = 51.71*voltage – 25.86
5. In this experiment, you will grip the bottle as hard as you can for 60 s. You want to record data during this time and compare the data from 0 -10 s with the data from 50 – 60 s. To do this, you will need to write a program that meets the following requirements:
1. The program must record 480 pressure sensor measurements and store them in a list.
2. The program should wait 0.125 seconds between measurements (so the program takes approximately 8 measurements per second).
3. The program must use the function above to calculate the pressure based on the HB voltage block.
4. The program should graph the pressure measurements to the screen. The pressure measurements should be scaled so that this graph occupies most of the screen. The stage backdrop should show appropriate axes for this graph.
5. The program should calculate the mean pressure for the entire 60 s period and display this value for the user.
6. The program should calculate the mean pressure during the period 0 – 10 s and display this value for the user.
7. The program should calculate the mean pressure during the period 50 – 60 s and display this value for the user.
6. Grip the bottle as hard as you can with one hand while you run your program. Remember not to start the program until you are gripping as hard as you can. Record your data in the table below.
7. Repeat this process for your other hand. Record your data in the table.

#### Processing the Data

1. In the space provided in the data table, subtract to find the difference between your 0–10 s average and your 50–60 s average for each hand.
2. Record the 0–60 s results for the other students in your group. Calculate and record your group average. Calculate and record the class average for 0–60 s.
4. Did your gripping power increase or decrease during the 60 s period? Why did it change?
5. How does your grip compare with the class average?
6. What did you learn about your strength in this experiment? Were you surprised?

#### Extension

Can you motivate yourself (or someone else) to grip harder? Use lights, motors, and/or sounds to make a robot that does different things depending on how hard the person is gripping. For example, maybe more LEDs light up the harder the person grips. Does the average grip pressure increase when someone uses your device?

## Where IS North?

#### Overview

Where IS north? It depends. Do you mean geographic north or magnetic north? The geographic (true) north pole is the point at 90° N latitude. It is aligned with the rotational axis of the Earth. The Earth is surrounded by a magnetic field with a north and south magnetic pole. The magnetic north pole is the point to which a compass needle points. It is currently in northern Canada, but moves at an average rate of 15 km per year due to complex fluid motion in the outer core of Earth. Depending on your location, the difference between magnetic north and geographic north, called magnetic declination, can range from 0° to 30°.

In this experiment, you will measure the magnetic field of the Earth. You will use this data to determine magnetic north. Knowing the direction of true north, you will calculate the magnetic declination at your location.

#### Materials

• computer with Birdbrain Robot Server and Scratch
• Hummingbird board
• Vernier BTA sensor adapter for Hummingbird
• Vernier Magnetic Field Sensor
• glue gun
• cardboard
• rubber bands

#### Procedure

1. Bend the magnetic field sensor so that the end is at a right angle to the handle of the sensor.
2. Connect the Vernier BTA sensor adapter to the Hummingbird. Then connect the magnetic field sensor to the adapter. Set the switch on the magnetic field sensor to 0.3 mT (high amplification).
3. You need to build a robot that will rotate the magnetic field sensor to a variety of angles between 0° and 360°. To do this, you should use two servo motors. One possible design is shown below. The top servo rotates from 0° to 180° while the bottom servo is at 0°. Then the bottom servo moves to 180°, and the top servo rotates from 0° to 180° again. In this way, the top plate can stop at any angle from 0° to 360°. There are other possible designs! Note: If you mount one servo on top of another, make sure that the axes of rotation of the two servos lie along the same line.
4. Once you have a robot that can stop at angles from 0° to 360°, mount the magnetic field sensor on it in the orientation shown below. DO NOT use hot glue on the magnetic field sensor! You can use rubber bands to gently attach it to your robot.
5. To read a measurement from the magnetic field sensor, you will use the HB voltage block. This block will read a voltage between 0 and 5 V.
6. The Vernier manual for the magnetic sensor provides a linear equation that you can use to convert voltage to magnetic field strength (in millitesla): magnetic field = (0.16 mT/V)*voltage – 0.32 mT
7. In this experiment, you need to record a number of magnetic field measurements. You should write a program that saves these measurements in a list. Your program should meet the following requirements:
1. The magnetic field sensor should stop every 15° between 0° and 360°.
2. At each stop, the program should record the angle in one list and the magnetic field sensor reading in another list.
3. The program should graph the magnetic field measurements to the screen. The measurements should be scaled so that this graph occupies most of the screen.
8. Look at your graph. Where is the magnetic field highest? This is the direction of magnetic north! Look through your lists to find the angle with the highest magnetic field. Be sure to record this value.

