Unit Overview
8.1 Contact Forces

Why do things sometimes get damaged when they hit each other?

Unit Summary

Oh, no! I’ve dropped my phone! Most of us have experienced the panic of watching our phones slip out of our hands and fall to the floor. We’ve experienced the relief of picking up an undamaged phone and the frustration of the shattered screen. This common experience anchors learning in the Contact Forces unit as students explore a variety of phenomena to figure out, “Why do things sometimes get damaged when they hit each other?”

Student questions about the factors that result in a shattered cell phone screen lead them to investigate what is really happening to any object during a collision. They make their thinking visible with free-body diagrams, mathematical models, and system models to explain the effects of relative forces, mass, speed, and energy in collisions. Students then use what they have learned about collisions to engineer something that will protect a fragile object from damage in a collision. They investigate which materials to use, gather design input from stakeholders to refine the criteria and constraints, develop micro and macro models of how their solution is working, and optimize their solution based on data from investigations. Finally, students apply what they have learned from the investigation and design to a related design problem.

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Unit Examples


Additional Unit Information

Building Toward the Following Standards and Practices
Performance Expectations

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

Physical Science PEs

  • MS-PS2-1 Apply Newton’s Third Law to design a solution to a problem involving the motion of two colliding objects. [Clarification Statement: Examples of practical problems could include the impact of collisions between two cars, between a car and stationary objects, and between a meteor and a space vehicle.] [Assessment Boundary: Assessment is limited to vertical or horizontal interactions in one dimension.]
  • MS-PS-2-2 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. [Clarification Statement: Emphasis is on balanced (Newton’s First Law) and unbalanced forces in a system, qualitative comparisons of forces, mass and changes in motion (Newton’s Second Law), frame of reference, and specification of units.] [Assessment Boundary: Assessment is limited to forces and changes in motion in one dimension and in an inertial reference frame and to change in one variable at a time. Assessment does not include the use of trigonometry.]
  • 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. [Clarification Statement: Emphasis is on descriptive relationships between kinetic energy and mass separately from kinetic energy and speed. Examples could include riding a bicycle at different speeds, rolling different sizes of rocks downhill, and getting hit by a wiffle ball versus a tennis ball.]
  • 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.

The following PE will be developed over three OpenSciEd units: OpenSciEd Unit 6.1: Why do we sometimes see different things when looking at the same object? (One-way Mirror Unit), OpenSciEd Unit 7.1: How can we make something new that was not there before? (Bath Bombs Unit), and OpenSciEd Unit 8.2: How can a sound make something move? (Sound Unit). This unit will address only the mechanical inputs that transmit signals to the brain through touch. The other units will address electromagnetic and other mechanical inputs (sound) and chemical inputs as well as the connection to signals processing in the brain. This unit, however, does make an important connection to how those signals are stored as memories and how damage to particular structures (axons on neurons) can cause memory loss in concussions.

  • 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. [Assessment Boundary: Assessment does not include mechanisms for transmission of this information.]
Disciplinary Core Ideas

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

PS2.A: Forces and Motion

  • 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).
  • 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.
  • 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.

PS3.A: Definitions of Energy

  • 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.

ETS1.B: Developing Possible Solutions

  • There are systematic processes for evaluating solutions with respect to how well they meet the criteria and constraints of a problem.
  • Sometimes parts of different solutions can be combined to create a solution that is better than any of its predecessors.

ETS1.C: Optimizing the Design Solution

  • 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.

LS1.D: Information Processing

  • 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.

Besides the disciplinary core ideas that are part of the foundation boxes for the target PEs in this unit, additional connections to the following DCIs are also developed and used in this unit:

PS3.B: Conservation of Energy and Energy Transfer

  • When the kinetic energy of an object changes, there is inevitably some other change in energy at the same time.

