This unit builds toward the following NGSS Performance Expectations (PEs):

• MS-PS2-1 Apply Newton’s Third Law to design a solution to a problem involving the motion of two colliding objects.
• MS-PS2-2. Plan an investigation to provide evidence that the change in an object’s motion depends on the sum of the forces on the object and the mass of the object.
• MS-PS3-1. Construct and interpret graphical displays of data to describe the relationships of kinetic energy to the mass of an object and to the speed of an object.
• MS-ETS1-2.* Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem.
• MS-ETS1-3.* Analyze data from tests to determine similarities and differences among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success.
• MS-LS1-8.* Gather and synthesize information that sensory receptors respond to stimuli by sending messages to the brain for immediate behavior or storage as memories.

*PEs marked with an asterisk are partially developed in this unit and shared with other units, as explained in the DCI column. 

This unit helps develop the following elements of Disciplinary Core Ideas (DCIs):

• PS2.A. For any pair of interacting objects, the force exerted by the first object on the second object is equal in strength to the force that the second object exerts on the first, but in the opposite direction (Newton’s third law). In Lesson 5, students figure out that the strength of the contact forces on each of the two objects in contact with each other are equal and in opposite directions of each other. This idea is reused in Lesson 6 and to a lesser extent Lessons 7-17.
• PS2.A. The motion of an object is determined by the sum of the forces acting on it; if the total force on the object is not zero, its motion will change. The greater the mass of the object, the greater the force needed to achieve the same change in motion. For any given object, a larger force causes a larger change in motion. In Lessons 2-4, students start building this idea. In Lessons 7-8, students explore how increasing the strength of a force applied from a spring scale launcher to a cart affects the speed of the cart when it is launched. They figure out that after increasing the mass of the cart a larger amount of force needed to be applied to it to get it to launch at the same speed. Students determine that when greater force is applied to a constant mass, the speed of the object will increase. Students reuse this idea in Lessons 9 and 11, with application in Lessons 12 and 13 to thinking around friction and air resistance. These ideas are also important in students’ design work.
• PS2.A. All positions of objects and the directions of forces and motions must be described in an arbitrarily chosen reference frame and arbitrarily chosen units of size. In order to share information with other people, these choices must also be shared. Students are introduced to frames of reference in Lesson 1 when they categorize all collisions that occur in any direction into one of three categories. Students identify a reference height to measure the deformation of an object in Lesson 4 and compare different units for measuring the size of a force in Lesson 5. They switch between measuring forces in different scales when they record peak contact forces in a collision in this lesson, and also in subsequent lessons. 
• PS3.A. Motion energy is properly called kinetic energy; it is proportional to the mass of the moving object and grows with the square of its speed. Students recall their previous use of kinetic energy in the context of particle level collisions from the Cup Design Unit in Lesson 2. In lesson 5, they identify the relative differences in kinetic energy for moving vs. non-moving carts with more or less mass or speed. In Lesson 7, they gather data to determine that the kinetic energy of an object is proportional to its mass and that it grows with the square of its speed.
• PS3.B. When the kinetic energy of an object changes, there is inevitably some other change in energy at the same time. In Lesson 8, students develop a model to show that energy is stored in part of the launcher system and is transferred out of the system when the cart is launched. In Lesson 12, students extend this model to show where energy is being transferred to the surroundings in a cart-box system.
• PS3.C. When two objects interact, each one exerts a force on the other that can cause energy to be transferred to or from the object. In Lesson 8, students identify contact forces as the mechanism for energy transfer in a collision. They apply this idea in Lesson 12 to explain that air resistance and friction transfer energy to the surroundings due to contact force interactions between two objects or an object and many particle collisions.
• ETS1.B. There are systematic processes for evaluating solutions with respect to how well they meet the criteria and constraints of a problem. Students develop criteria and constraints for their sensor system design in Lesson 14, use these to inform their design in Lesson 15, and give feedback to other groups based on evaluations of each others’ designs in Lesson 16.
• ETS1.B. Sometimes parts of different solutions can be combined to create a solution that is better than any of its predecessors. Students consider how aspects of different designs could be combined in Lesson 16.
• ETS1.C. Although one design may not perform the best across all tests, identifying the characteristics of the design that performed the best in each test can provide useful information for the redesign process—that is, some of those characteristics may be incorporated into the new design. Students analyze the performance of their designs and iterate on them in Lesson 15.
• LS1.D. Each sense receptor responds to different inputs (electromagnetic, mechanical, chemical), transmitting them as signals that travel along nerve cells to the brain. The signals are then processed in the brain, resulting in immediate behaviors or memories. Students develop this idea through a reading in Lesson 5. The struck-through portion of this DCI is developed in Unit 6.1 and Unit 7.1.

