The Physics Classroom » Concept Builders » Newton's Laws » Fnet = m•a » Teacher Notes

Notes:

The Fnet = m•a Concept Builder is an adjustable-size file that displays nicely on smart phones, on tablets such as the iPad, on Chromebooks, and on laptops and desktops. The size of the Concept Builder can be scaled to fit the device that it is displayed on. The compatibility with smart phones, iPads, other tablets, and Chromebooks make it a perfect tool for use in a 1:1 classroom.

This Concept Builder uses a technology known as WebGL. WebGL is a JavaScript-based technology that is built into most modern browsers on mobile devices as well as desktop/laptop browsers. Older browsers may not be WebGL compliant or may not offer it as a standard feature but allow it to be enabled by the user of the device. For instance, the Safari browser has an Enable WebGL option in its Develop menu. Without enabling WebGL, the use of WebGL JavaScript code is not functional and would not work in such browsers. We expect that future browsers will all be WebGL-compliant and will sealessly work with this Concept Builder. Additional information about WebGL can be found on Wikipedia.

 

Teaching Ideas and Suggestions:

This Concept Builder is intended for use near the middle to late stages of a learning cycle on Newton's second law of motion. Most first-year Physics students quickly learn how to use Newton's second law equation to quickly calculate the net force from knowledge of the mass and acceleration. It takes a little longer before they become accustomed to analyzing a force diagram with known force values for individual forces and do the two-step analysis of determining the net force from the diagram and then the acceleration from the Fnet = m•a equation. But in our experience, it takes even longer for Physics students to work the same problem in reverse, using knowledge of mass and acceleration to determine the net force and then using the net force to calculate the value of an unknown force in a force diagram. It is this latter skill that this Concept Builder targets.

Every question in the Concept Builder includes a given mass and acceleration. Students are presented with the challenge of determining a set of north, south, east, and west vectors which will produce such an acceleration. They are provided with a force diagram and must drag out force vectors on the diagram to a given magnitude in order to produce the specified acceleration. Once they tap on the Check Answer button, the motion of the mass is animated and feedback is provided. 

This Concept Builder was intended as an in-class activity. After some early lab work, some discussion of the second law, and some practice using it in various problem-solving contexts, allow students an opportunity to interact with the questions. The Concept Builder includes three levels of difficulty. Teachers using the Concept Builder with their classes should preview the activity (or view the Questions in the separate file) in order to judge which levels would be most appropriate for their students. The questions for each level are unique to that level. Our summary of the three levels is as follows:  
 
  • Apprentice Level: Question Groups 1-3. Mass is set to 1-kg. Accelerations are along the axes.
  • Masters Level: Question Groups 4-6. Mass is randomly selected. Accelerations are along the axes.
  • Wizard Level: Question Groups 7-9. Mass is randomly selected. Accelerations are at angles to the axes.

In order to complete a level, a student must correctly analyze each question at that level. If a student's analysis is incorrect, then the student will have to correctly analyze the same or very similar question twice in order to successfully complete the level. This approach provides the student extra practice on questions for which they exhibited difficulty. As a student progresses through a level, a system of stars and other indicators are used to indicate progress on the level. A star is an indicator of correctly analyzing the question. Once a star is earned, that question is removed from the cue of questions to be analyzed. Each situation is color-coded with either a yellow or a red box. A red box indicates that the student has incorrectly analyzed the question and will have to correctly analyze it twice before earning a star. A yellow box is an indicator that the question must be correctly analyzed one time in order to earn a star. Once every question at a level has been analyzed, the student earns a medal which is displayed on the Main Menu. This system of stars and medals allows a teacher to easily check-off student progress or offer credit for completing assigned levels.

The most valuable (and most overlooked) aspect of this Concept Builder is the Help Me! feature. Each question group is accompanied by a Help page that discusses the specifics of the question. This Help feature transforms the activity from a question-answering activity into a concept-building activity. The student who takes the time to use the Help pages can be transformed from a guesser to a learner and from an unsure student to a confident student. The "meat and potatoes" of the Help pages are in the sections titled "How to Think About This Situation:" Students need to be encouraged by teachers to use the Help Me! button and to read this section of the page. A student that takes time to reflect upon how they are answering the question and how an expert would think about the situation can transform their naivete into expertise. 
 
 
 

 

Related Resources

There are numerous resources at The Physics Classroom website that serve as very complementary supports for the Fnet = m•a Concept Builder. These include:
  • Minds On Physics Internet Modules:
    The Minds On Physics Internet Modules include a collection of interactive questioning modules that help learners assess their understanding of physics concepts and solidify those understandings by answering questions that require higher-order thinking. Assignment NL9 of the Newton's Laws module provides a great complement to this Concept Builder. It is best used in the middle to later stages of the learning cycle. Visit the Minds On Physics Internet Modules.

    Users may find that the App version of Minds On Physics works best on their devices. The App Version can be found at the Minds On Physics the App section of our website. The Newton's Laws module can be found on Part 2 of the six-part App series. Visit Minds On Physics the App.

     
 

Additional resources and ideas for incorporating Fnet = m•a into an instructional unit on Newton's Laws can be found at the Teacher Toolkits section of The Physics Classroom website.  Visit Teacher Toolkits.