Unit 4.2 Why do objects change their motion?

By the end of this unit, students will be able to:

• Conduct an investigation to provide evidence of the effects of balanced and unbalanced forces on the motion of an object.
• Explain how forces are connected to accelerations/changes in velocity utilizing appropriate words to describe the types of forces.
• Use mathematical representations and proportionality to support a claim regarding position, velocity, and time.
• Analyze data to support the claim that Newton's second law of motion describes the mathematical relationship among the net force on an object, its mass, and its acceleration.

These materials provide an introduction to Newton's three laws of motion and make connections to position, velocity, and acceleration, common topics in any lower-level physical science course. This module is a crucial building block toward our ultimate goal of understanding motions related to a Rube Goldberg Machine and similar obstacle-course challenges (this unit's motivating question), and in particular, students will be participating in a jigsaw activity to learn about and analyze forces at work that they will apply in the end-unit Rube Goldberg Challenge. In addition, prior concepts related to energy (unit 2), density (unit 3) and motion (unit 4.1) are synthesized to help explain Newton's Laws. Students also return to systems thinking and the Hunga Tonga volcanic eruption, which are topics first discussed in Unit 2 (Unit 2.5 on System Mapping in particular).

The materials in this unit should take about 150 min of class time. Most lab exercises rely on small group work and are best suited to smaller classes or a lab meet-up outside of a traditional lecture room. However, most of the "lab" activities in this unit can be adapted to be demonstrations in a more traditional lecture-style class with think-pair-share activities.

Teaching Materials:

All Slides: Unit 4.2 All Slides v2 (PowerPoint 2007 (.pptx) 5.6MB Aug30 24)

Motion Diagrams Handout: Word: Unit 4.2 Motion Diagrams Handout v2 (Microsoft Word 2007 (.docx) 567kB Aug30 24)
PDF: U4.2 Motion Diagrams Handout.pdf (Acrobat (PDF) 486kB Jul12 24)

Forces Jigsaw Activity: Full resource: Unit 4.2 Forces jigsaw All v2 (Microsoft Word 2007 (.docx) 949kB Aug30 24)
Applied & Normal: Unit 4.2 Forces jigsaw applied normall v2 (Microsoft Word 2007 (.docx) 232kB Aug30 24)
Electric & Magnetic: U4.2 Forces Jigsaw Electric & Magnetic.docx (Microsoft Word 2007 (.docx) 447kB Jul12 24)
Friction & Air Resistance: U4.2 Forces Jigsaw Friction & Air Resistance.docx (Microsoft Word 2007 (.docx) 452kB Jul12 24)
Tension & Spring: U4.2 Forces Jigsaw Tension & Spring.docx (Microsoft Word 2007 (.docx) 499kB Jul12 24)
Gravity & Buoyancy Unit 4.2 Forces jigsaw gravity buoyancyl v2 (Microsoft Word 2007 (.docx) 168kB Aug30 24):

Forces Stations Lab: Student handout: U4.2 Forces Lab - Student.docx (Microsoft Word 2007 (.docx) 264kB Jul12 24)
Instructor Notes:

U4.2 Forces Lab - Instructor Copy.docx -- educator-only file

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Other Materials: Golf ball & Ping Pong ball (or any two objects that have similar shape but very different masses so they will fall with the same acceleration when dropped), computer and projector, white board or chalk board with markers/chalk.

Scientist Spotlight Full Resource (In this unit: Edward Bouchet): Scientist Spotlight Slides (PowerPoint 2007 (.pptx) 4.6MB Jul8 24)

Reflection Assignment: U4.2 Reflection Assignment and Rubric.docx (Microsoft Word 2007 (.docx) 543kB Jul12 24)

The Lab(s) is/are assessed as a Science Journal, as always. Science Journal TIDeS Instructions (Microsoft Word 2007 (.docx) 2.9MB Aug30 24)

Pre-Class Assignment(s):

Complete Scientist Spotlight Edward Bouchet - spend a few additional minutes online learning more about him. Be prepared to share something surprising or interesting that you learned about him.

In Class, Part 1 (80 min)

Introduction (15 min):

• Students read about Edward Bouchet before class. Ask the class to share something surprising or interesting that they learned about him, then recap what we just learned about velocity and acceleration.
• Class discussion/think-pair-share. Anything that has an acceleration must be experiencing an unbalanced force. A force is anything that can cause an object to change its motion.

Discovering Forces Jigsaw and Lab (65 min):

• Students learn about pairs of forces in small groups, then groups reform as teams of five that revisit many of the same stations that they saw in Unit 2.3 (Energy Types and Transformations). This time, instead of energy transformations, students look for forces. Students work as a team to develop a catalog of forces (tension, weight/gravity, applied, elastic/spring, normal, buoyant, friction/air resistance/drag), annotate sketches of forces at work, and write a sentence explaining how they think the forces are acting to cause a change in motion. 
• Students complete a discovering forces Jigsaw Activity: Each group will learn about two forces, then form a new group and teach the new group about their two forces. In new groups, identify forces with familiar objects, and work as teams to build a catalog of forces with examples.
• Short lecture time to recap our catalog of forces. This includes class demonstrations to help students conceptualize the mechanism of the normal force as a microscopic spring force (see slides) and a return to pirates (pirates of caribbean) to discuss buoyant forces.
• Revision/completion time: Back in small groups (mix up the groups so students can compare among different groups), students revise their explanations.