PS3.C: Relationship Between Energy and Forces

  • When two objects interact, each one exerts a force on the other that can cause energy to be transferred to or from the object.
Focal Science & Engineering Practices

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:

  • Planning and Carrying Out Investigations

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

  • Analyzing and Interpreting Data
  • 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. 

Focal Crosscutting Concepts

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

  • Systems and system models
  • Energy and matter
  • Structure and function
  • Stability and change

For more detailed information about the focal Crosscutting Concepts in this unit, read the unit front matter in the Teacher Edition. 

Unit Information
What are prerequisite math concepts necessary for the unit?

Because 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 direct and inverse relationships (e.g., as force increases, change in motion increases; but as mass increases, change in motion from a given force decreases).

But 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 the following experiences from grade 7 CCS math to use in this unit in Lessons 7 and 10:

  • Working with unit rates and ratios is within the 6th and 7th grade Common Core Math Standards. 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 10 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 encountered working with squared relationships in 6th grade in finding the surface area of a cube with sides of length s, and in 8th grade they will be working with squaring the side lengths of a right triangle in their work with the Pythagorean theorem. Coordinate with your math teachers to determine where you students will be at in their familiarity with thinking about relationships like these.
  • Students calculate and use a type of ratio called a scale factor in Lesson 7. 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.

Because data from investigations are often scattered due to source of error, even when the expected trend in the data is linear, students are introduced to lines of best fit in Lesson 4 in this unit. They reuse this construct in the Lesson 10 assessment. Students do not have to have encountered this idea in previous mathematics instruction. Lesson 4 assumes that this may be the first time students encountered this idea.

What modifications will I need to make if this unit is taught out of sequence?

This is the first unit in 8th grade in the OpenSciEd Middle School 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. 

Before this unit, students should understand the following ideas from their experiences in OpenSciEd Unit 6.1: Why do we sometimes see different things when looking at the same object? (One-way Mirror Unit) and in Unknown material with identifier: cl:

  • The brain pieces together the information to tell us what we “see.”
  • Large groups of cells work together to form tissues or organs that work together to perform particular body functions.

They should also understand the following ideas from their experiences in Cup Design Unit and Storms Unit:

  • Motion energy is properly called kinetic energy (part of PS3.A).
  • Temperature is a measure of the average kinetic energy of particles of matter (part of PS3.A).
  • Energy is transferred in particle-level collisions within and between gases, solids, and liquids (e.g. through conduction).
Unit Acknowledgements
Unit Development Team
  • Michael Novak, Unit Lead, Northwestern University
  • Susan Kowalski, Field Test Unit Lead, BSCS Science Learning
  • Zoë Buck Bracey, Writer, BSCS Science Learning
  • Joel Donna, Writer, University of Wisconsin – River Falls
  • Shelly Ledoux, Writer, The Dana Center at University of Texas – Austin
  • Dawn Novak, Writer and Reviewer, BSCS Science Learning
  • Whitney Smith, Writer, BSCS Science Learning
  • Tara McGill, Review, Northwestern University
  • Christina Schwarz, Unit Advisory Chair, Michigan State University
  • Thomas Clayton, Teacher Advisor, Columbia Middle School, Berkeley Heights, NJ
  • Amanda Leighton, Teacher Advisor, Haddonfield Middle School, Haddonfield, NJ
  • Katie Van Horne, Assessment Specialist
Production Team

BSCS Science Learning

  • Stacey Luce, Editorial Production Lead and Copyeditor
  • Valerie Maltese, Marketing Specialist & Project Coordinator
  • Renee DeVaul, Project Coordinator
  • Chris Moraine, Multimedia Graphic Designer
Unit External Evaluation
NextGenScience’s Science Peer Review Panel

An integral component of OpenSciEd’s development process is external validation of alignment to the Next Generation Science Standards by NextGenScience’s Science Peer Review Panel using the EQuIP Rubric for Science. We are proud that this unit has earned the highest score available and has been awarded the NGSS Design Badge. You can find additional information about the EQuIP rubric and the peer review process at the nextgenscience.org website.