An Introduction to Free Body Diagrams

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Students will be able to create and describe free body diagrams.

Big Idea

Students use the vector nature of forces to draw free body diagrams.


My students are already comfortable defining force from the first lesson in this unit and they are also aware of the vector nature of forces from the second lesson. Because the students have shown via assessments that they've mastered these concepts, we are ready to move forward. In today's lesson, the students will need to apply their knowledge in order to be successful drawing and explaining free body diagrams (FBD).

FBD Pre-Test

10 minutes

As students enter the room, I have each person pick up a copy of the free body diagram pre-test. I have them work individually and silently through the activity so that they aren't able to get answers from a peer. Once time is up, I have students turn the paper upside down and put it aside for later. My instructions are purposely vague because I want students to hold onto their work until the end of class without referring back to it. 

Students usually aren't so happy when I use pre & post tests because they are uncomfortable with anything they have not yet mastered. While I don't expect my students to have any clue how to draw proper free body diagrams, I still find value in giving this assessment because it shows me and the students their initial level of knowledge. At the end of class today, the students will be able to make updates to these pre-tests and see evidence of their learning.


Direct Instruction

15 minutes

It's time for students to take out their science notebooks and get ready to learn the fundamentals of free body diagrams. My students are operating under the expectation that they must write down key points from the presentation and we'll work through several examples that also need to be recorded. This expectation of how to take notes has been outlined and ingrained in their learning since freshman year, but we've practiced it quite a bit throughout earlier lessons in the course as well.

I display my free body diagram basics presentation (also available in PPT) to help the students understand what they need to write down, but I also have a hard copy of the document in my hand. By the end of the presentation, students should understand what a FBD is, how it can be useful, and several examples of how it can be applied to different situations. The printed copy includes notes (viewable only when the file is downloaded) that I've written to myself to make sure I accomplish the learning goals as we work through the presentation. The nice thing about having a hard copy in my hand is that if I'm teaching and think of a great side-note or new example, I have the ability to write it down immediately.

While I describe this section as "direct instruction," I usually have a lot of interaction with my students throughout the presentation. I am constantly moving throughout the room to change my proximity and am listening to my students as they work through the examples with the people around them. The students will ask questions, participate in problem-solving, and connect to real-world examples to stay engaged the entire time.     

Collaborative FBD Practice

10 minutes

Now that students have a better understanding of free body diagrams, I ask students to get into pairs. I make the pairs using a random partner generator, and this forces students to work with someone that they might not otherwise choose. It's a fun way to pair students and the element of suspense adds a bit of excitement.

Once everyone knows who their partners are, I ask student to move to sit with that person and bring their pre-tests with them. Instead of having students simply revise and correct their own forces pre-test, I assign each pair 1 or 2 problems from the pre-test. It is then that pair's responsibility to revise and understand (to the best of their ability) their assigned problems. Some pairs may not need to make any revisions, but I still ask them to use this time to discuss the "why" behind their diagrams.  

Before I give students time to collaboratively work, I inform the class that they'll be sharing their solutions with the rest of the class before the end of the hour. While we always strive for accuracy in AP Physics, it's more the process and justification that I want students to focus on during their work time. Students need to show their thought processes through calculations and work. They should also be able to explain their answers and support those answers with clearly articulated and well supported ideas.

Sharing FBD Solutions

15 minutes

Starting with whichever group was assigned the first problem, each pair will come to the front of the room and share their solution. I expect that students not only show the solution to the rest of the class using the document camera, but students must also explain each force they've drawn on the diagram. The students who are not sharing should be checking their pre-tests with the solution that is being provided. I sit in the back of the room and use the answer key to double check what the students are sharing while they are at the front of the room.

In the event that the students need guidance, I might ask probing questions such as "Why did you leave out friction?" or "How did you know to include air resistance in this example?" I also look to the rest of the class to provide support to students presenting by asking general questions to the class such as "What do you all think" or "How can we help them?" If a pair is completely off track and the rest of the class is unable to provide adequate support, I join the group at the front of the room and walk the entire class through the problem. This doesn't happen frequently and it's my last resort in an activity like this.

The lesson ends when the last pair presents, but I do give the class some finality by asking students how confident they are feeling on the material. They show their confidence by full extending their arms if they have a high confidence, partially extending their arms if they are moderately confident, and not raising their hands at all if they are completely lost. Most students fully extended their arms after this activity, so I feel confident we are ready to move on to Newton's First Law in the next lesson.