#### Processing the Data

1. Consult a map to estimate the direction of north for your location. How does this compare to the direction you identified in this experiment? If a compass is available, use this to measure the direction of magnetic north and compare it to your results.
2. The difference between the measured magnetic north and true north is called magnetic declination. What is the magnetic declination for your location? What modifications would be needed on a compass in your location to keep you on course when following a map?

#### Extensions

1. Modify your program so that it automatically calculates the angle of magnetic north. Then make the robot point the magnetic field sensor in this direction. Use LEDs to show the user the direction of magnetic north.
2. Research current theories on why the magnetic north pole moves.
3. Scientists have found that the magnetic field of the Earth is continually changing. What would be the implications of a big change?

## Physics of Pinball

The eighth graders of Vickery Creek Middle School demonstrate their knowledge of physics by creating pinball machines! They learn about simple and complex machines in physical science, and then they apply this knowledge in their STEAM elective.

This project starts with an introduction to how pinball illustrates Newton’s Laws of Motion. Roanoke Pinball Museum: Key Concepts is a helpful resource. Discovery: How It’s Made: Pinball Machine shows the construction of an actual pinball machine.

Next, students are challenged to build a pinball machine with the Hummingbird. The game must have a clear, engaging theme and incorporate at least 3 simple or compound machines to apply Newton’s laws of motion. Students can use the attached planning document to design their machine.

As students build their game, the DIY Cardboard Pinball Machine video may be helpful. In addition, this video shows how to use the Hummingbird servo to construct a basic flipper that uses a linkage. Students can experiment with modifying this linkage to get the movement that they want for their game.

When students complete their games, invite the school community to come play!

Differentiation: Students ready for an additional challenge can add sound effects and score keeping through Scratch or Snap!.

Assessment: The rubric found in teacher materials can be used to evaluate projects.

Check out this pinball machine from CreArtBot:

## Human Body Systems

In 2017, year 3 students in Hong Kong used the Hummingbird to learn about human body systems. This six-week inquiry-based project was designed by Leigh Thomas at Renaissance College using Agency by Design resources from Project Zero of the Harvard Graduate School of Education. The purpose of this project was for students to study a body system in detail and create a representation that illustrated the function and importance of that system. Children needed a deep understanding of the body system that they were investigating in order to be able to extrapolate it and represent it in a meaningful way. Many children made connections to other human body systems as they explored their system and represented it. For example, the skeletal system group used the servo motors to show how the ribcage moved and connected this to the breathing in and out of the lungs and the movement of the diaphragm.

### Lesson Procedures:

This project incorporates several Agency by Design resources from Project Zero of the Harvard Graduate School of Education.

1. Prior to beginning the project, select which human body systems you would like your students to focus on. Possible systems include the following:
• Circulatory System
• Skeletal System
• Respiratory System
• Muscular System
• Nervous System
2. Explain the project to students. Each group will choose one of the human body systems and will construct a robotic diorama to represent that body system.
3. Divide the students into groups of 3 or 4. Allow each group to choose a body system for their project based on their interest.
4. Provide students with time to research their chosen body system. This is done in the first two weeks of the project with support from the teachers.
5. Have each group complete the Parts, Purposes, and Complexities thinking routine about their body system. The students can use this step in the process to think deeply about the parts of their system, the purposes of each part, and how they work together to perform a complex function.
6. Groups should create a sketch of how they plan to represent their body system using a robotic model.
7. Groups should complete an initial prototype in 1-2 hours. They then use the Imagine If thinking routine to imagine how they can improve their prototype.
8. Groups build and program their systems over a period of two weeks. At the beginning of each session, students should reflect on their work from the previous day and plan their next steps. They should document their process, either on paper or in a digital portfolio.
9. Groups should use the Think, Feel, Care thinking routine regularly throughout the unit to think about one person in their group and reflect on how that person is feeling, what they are thinking, and what they care about. This helps students to think deeply about other children in their group in an empathetic way and alerts you to any subtle social issues that may need to be addressed.
10. Once the allotted time for building and programming is complete, each group should present their work to the class and/or the larger school community.

## Mechanisms: Cable-Driven Robots

In this lesson, you will be building a cable-driven robot. Watch this video to see what it will look like.