**Disciplinary core ideas are reproduced verbatim from A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas. DOI: https://doi.org/10.17226/13165. National Research Council; Division of Behavioral and Social Sciences and Education; Board on Science Education; Committee on a Conceptual Framework for New K-12 Science Education Standards. National Academies Press, Washington, DC. This material may be reproduced and used by other parties with this attribution. If the original material is altered in any way, the attribution must state that the material is adapted from the original.

While this unit engages students in multiple SEPs across the lesson-level performance expectations for all the lessons in the unit, there is one focal practice that this unit targets to support students’ development in a learning progression across the 8th grade year for the SEPs:

• Analyzing and Interpreting Data

In addition, there are three supporting practices that students will utilize over the course of the unit:

• Asking Questions and Identifying Problems
• Developing and Using Models
• Planning and Carrying Out Investigations
• Constructing Explanations and Designing Solutions
• Engaging in Argument from Evidence

This unit is designed to be taught at the start of 8th grade. If it is taught in 6th or 7th grade, be prepared to provide students greater support in the following SEPs. For more detailed information about the focal Science and Engineering Practices in this unit, read the unit front matter in the Teacher Edition. 

While this unit engages students in multiple CCCs across the lesson-level performance expectations for all the lessons in the unit, there are two focal CCCs that this unit targets to support students’ development in a learning progression for the CCCs across the 8th grade year:

• Structure and Function
• Stability and Change

In addition, there are four supporting crosscutting concepts that are key to the sensemaking that students will utilize over the course of the unit:

• Patterns
• Cause and Effect
• Systems and System Models
• Energy and Matter

Which elements of the Nature of Science are developed in the unit?

• Science investigations use a variety of methods and tools to make measurements and observations. (NOS-SEP)
• Advances in technology influence the progress of science and science has influenced advances in technology. (NOS-CCC)

How are they developed?

• Students use their fingers as force sensors and a peak contact force collar to measure the contact forces on two colliding objects, and they use a variety of materials such as a rubber band to measure the amount of friction from different surfaces and dissect various cushioning materials. They collect data using push-pull force scales, rulers, slow motion videos, reflected laser beams, and computational simulations. As the unit progresses, they also explore a variety of ways different systems can be used to measure forces.
• Students explore how micro:bit technologies allow them to measure forces much more easily and recognize that these devices require significant understanding of science, particularly electricity.

Which elements of ETS are developed in the unit?

• Engineering advances have led to important discoveries in virtually every field of science and scientific discoveries have led to the development of entire industries and engineered systems.
• Science and technology drive each other forward.

How are they developed?

• Students use their science understanding from the unique to engineer new solutions and use new technologies to drive forward their science thinking.

