Free Body Diagram, Day #1

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Objective

Students will be able to identify the forces acting on an object, draw them in a free body diagram and identify whether the object is in a state of equilibrium.

Big Idea

Free body diagrams are a way to represent the forces that act on an object.

Paired Reading

10 minutes

The goal of this lesson is to introduce forces and free body diagrams to students. Throughout the lesson, students develop and use free body diagrams to model forces and how they act on an object. To begin class, I have students do a paired reading with one partner on Forces and Force Diagrams. To complete the paired reading, I ask students to switch off roles each paragraph as reader or writer. I ask the groups to write down at least one important fact from each paragraph on their own copy of the reading to help when we have a discussion about free body diagrams.

Free Body Diagram Guided Notes

35 minutes

After students have finished reading, I like to start out with a discussion about their homework about definitions of forces and the reading before getting into the guided notes. I ask students what definitions they came up with for the forces and I write those on the board so we have a reference for the notes later on. Then I ask them what important facts they pulled out of the reading. When we discuss the facts, I hope to get the students to focus on the idea that forces have direction and are shown with arrows on a diagram as well as that forces can go in any direction. I also like to have them discuss the steps of how to draw a free body diagram from the example that the reading provides.

After the discussion, I ask students to turn to the Free Body Diagram Notes in their packets where we practice drawing free body diagrams. For the first example, I start out by telling them that any diagram that we draw has weight pointing directly down. Then I tell them that in this situation there is a surface and I ask them which force they think would also be on the diagram. After students answer normal force, I show that the normal force is always perpendicular to the surface. Then I ask if they think there is any other forces acting on the object. Once we have ruled out any other forces acting on the object, I ask them how many forces are acting on the object and in which direction. Since there are two forces and they are in opposite directions, I tell the students that there is no net force acting on the object and that when there is no net force the object is in a state of equilibrium. 

I guide them through the next three examples similar to the first example, as shown in the FBD Notes KEY. Then I have them try the last two on their own and use them as a Think-Pair-Share. First, I give them 2-3 minutes to work through the two problems on their own. Then I give them 2-3 minutes to compare with a partner and talk about how they got their answers. Finally, I ask for student volunteers to tell me one force on either of the examples so they can check and see how they did before doing some on Worksheet #1.

Free Body Diagram Worksheet #1

20 minutes

After the notes, I ask students to work at their tables on Worksheet #1 Free Body Diagrams where they practice looking at situations and drawing free body diagrams. Then they practice identifying if the object has a net force acting on the object and if the object is in a state of equilibrium. While students are working on the worksheet with their table groups, I walk around to answer any questions students might have. Below you can see one of my students work on two very similar problems from the worksheet.

Forces Exit Slip

5 minutes

To end class, I give students the Forces Exit Slip. I ask them to be able to match the definitions to the names of the forces and draw a free body diagram for a situation. I use this as a formative assessment to help me to see what I need to focus on the next lesson. Some student examples include Forces Exit Slip- Excellent Student Work and Force Exit Slip- Needs Practice Student Work.