The mechanism includes the following parts:

• Links are connected by rotating joints.
• A hollow guide is attached to each link.
• A cable passes through all the guides. This cable is attached to a motor.

The motor pulls on the cable to move the links. As the motor pulls the cable, the guides become closer together, which makes the links rotate about the joints. Note that the cable can only move the mechanism in one direction, to bend the linkage at the joints.

#### Materials Needed

##### Paper Templates (See Teacher Materials)

When printing the templates, be sure to print them the actual size (no scaling) on 8.5” x 11” paper. You will use the templates to cut cardboard as shown in the instructions below. Be sure to use cardboard that is less than ⅛” thick.

• Servo Unit
• Box Unit
##### Other Materials
• servo motor with bag of accessories
• straws cut into 2” pieces (12 pieces)
• yarn or cord
• optional: Tapestry needle

#### Building a Cable-Driven Robot

1. Use this video to assemble the servo motor unit.
2. Attach the servo motor to your Hummingbird board and set it to 0°.
3. Next, assemble the cable-driven mechanism using the video below. You may find it helpful to use a tapestry needle to thread the cable through the straws.
4. Reattach the servo to the Hummingbird board. Gradually increase the angle of the servo and observe the movement of the links.
5. Continue to increase the servo angle until the links stop moving. This is the maximum angle that you should use with your mechanism. Record this angle.
6. Write a program to move the servo back and forth between 0° and the maximum angle.
7. The motor acts to bend the linkage. What provides the energy to straighten the linkage? How could you use another servo to straighten the linkage?
8. We used a cable-driven mechanism to create this giraffe. What are some other ways that you might use this mechanism in a robot?

#### How the Mechanism Operates

You can use geometry to investigate how moving the cable causes the cable-driven linkage to bend at the joints.

1. The diagram below shows one joint of the cable-driven robot. We can approximate this as the triangle JAB. Assuming that the joint is midway between the two guides, what type of triangle is JAB?
2. Use your mechanism to measure the length of line segments JA and JB. Do these measurements support your answer to (1)? Explain.
3. Angle AJB in the diagram measures 110°. Draw a scaled version of triangle JAB using the measurements that you have taken so far and this angle. Lengths in the scaled version should be 10 times larger than the real-world lengths.
4. Measure the scaled length of line segment AB. What real-world length does this correspond to?
5. When the motor pulls on the cable, it acts to shorten the line segment AB. Suppose that the length of AB decreases by 20%. What is the new scaled length of this line segment?
6. Construct a new scaled version of triangle JAB using the decreased length. Measure angle AJB.
7. How has the change in cable length affected angle AJB? Explain what this means for a cable-driven robot.

You can create a cable-driven robot with multiple linkages. In this part of the lesson, you will connect two linkages to your servo motor unit. Examples with even more linkages are shown below.

1. Make sure your servo motor is set to 0°.
2. Use this video to modify your cable-driven mechanism to include two linkages. You will need a second cardboard box as a spacer. You may have one from a previous lesson, or you can make a second box using the instructions and paper template above. Alternatively, you can team up with a neighbor for this section!
3. Reattach the servo to the Hummingbird board. Gradually increase the angle of the servo and observe the movement of the links.
4. Continue to increase the servo angle until the links stop moving. This is the maximum angle that you should use with your mechanism. Record this angle.
5. Write a program to move the servo back and forth between 0° and the maximum angle.
7. We used two cable-driven linkages to create this bird. What are some other ways that you might use this mechanism in a robot?

#### Creating Cable-Driven Robots

You have already seen the giraffe and bird examples, but the you can create many other cable-driven robots! This video shows a cable-driven robotic hand.

In this lesson, the linkages were created from cardboard, but you can use other materials. This blooming flower was created with stiffened felt.

Now it is time to create your own cable-driven robot! How many linkages will you need? Will you connect them to the same servo or to different servos?

## Mechanisms: Gear Trains

In this lesson, you will be building mechanisms with gears. Watch this video to see an example.

A gear train is a mechanism that consists of two or more gears. Gears are disks with teeth that mesh together. The diagram below shows a gear train with two gears. This mechanism has the following parts:

• The drive gear is rotated by a motor.
• The teeth of the drive gear mesh with those of the driven gear.

As the drive gear rotates, its teeth turn the driven gear. How fast the driven gear rotates depends on the number of teeth it has relative to the number of teeth of the drive gear.