This unit builds toward the following Computer Science Teachers Association (CSTA) Standards:

• 2-CS-02 Design projects that combine hardware and software components to collect and exchange data.
• 2-CS-03 Systematically identify and fix problems with computing devices and their components.
• 2-DA-08 Collect data using computational tools & transform the data to make it more useful and reliable.
• 2-DA-09 Refine computational models based on the data they have generated.
• 2-AP-10 Use flowcharts and/or pseudocode to address complex problems as algorithms.
• 2-AP-11 Create clearly named variables that represent different data types & perform operations on their values.
• 2-AP-12 Design and iteratively develop programs that combine control structures, including nested loops and compound conditionals.
• 2-AP-13 Decompose problems and subproblems into parts to facilitate the design, implementation, and review of programs.
• 2-AP-15 Seek and incorporate feedback from team members and users to refine a solution that meets user needs.
• 2-AP-16 Incorporate existing code, media, and libraries into original programs, and give attribution.
• 2-AP-19 Document programs in order to make them easier to follow, test, and debug.
• 2-IC-21 Discuss issues of bias and accessibility in the design of existing technologies.

CSTA standards are reproduced verbatim from: Computer Science Teachers Association (2017). CSTA K-12 Computer Science Standards, Revised 2017. https://csteachers.org/k12standards/.

The computer science integrated version of Unit 8.1 keeps the same core science content and performance expectations as the original, but adds lessons and activities where students learn and apply CS tools like micro:bits, sensors, and block-based programming. These additions expand Lesson Set 2 and significantly modify Lesson Set 3 to support a new engineering design project focused on building sensor systems that measure and respond to forces.

For a complete overview of the differences between 8.1 Contact Forces + Computer Science and the original unit download the overview document.

This unit is anchored in the phenomenon of cell phones breaking, expanding along the way to bring in other collisions and applications in which forces are measured. This unit begins with students considering national statistics on the frequency and cost of cell phone breakage. Students share situations in which they have seen cell phones break. Students then contrast these situations with other situations where something else collided with another object and either broke or, surprisingly, did not break. Students then attempt to identify the factors that contribute to damage occurring in some collisions and not others, as well as try to explain what is happening during the collision that causes some items to become damaged in a collision when others are not. Students then develop a Driving Question Board (DQB) to guide future investigations.

This introduction, using a commonly broken and widely used device, allows students to investigate ideas regarding energy and forces in a collision. The ideas of deformation and breaking point examined in Lesson Set 1 apply widely to phone use, as some collisions result in damage while others surprisingly do not. In Lesson set 2, students examine other collisions and force measurement more generally.

Lesson Set 3 builds on the force measurement questions in Lesson Set 2 to give students an opportunity to design, build, and test a sensor system for applications where force measurement is needed.

Each OpenScied unit’s anchoring phenomenon is chosen from a group of possible phenomena after analyzing student interest survey results and consulting with external advisory panels. We also chose cell phone breakage as the first anchoring phenomenon for this unit for these reasons:

• This anchor ranked higher than the top alternative (related to bike helmets) in a pre-field release student survey.
• +80% of teenagers own cell phones, and those who don’t are around classmates who do. Nearly all students, therefore, have multiple interactions with people on a daily basis who have these devices, even if they do not own one themselves.
•  A subsequent pilot study of this anchor confirmed that witnessing the type of phenomena referenced in the anchor, some sort of collision that caused someone’s cell phone to get damaged, was an extremely common occurrence.
• Cell phones are devices that people commonly buy protective cases for.
• Two pre-field test pilots of the cell phone damage) anchor produced driving question boards that had a majority of the students’ questions on them and ideas for investigations to answer those questions, which were anticipated by the unit development team, and were specifically targeted in the field test version of the storyline.

We chose to have a more open-ended design problem for these reasons:

• First, students engage with a variety of other collisions and possible force measurement applications in Lesson Set 2, so students should ideally have the opportunity to focus on an application that interests them.
• Second, students are familiar with a variety of sensors but may find one more interesting or useful. A more open-ended project allows students to select a sensor system they feel they can be successful with.
• Third, teachers always have the ability to limit the sensors used or the design problem if they are have logistics concerns, but starting with a more open-ended project (with enhanced scaffolds from the field test) is intended to support student creativity.

