### Notes:

The Explosion - Law Enforcement 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 Interactive 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.

### Teaching Ideas and Suggestions:

This Concept Builder is intended for use in the middle to later stages of a learning cycle on Momentum Conservation. Like most of our Concept Builders, it makes for an exceptional formative assessment of student understanding of the momentum conservation. All questions pertain to the same type of physical scenario: two wheeled carts are at rest on a track. An explosive-like impulse propels the two carts in opposite directions. The before-explosion momentum of the system of two carts is 0. Thus for total system momentum to be conserved, the after-explosion momentum of the two carts must also be 0. For this to be the case, the two carts must have equal and opposite momenta values. As such the learner must inspect the relative mass and velocities of the two carts in the After Explosion snapshot of the carts.

Each question indicates that the mass of an unloaded cart is equal to the mass of the bricks that are loaded onto the cart. If it is helpful, a student could call the cart a 1-kg cart. As such, a cart loaded with a brick has a mass of 2 kg and a cart loaded with two bricks has a mass of 3 kg. Such designations would allow students to work with simple numbers that are true to the situation. The velocities are either 1-unit, 2-units, or 3-units. A similar designation could be made to refer to these as 1 m/s, 2 m/s, and 3 m/s. Doing so allows a student to work with numbers and perform quick and simple calculations of momentum from the product of mass and velocity.

This Concept Builder was intended as an in-class activity. The Concept Builder includes 18 different questions which are used in each of the three difficulty levels. For the Apprentice difficulty level, students must answer three questions correctly. The 18 total questions are divided into three question groups with six questions each. The Master difficulty level requires the student to answer six questions correctly; the 18 questions are divided into six question groups with three questions each. Finally, the Wizard difficulty level divides the 18 questions into nine question groups with two questions each; students must analyze nine questions correctly to complete the Wizard difficulty level. 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 activities would be most appropriate for their students.

In order to complete an activity, a student must correctly analyze each situation at that level. If a student's answer is incorrect, then the student will have to correctly answer the same or very similar question twice in order to successfully complete the difficulty level. This approach provides the student extra practice on situations for which they exhibited difficulty. As a student progresses through a difficulty level, a system of stars and other indicators are used to indicate progress on the activity. A star is an indicator of correctly analyzing the situation. Once a star is earned, that situation is removed from the cue of situations 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 situation and will have to correctly analyze it twice before earning a star. A yellow box is an indicator that the situation must be correctly analyzed one time in order to earn a star. Once every situation for an activity has been successfully analyzed, the student earns a medal that 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 difficulty levels.

The most valuable (and most overlooked) aspect of this concept-building tool 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 Concept Builder 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 Explosions - Law Enforcement 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. Assignments MC4, MC5, and MC6 from the Momentum and Collisions module make for a great complement to this Concept Builder. They are 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 Momentum and Collisions module can be found on Part 3 of the six-part App series. Visit Minds On Physics the App.

• Curriculum/Practice: Several Concept Development worksheets at the Curriculum Corner will be very useful in assisting students in cultivating their understanding, most notably ...

Action-Reaction and Momentum Conservation

Momentum Problem-Solving

Visit the Curriculum Corner - Momentum.

• Physics Interactives: Our Physics Interactives section of the site includes a collection of interactive simulations that engage students in the manipulation of variables and the observation of the effects of such changes. We can recommend several simulations as appropriate pre-cursors to this Concept Builder, most notably ...

Collision Carts

Exploding Carts

Visit the Physics Interactives - Momentum section.

Additional resources and ideas for incorporating the Explosions - Law Enforcement Concept Builder into an instructional unit on Momentum and Collisions  can be found at the Teacher Toolkits section of The Physics Classroom website.  Visit Teacher Toolkits.