### Materials Needed

#### Paper Templates (See Teacher Materials)

When printing the templates, be sure to print them the actual size (no scaling) on 8.5” x 11” paper. You will use the templates to cut cardboard as shown in the instructions below. Be sure to use cardboard that is less than ⅛” thick.

• Motor Unit
• Box Unit

#### Other Materials

##### Building the Gear Train Mechanism
• Gear motor plus plastic brick adapter
• Pipe cleaner
• 3 Technic friction axle pegs
• 1 Technic 13M beam
• 2 Technic gears with 40 teeth
• 1 3M Technic axle
• 1 Technic bushing
##### Additional Materials for Investigating the Gear Ratio
• Pipe cleaner
• 1 Technic gear with 24 teeth
• 1 Technic gear with 8 teeth
• Stopwatch
##### Additional Materials for Extending the Gear Train
• More axles, bushings, and gears

#### Building the Gear Train Mechanism

1. You will need a motor unit for this lesson. You may have already built one. If not, you can use these instructions to assemble the motor unit.
2. Next, use this video to assemble your gear train mechanism.
3. Attach the motor to motor port 1 on your Hummingbird board. Write a simple program to turn on the motor. Observe the movement of the gears.
4. Does the drive gear rotate clockwise or counterclockwise? Does the driven gear rotate clockwise or counterclockwise?

#### Investigating the Gear Ratio

1. Glue a small piece of pipe cleaner to the bushing, as shown in the picture below.
2. Set the motor speed to 40 and measure the amount of time that it takes the driven gear to make ten rotations.
1. Start the stopwatch when the pipe cleaner passes the black beam.
2. Stop the stopwatch when the pipe cleaner has made ten complete rotations. The pipe cleaner should be in the same position that it was when you started the stopwatch.
3. Enter your measurement in the table below and compute the time required for one rotation of the driven gear.
3. What is the time required for one rotation of the drive gear? Explain your answer.
4. Replace the driven gear with a gear with 24 teeth. This video will show you how. Make sure that the teeth of the drive gear mesh with the teeth of the new driven gear.
5. Set the motor speed to 40 and measure the amount of time that it takes the driven gear to make ten rotations.
1. Enter your measurement in the table and compute the time required for one rotation of the driven gear.
6. Replace the driven gear with a gear with eight teeth. Make sure that the teeth of the drive gear mesh with the teeth of the new driven gear.
7. Set the motor speed to 40 and measure the amount of time that it takes the driven gear to make ten rotations.
1. Enter your measurement in the table and compute the time required for one rotation of the driven gear.
8. The gear ratio is defined as the proportion that relates the number of teeth on the driven gear to the number of teeth on the drive gear.
1. Compute gear ratio for each driven gear and enter it in the table.
9. Based on your data, write an equation that predicts the time for one rotation of the driven gear based on the gear ratio and the time for one rotation of the drive gear. You should be able to defend your equation.

#### Extending the Gear Train

A gear train can include more than two gears. Gears in between the drive gear and the driven gear are called passive gears. For example, the picture below shows a gear train with three gears. The 24-tooth gear in the middle is a passive gear.

Try building different gear trains. How does the number of gears in the train influence the direction of rotation of the driven gear?

In this lesson, the gear ratio was always greater than or equal to 1. However, this is not a requirement. Try using a smaller gear as the drive gear!

#### Using Gears to Create Robots

Gears are used in a robot design to increase or decrease the speed of a motor. Increasing the speed of the motor decreases the torque that it can apply; this means that the motor cannot apply as much force to rotate an object. Decreasing the rotation speed of the motor increases the amount of torque that it can apply. If you need your robot to rotate something heavy, you will need to use gears to decrease the rotation speed.

Gear trains are often used to in vehicles; for example, this video shows how gears are used in a car. However, gears can also be used in many other ways. The video below shows how gears were used to create a robotic Etch-A-Sketch and ballerinas that rotate at different speeds.

Now it is time to use gears to create your own robot! Do you want to increase or decrease the rotation speed of the motor? How many gears do you need in your gear train?

###### News

In the last two lessons, you learned to make mechanisms with cranks, rods, and pistons. These mechanisms are special cases of a more general type of mechanism called a linkage. In this lesson, you will explore other types of linkages. Watch this video for an example.

