Manipulating Forces

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Students use cut-out manipulatives to apply the concept of Newton's First Law to various situations.

Big Idea

An object changes its current motion when an unbalanced force acts it.

Context and Supplies

In the last lesson, students applied how net force affects motion via free body diagrams (FBD).  They also learned about the types of forces and are familiar with Newton's First Law.

The major goal of this lesson is to have students create free body diagrams to visualize the net force on an object given individual forces or individual forces given the net force.  In doing this they are applying Science Practice 5: Using mathematics and computational thinking and Math Practice 2: Reason abstractly and quantitatively.  The concepts of net force is also foundation to understanding HS-PS2-1, which is the mathematical relationship between the net force acting on an object and its acceleration.

Students manipulate cut-out arrows to create a FBD given a description.  For supplies, this lesson requires a projector to display a Power Point and a dozen sets of cut-out paper arrows and labels which are prepared for them ahead of time.



Student FBD Activity

45 minutes

I like to include activities that have students think about forces by manipulating objects.  I find that the act of physically moving objects around helps many students.  For this activity, my students  practice making FBDs using  paper arrows that they have to position around a picture of a box.  They work with a partner so that they can discuss the solution to a series of timed examples which are displayed with the Manipulating Forces - Power Point.  They also have to relate the net force to the actual motion of the object and understand how that motion is or is not changing.  

With the  opening slide displayed on the board, I let students know that they have 16 challenges to complete and that they only have 2 minutes per challenge.  I hand out the Student Answer Sheet, each student gets their own.  Students work with the person they sit next to (groups of two) and each group gets one set of the Manipulating Forces - Cut Outs.  I have them lay out the shapes and organize the materials on their desk.  Included with the arrows are labels.  Students should not write on the arrows, but place the labels next to the arrows to indicate the type of force.

Once students are settled and ready, I project the first challenge which is a box at rest on a tabletop.  A two-minute timer animation starts automatically on the Power Point so that students see how much time is left.  Students read the situation and engage in discussion as they position the manipulatives to work out the forces that must be acting on the object.  Once they have settled on a configuration for their forces, they record their result on their answer sheet.   If they get stuck, a hint is displayed one-minute into the challenge.  

While time runs, I circle the classroom and help students who struggle.  A chime sound is made when the time is up and I reveal the correct answer.  Students score themselves using the rubric that is part of the presentation.  I tell the students that they should fix any mistakes as this sheet will be a useful study tool.  This cycle repeats 15 more times with challenges getting increasingly difficult.


5 minutes

At the end of the challenges, I collect the students' answer sheets so that I can assess students responses and understanding later.  To be funny, I point out to the students that physicists love boxes as evidenced by the fact that over 80% of all examples involving FBDs use boxes.  Assuming there is no love affair with boxes and that most objects in this world are NOT boxes,  I call on a few students to explain why they think physics books so often use boxes in their examples.  Usually a few are able to explain that it is because a box is a simple object that makes for a simple situation so other details or factors don't interfere with the core idea of net force.

Then I hand out the homework, which is more Free Body Diagram Practice problems.  However, this homework assignment does not have a single box!  Students will apply their understanding of FBDs and net force to real life situations.