In Class, Part 2: System Mapping and Summary (70 min)

Today's class is a guided discussion with the students to infer Newton's three laws based on observations they've made in this unit and also drawing back on the prior unit (4.1). The students have previously used system mapping in Unit 2.6, so this application of familiar methods with new material will help to reinforce system mapping.

Introduction to Newton's Three Laws (20 min): 

• Note by experimentation that a golf ball and ping pong ball both dropped from the same height land at exactly the same time - acceleration of gravity does not depend on mass. (side note: dropping objects off tall heights, such as differently sized pumpkins dropped off the roof of a building, also make this same point much more dramatic but require more prior planning.) Discussion questions:
  • Does the acceleration vary if the ball is dropped (as in this short experiment) or launched (as in our rocket launchers)?  
  • This means this is kinda like.... a volcano. Use same techniques to determine how fast the stuff coming out of the Hunga Tonga volcano would need to be to make it to the stratosphere. 
  • If acceleration is the same for everything in freefall, is force the same? (no!) What is different about the two rockets that causes the force to be different? 
• Ask students to pull up their Unit 2.6 System diagrams. Where do collisions fit in here? Discussion leads to students discovering Newton's Third Law: When two objects touch they exert equal magnitude forces on each other.

Discover Newton's Three Laws, Ok, Go! Style (30 min)

• Watch the OK, GO! music video called Upside Down, Inside Out multiple times through. Discussion questions - instructor notes for all discussion questions are in the notes section of the powerpoint slides.
• Think-pair-share: Describe what you see around 1:50 in the video. What kinds of motion result when the objects aren't experiencing an unbalanced force? Remember: Anything that is changing its motion is experiencing an unbalanced force. What kinds of forces can you observe? What do collisions do? Are forces acting on one object during a collision, or just both? During a collision is one object pushing harder on another object? 
• Watch the subtle motion that happens at 0:24 in the video - what is happening at that moment? Describe the forces that are acting just before and just after the subtle motion.
• Optional: The introduction describes that the video was shot in "zero gravity," but the weight/gravity force is never zero, even for astronauts in outer space. How can we reconcile what we see with the fact that there IS a gravity force? 
• Newton's First Law states: An object will travel in a straight line with a constant (or zero) velocity unless it is acted on by an unbalanced force. Watch the video one complete time through (again). What observations can you make that are consistent with this?
• Newton's Second Law states: An object will experience an acceleration (i.e., change its motion) if it is being acted upon by an unbalanced force. Watch the video one complete time through (again). What observations can you make that are consistent with this?
• Newton's Third Law states: When objects collide, they exert equal and opposite forces on each other. Watch the video one complete time through (again). What observations can you make that are consistent with this?

Apply Newton's Laws and Motion Concepts to explain what is happening to skydiving (20 min)

• After watching this video, this group exercise follows the same format that we'll ask the students to use to explain their Rube Goldberg machines in Unit 4.3. This assignment is a bit of practice for what's coming up at the end of the semester, so students have time to get feedback.

This unit is a crucial building block for the students on their journey to the final Rube Goldberg Challenge project and to answering this unit's motivating question. There's not much that can be skipped here - these materials typically occupy several weeks in a traditional physics course, so the materials are already condensed.

A student worksheet is provided as part of the resources for the Discovering Forces Stations Lab. Instructors can choose to use this worksheet in lieu of the science journal format. We will often but not always provide student worksheets. In classes with time constraints, worksheets can be faster, but they do not ask your students to engage as deeply with the practices of science.

As always, there is a reflection for the end of this unit. Reflection prompt: Given traditional readings and descriptions of Newton's Three Laws of motion, explain what they mean in everyday language and give supporting evidence from this unit for each of the three laws.

Students complete another Scientist Spotlight in this unit. The goal of these is to showcase an array of scientists in fields relevant to the topics of the day, some from long ago and others young and active today, together representing a diversity of people who have all overcome some challenge in pursuit of their scientific passion.

This video called Buoyancy - Pirates of the Caribbean courtesy the @Sblankman76 youtube channel explains why a scene from Pirates of the Caribbean could never work because of the Buoyant Force.

This is a short GoPro skydiving video called GoPro: Helicopter Skydive from the @GoPro youtube channel: https://www.youtube.com/watch?v=CzW_5x1M4Uc

The demonstration for the Normal Force is outlined in the instructor guide for this textbook: Knight, Randall D. Physics for Scientists and Engineers: A Strategic Approach. Prentice Hall, 2021.

The reflection prompt requests students to read about Newton's three laws of motion. Students should read three sections in this open-source physics textbook (full citation below). They should skip the numerical examples in pink background, but the text and descriptions are good. Sections to read:

Urone, Paul Peter, and Roger Hinrichs. "College Physics (OpenStax)." (2012).

• 4.1 Development of Force Concept
• 4.2 Newton's First Law of Motion: Inertia
• 4.3 Newton's Second Law of Motion: Concept of a System
• 4.4 Newton's Third Law of Motion: Symmetry in Forces