The diagram below shows a four-bar linkage. This mechanism has the following parts:

• Four links that are connected by four rotating joints.
• Three of the links can move, but the fourth is fixed in position. This link is called the ground link.
• One of the moving links is a crank that is rotated by a motor.

As the crank rotates, it causes two other links to move while the ground link remains fixed in position. Varying the lengths of the links produces different patterns of movement.

#### Materials Needed

##### Paper Templates (See Teacher Materials)

When printing the templates, be sure to print them the actual size (no scaling) on 8.5” x 11” paper. You will use the templates to cut cardboard as shown in the instructions below. Be sure to use cardboard that is less than ⅛” thick.

• Motor Unit
• Box Unit
##### Other Materials
###### Building the Four-Bar Linkage Mechanism
• motor unit with gear motor plus plastic brick adapter
• pipe cleaner
• 5 Technic friction axle pegs
• 4 Technic beams, two 5M and two 13M
• 11 Technic friction pegs
• 4 Technic 13M beams
• 2 Technic 5M beams
• 2 Technic 3M beams
• 1 Technic 9M beam
• motor unit with gear motor and plastic brick adapter
• small box
• pipe cleaner
• marker

#### Building the Four-Bar Linkage Mechanism

1. You should complete the crank lesson prior to this one. If you have not already completed the crank lesson, do that first. This lesson will use the motor unit from the crank lesson.
3. Attach the motor to motor port 1 on your Hummingbird board. Write a simple program to turn on the motor. Observe the movement of the four-bar linkage.

#### Grashof Condition

Not every set of link lengths will produce a working four-bar linkage. The lengths must satisfy an equation known as the Grashof condition. This equation is defined in terms of the following variables:

• s is the length of the shortest link
• l is the length of the longest link
• p is the length of one of the intermediate links
• q is the length of the other intermediate link

1. Measure the lengths of all four of the links in your mechanism (including the ground link). Which links are s and l?
2. You have seen that your linkage can move, so you know that it should satisfy the Grashof condition. Use your measurements to show that it does.
3. Next, move the pin that connects the two 13M beams. Place this pin so that it is in the center hole for both of these beams. You may have to rotate the crank to reconnect the two beams.
4. How have the link lengths changed for your mechanism? Does the linkage still meet the Grashof condition? Predict whether or not the linkage will be able to move.
6. Again, move the pin that connects the two 13M beams. Place this pin so that it is in at the end of one beam and in the fifth hole of the other (close to the joint of the ground link). You may have to rotate the crank to reconnect the two beams.
7. How have the link lengths changed for your mechanism? Does the linkage still meet the Grashof condition? Predict whether or not the linkage will be able to move.

1. Use this video to transform your four-bar linkage into a scissor linkage. You will need four more 13M beams and six more connecting pins.
2. This mechanism is based on a four-bar linkage, but it uses additional links to increase the amount of movement produced by the linkage.
3. The picture below shows a scissor linkage at two different positions.
1. As the red point on the linkage moves from height h1 to height h2, how far does the top of the linkage move? Defend your answer using congruent triangles.
4. We used the scissor linkage to animate a fish leaping out of the water. What are some other ways that you might use a scissor linkage in a robot?

1. For the next part of this lesson, you will need two motor units. You can use these instructions to assemble a second one, or you can team up with a neighbor!
2. Use the two motor units to create a new linkage for drawing. You will need a small box, a pipe cleaner, paper, a marker, two 5 M beams, two 13 M beams, two 3M beams, one 9M beam, and 11 connecting pins.
3. Attach the motors to motor ports 1 and 2 on your Hummingbird board. Write a program to turn on the motors. What does your robot draw?
4. Vary the speeds of the motor. Use a different piece of paper for each speed combination and label it with the speeds you used. This will enable you to compare your drawings more easily.
5. How do the speeds of the two motors affect what your robot draws?
6. What does the robot draw when the two speeds are equal? How is this different from the drawings when the speeds are different?
7. Your drawing robot is a five-bar linkage. Draw a picture of it and label the five links (don’t forget the ground link!).
8. A five-bar linkage can move in more different ways than a four-bar linkage. That is why you can use two cranks to make two of the links rotate independently. What would happen if you tried to attach motors to two of the links in the four-bar linkage?

#### Using Linkages to Create Robots

In this lesson, you have used different kinds of linkages to create several different robots, but there are so many more possibilities! Theo Jansen even uses linkages to create enormous works of art!