This unit is broken into three lessons, each of which helps make progress on a sub-question related to the driving question for the entire unit. Lessons 1-6 focus on developing science ideas about force interactions between colliding objects. Lessons 7-13 focus on the relationship between forces and energy transfer in collisions, and how we can use sensors to measure those relationships. Lessons 14-17 focus on design solutions that measure forces using sensor technology. The icons in the image below represent where computer science concepts and practices are explored and utilized in the unit.

This unit is designed to be taught as the first unit of 8th grade. It is designed to be taught after students have experienced Unit 6.2 and Unit 6.6 or equivalent units. As such, work in this unit can leverage ideas about the particle nature of matter, how energy can be transferred through particle-level collisions (conduction), how neurons work together to transfer signals from our senses to our brain. It is also designed to be taught after students have experienced the Unit 7.2, as it leverages ideas about the role of criteria, constraints, stakeholders, and tradeoffs in the engineering design process.

This unit provides some supports to introduce micro:bits and Makecode, but does so relatively quickly with some recommendations for additional supports if students need them. This introduction will go smoother if students have prior experiences with computer science in OpenSciEd from OpenSciEd Unit 6.3: Why does a lot of hail, rain, or snow fall at some times and not others? (Storms Unit – CS Unit), OpenSciEd Unit 6.5: Where do natural hazards happen and how can we use science, technology and engineering to detect, warn, or protect people from them? (Natural Hazards + Computer Science), and/or OpenSciEd Unit 7.6: How do changes in Earth’s system impact our communities and what can we do about it? (Droughts and Floods + Computer Science).

This unit is designed to be taught prior to OpenSciEd Unit 8.2: How can a sound make something move? (Sound Unit) or OpenSciEd Unit 8.2: How can a sound make something move? (Sound + Computer Science). Either unit will leverage ideas about how forces transfer energy across a system and that all solid matter can be elastically deformed in a collision that are developed in this unit. It is also designed to be taught prior to OpenSciEd Unit 8.3: How can a magnet move another object without touching it? (Magnets Unit). That unit will leverage ideas about how every force is part of a force pair, that are two equal and opposite forces on two different objects.

This is the first unit in 8th grade in the OpenSciEd Scope and Sequence. Given this placement, several modifications would need to be made if teaching this unit earlier or later in the middle school curriculum. These include the following adjustments:

• If taught before the Unit 6.2, supplemental teaching of the following would be required:

• • Energy transfer as the result of two colliding objects at the particle level.
  • Understanding of the role of independent and dependent variables, along with controlled variables, in an investigation.
  • What criteria and constraints are, and how they can be used to inform design decisions.

• If taught before Unit 6.5, supplemental teaching of the following would be required:

• • What a stakeholder is, and the role of stakeholders in the iterative design process.
• If taught before Unit 6.3 + Computer Science, Unit 6.5 + Computer Science, Unit 7.6 + Computer Science, examination of the tilt sensor code and other extensions recommended in Lesson 11 might be beneficial to give students more practice with programming.

In general, this unit is taught using a conceptual approach to describing the relationship among force, mass, and change in motion during collisions, students need only have experience with qualitatively reasoning about positive and negative associations (e.g., as force increases, change in motion increases; but as mass increases, change in motion from a given force decreases). Because the focus of MS-PS3-1 is on quantitative understanding of the relationship of the kinetic energy of an object to the mass of an object and to the speed of an object, students will need to leverage experiences from grade 7 Common Core Mathematics Standards in Lessons 7 and 13.

In Lesson 7, students will be working with unit rates and ratios. By the beginning of 8th grade, students should be well-versed in how to do this calculation. It will be leveraged in Lessons 7 and 13 of this unit when students recognize that the relationship between mass and kinetic energy is directly proportional. Such a relationship is one they have encountered in graphs many times in Common Core Mathematics since 6th grade.

Recognizing the relationship between speed and kinetic energy as nonlinear will also be straightforward, but describing the change in kinetic energy as being related to the square of the speed of an object will be challenging. Students will have worked with squared relationships in 6th grade when finding the surface area of a cube with sides of lengths, and in 8th grade, they will be squaring the side lengths of a right triangle in their work with the Pythagorean theorem. Coordinate with your math teachers to determine where your students will be in their familiarity with thinking about relationships like these.

• In Lesson 7 students calculate and use a type of ratio called a scale factor. They will use this idea again in the Lesson 13 assessment. Students will have encountered this concept before in math class in one or both of these contexts:

• • 7.G.A.1 Solve problems involving scale drawings of geometric figures, including computing actual lengths and areas from a scale drawing and reproducing a scale drawing at a different scale.
  • 7.RP.A.2 Recognize and represent proportional relationships between quantities.
• There are multiple connections between the work students will be doing in Lesson 7 and the work they will be doing in math class this year (grade 8). These include the following:
  • 8.F.B.5 Describe qualitatively the functional relationship between two quantities by analyzing a graph (e.g., where the function is increasing or decreasing, linear or nonlinear).

• • 8.EE.A.1 Know and apply the properties of integer exponents to generate equivalent numerical expressions.
• In Lesson 4, students are introduced to lines of best fit. It is assumed that Lesson 4 is the students’ first introduction to the idea.

The following are example options to shorten or condense parts of the unit without completely eliminating important sensemaking for students:

• If all students are very familiar with micro:bits and Makecode, Lessons 9 and 11 may be condensed by skipping the sensor stations and having students only make sense of the flex sensor code and digital load cell code before moving on to Lesson 12.
• If there is not time for a full engineering design project, students could be asked to iterate on the sensor systems in Lesson 12, then omit Lessons 14-17.
• Lesson 17 is listed as optional since students have some opportunities to share their work with peers in Lesson 16.

To extend or enhance the unit, consider the following:

• Lesson 3: Consider letting students investigate the deformation of a table and other rigid materials in small groups using the laser setup. If this option is utilized, consider all proper safety precautions when using glass with students, such as safety goggles, gloves for potential sharp edges, and proper distribution and cleanup procedures that minimize encounters with any potential broken glass or other materials. See the materials preparation section of this lesson for more guidance.
• Lesson 3: Add in additional slow-motion videos in areas of student interest, such as a football making contact with the ground for classrooms that have several students engaged in football.
• Lesson 4: Expand the investigation to allow multiple groups to test multiple conditions. This would involve an increased number of materials and increased class time.
• Lesson 5: Allow students to spend more time at each investigation station. Ask students to test out each station with increased mass, increased speed, and with a variety of moving and non-moving carts.
• Lesson 6: Ask students to also revisit the related phenomena. Ask students to pick a related phenomenon and explain the outcomes of the related phenomenon (damage, no damage) using our science ideas. At this point, students should be able to construct a partial explanation for their related phenomena.
• Lesson 13: Ask students to revisit their related phenomena once again and attempt to explain the outcomes of the collisions. At this point, students should be able to explain the forces on each object and the energy transfer that occurs in the collision.
• Lessons 16-17:  Give students more time to iterate on their designs after receiving feedback in these lessons.

• Dan Voss, Unit Lead, Northwestern University
• Nicole Vick, Unit Lead, Northwestern University
• Michael Novak, Field Test Unit Lead, Northwestern University
• Nicole Vick, Field Test Unit Lead, Northwestern University
• Jacob Noll, Writer, Northwestern University
• Diego Rojas-Perilla, Coherence Reviewer, BSCS
• Cari Williams, Field Test Coherence Reviewer, Independent Consultant
• Dominique Poncelet, Co-Design Teacher, Oklahoma City Public Schools
• Kris Grymonpre, Co-Design Teacher, Boston Public Schools
• Hillary Paul Metcalf,  Professional Learning Designer, Center to Support Excellence in Teaching at Stanford University
• Rachel Zulick, Professional Learning Designer, Center to Support Excellence in Teaching at Stanford University
• Cari Williams, Computer Science Integration Advisor, Independent Consultant

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