As examples, this video shows a linkage that was used to make the wings of a penguin flap, and the video below shows a robot arm that incorporates a number of linkages. How would you describe the linkages shown in these videos?

Now it is time to use linkages to create your own robot! How can you modify the linkages you used in this lesson to make something new?

## Mechanisms: Cranks with Pistons

In this lesson, you will be extending your crank mechanism to create a crank and piston mechanism. Watch this video to see what it will look like.

This mechanism has four parts:

• The crank is attached to a motor that rotates it.
• The rod is attached to the crank and the piston at joints that are free to rotate.
• The guide is fixed in place; its purpose is to make the piston move in a line. The piston is free to move up and down in a line but cannot rotate.

As the crank rotates, the piston moves up and down in a linear reciprocating motion. A crank and piston system transforms rotational motion into linear motion. The linear motion can be vertical or horizontal (or in another direction), depending upon the orientation of the guide.

#### Materials Needed

##### Paper Template (See Teacher Materials)

When printing the template, be sure to print it the actual size (no scaling) on 8.5” x 11” paper. You will use the template to cut cardboard as shown in the instructions below. Be sure to use cardboard that is less than ⅛” thick.

• Piston Unit
##### Other Materials
• crank mechanism (from crank lesson)
• 1 Technic friction axle peg
• 1 Technic 13M beam
• pipe cleaner
• ruler or tape measure
• stopwatch

#### Building the Crank and Piston Mechanism

1. You will need a crank mechanism for this lesson. If you have not already completed the crank lesson, do that first.
2. Next, use this video to assemble your crank and piston mechanism.
3. Attach the motor to motor port 1 on your Hummingbird board. Write a simple program to turn on the motor. Observe the movement of the mechanism.

#### Graphing the Position of the Piston

Think about starting a timer when you turn on the motor. As the seconds pass, the crank rotates and the piston moves up and down. We could make a graph with time on the x-axis and the position of the piston on the y-axis. This graph would look something like the curve shown below.

The figure above shows only a single rotation of the crank. As the crank rotates again and again, this curve would be repeated. This type of periodic motion is called a wave.

1. The highest point of a wave is called the peak, and the lowest point is called the trough. Label one peak and one trough on the graph above.
2. The distance between the peak and the trough is called the wave height. Label the wave height on the graph above.
3. How can you find the wave height for the piston? Measure this value and then compare your method and answer with your classmates.
4. A wave is often described by its amplitude instead of its wave height. The amplitude is half of the wave height. Find the amplitude of the piston wave.

#### Changing the Length of the Crank

Now you will investigate how you can change the piston wave by changing the length of the crank. You can change the length of the crank by using the other holes along the length of the crank.

1. Move the connecting pin at the end of the crank to the neighboring hole.
2. Measure the amplitude of the piston wave.
3. Change the length of the crank again. This time, place the connecting pin between the two connecting pins that connect the crank to the motor adapter.
4. Measure the amplitude of the piston wave.
5. How is the amplitude of the piston wave related to the length of the crank?
6. Can the rod ever be shorter than the crank? Why or why not?

#### Period of the Piston Wave

The time period between one peak and the next is called the period of a wave.

1. Set the speed of the motor to 20.
2. Use a stopwatch to measure how long it take the crank to rotate 10 times.
3. What is the period of the wave?
4. Complete the table below.
5. How is the period of the wave related to the speed of the motor? Predict the period for a speed of 50 and give evidence for your answer.

#### Using Cranks and Pistons to Create Robots

Crank and piston mechanisms are used in robots to produce linear movement in a particular direction. For example, this video shows a project in which a piston is used to move a character up and down. Can you identify the parts of the mechanism in the video? This robotic turtle also uses a crank and piston. What might be inside the turtle’s shell?

Now try it out in your own robot! How far do you want to move the piston? How long do the crank and connecting rod need to be to make this happen? Remember, the piston does not have to move only vertically or horizontally. It can move along a straight line in any direction!

• Crank and Slider Mechanism: This website animates the movement of a crank and piston mechanism and describes its parts.
• Diesel Engine: This video shows how a diesel engine uses a crank and piston mechanism in a car or truck. In this case, the explosion of the fuel produces the linear motion of the piston, and the mechanism transforms this motion to rotate the wheels of the vehicle.

## Mechanisms: Cranks

In this lesson, you will be building a crank mechanism. Watch this video to see what it will look like.

This mechanism has three parts:

• The crank is attached to a motor that rotates it.
• The rod is attached to the crank using a connecting pin. The rod is free to rotate about this pin.
• The guide is fixed in place; its purpose is to keep the rod in place horizontally. The rod is free to rotate and move vertically.

As the crank rotates, the rod moves back and forth. This type of back and forth motion is called reciprocating motion.

#### Materials Needed

##### Paper Templates (See Teacher Materials)

When printing the templates, be sure to print them the actual size (no scaling) on 8.5” x 11” paper. You will use the templates to cut cardboard as shown in the instructions below. Be sure to use cardboard that is less than ⅛” thick.

• Motor Unit
• Box Unit
##### Other Materials
• Gear motor plus plastic brick adapter
• 3 Technic friction axle pegs
• 2 Technic beams: 5M and 13M
• Pipe cleaner
• Cardboard
• Construction paper
• Pencil

#### Building the Crank Mechanism

1. You will need a motor unit for this lesson. You may have already built one. If not, you can use these instructions to assemble the motor unit.
2. Next, use this video to assemble your crank mechanism.
3. Attach the motor to motor port 1 on your Hummingbird board. Write a simple program to turn on the motor. Observe the movement of the crank and the rod.
4. Change the speed of the motor. How does this change the movement of the rod?

#### Calculating How Far the Crank Moves

You can use geometry to calculate how far the crank will move horizontally and vertically.

1. This diagram shows an idealized version of the crank mechanism. The length of the crank is c, and r is the length of the rod. G is the point where the guide keeps the rod in place. The rod must always pass through this point.
2. Draw the crank and guide when the rod is at its highest point. What are the coordinates of the top of the rod in terms of c and r?
3. Draw the crank and guide when the rod is at its lowest point. What are the coordinates of the top of the rod in terms of c and r?
4. In terms of c and r, what is the vertical distance that the top of the rod moves between its highest and lowest points?
5. Measure c and r for your crank mechanism. The length of the rod should be measured from the center of the connecting pin to the center of the hole at the end of the rod. The length of the crank should be measured from the center of the motor to the center of the connecting pin at the end of the crank, as shown in the picture below.
6. Use your measurements to calculate the vertical distance that the rod moves.
7. Next, you will find the horizontal distance that the rod moves as it travels. The crank mechanism is shown below with the crank lying along the x-axis. The distance h is the maximum distance that the top of the rod moves to the left of the y-axis.
8. What similar triangles can you find in the diagram above? Use the properties of similar triangles to fill in the blanks in the equation below.
9. Consider the triangle below point G. We will call the height of this triangle a. Measure the value of a for your crank mechanism by measuring the vertical distance from the center of the motor to the guide.
10. Now that you have values for a and c, find the length of the third side of the triangle in the diagram above.
11. Now you have enough information to find h! Use your equation from (8) to solve for h. What is the total horizontal distance that the end of the rod moves?

#### Finding the Path of the Rod

Next, you will trace the path of the end of the rod. This way, you can get a better idea of the type of movement caused by a crank. This process is shown in this video and described below.

1. Tape a piece of paper onto a piece of cardboard about the same size. Place this behind the end of the rod.
2. Place a pencil through the top hole of the rod.
3. As the rod moves, use the pencil to trace its path. Try not to affect the motion of the rod; just follow it with the pencil.
4. Remove the paper and the pencil. Darken the path you traced and measure its maximum width and height.
5. How do these values compare with the ones that you calculated above?

#### Changing the Length of the Crank

Now you will investigate how you can change the motion of the mechanism by changing the length of the crank. You can change the length of the crank by using the other two holes along the length of the crank.

1. Use this video to change the length of the crank. Move the connecting pin to the next hole on the crank.
2. Trace the path of the rod. Measure the maximum width and height of the new path.
3. Change the length of the crank again. This time, place the connecting pin between the two pins that connect the crank to the motor adapter.
4. Trace the path of the rod. Measure the maximum width and height of the new path.
5. How does changing the length of the crank affect the path of the rod?

#### Using Cranks to Create Robots

Cranks can be incorporated into a robot design in many different ways. Two examples are shown in this video. Can you identify the parts of the mechanisms in this video? In the fish example, how has the crank mechanism been modified?

Now it is time to use a crank to create your own robot! How can you use reciprocating motion to make make something interesting? Do you need to modify the crank, rod, or guide for your design?