Something About Sigma: Calculating Net Force

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Students will determine the net force acting on a single object and relate that to its motion.

Big Idea

Free body diagrams are a useful tool to understand all of the forces acting on an object and the resulting objects motion.


In this lesson I have three goals for students' learning. Each of these goals match a section of the lesson (listed is in parentheses):

Students are to...

  1. Develop a new mental model that constant velocity does NOT require a net force (Homework review).
  2. Use free-body diagrams to visually display forces (Net Force and Free-body Diagrams)
  3. Identify and understand the following forces: normal, friction, tension, air resistance and gravity. (Force Scattegories)

This lesson is a mixture of lecture, game play and independent student work.  Lecture, or direct instruction, is the easiest and quickest way to pass on content.  However, it is not effective for long term learning.  So I couple the short lecture with game play and practice problems so that the students are active in their learning.

I expect students, when given individual forces, to use free body diagrams to determine the net force on an object.  In doing this they are applying Science Practice 5: Using mathematics and computational thinking and Math Practice 2: Reason abstractly and quantitatively.  Net force is also foundational to understanding HS-PS2-1 which is the mathematical relationship between the net force acting on an object and its acceleration.

In order to gauge student understanding, I give a formative assessment at the beginning of class and another at the end.  I use Socrative for this assessment, as it is very easy to look at and analyze the results.  To see how I use this tool, go to Using

Homework Review

10 minutes

I start the class by projecting Net Force Goals on the board. Next, I instruct students to complete the 10 question formative assessment (here is the Excel file to upload to Inertia and Net Force).  Some of the questions are from yesterday's Socrative assessment given during the inertia activities.  I'll use today's results to assess if there has been growth in the students understanding of Newton's 1st Law.

While students are working on this, I walk around the class and record a grade in my grade book for students' homework. Sometimes I collect and correct homework as a way to assign a grade and to determine what students know. In this instance I am only checking for completeness and assigning a grade for effort.  I usually grade homework out of 6 points so if they have a complete assignment, they get 6 points, half done gets a 3, etc. The reason for this is partly to save myself time in grading.  But I make it clear to students that full effort on homework will benefit both their understanding of the material and their grades on quizzes and tests.

Then I display the Inertia Homework Worksheet Solutions on the screen at the front of the classroom using my document camera, but I keep the answers covered.  I call on students to provide their answers. If a student does not have a correct answer, I'll gently guide them to the correct answer by asking leading questions or asking about their thinking. Once the correct answer has been given by the students, I reveal the answers.  I like to give the students multiple ways to check their answers as some people register visual information more effectively than auditory and vice versa. 

I inform the students that they are expected to correct their sheets as we work through the answers. It is important that students correct their own misunderstanding. Students keep this sheet for future reference and for preparing for the quiz on this material.

Net Force (sigma F) and Free-Body Diagrams

20 minutes

After reviewing the homework, I bring up the Net Force Powerpoint to introduce the concept of free body diagrams (FBD) and how it relates to net force.  I tell the students that this concept involves simple addition and subtraction and will be vital to their application of more advanced concepts which are coming in the next few days.  Students take notes during this lecture which is about 10 minutes long. I try to limit the amount of lecturing I do, but it is still a good way to present information. Furthermore, students will experience this teaching method often in college. I expect students to take detailed notes that include most everything projected as well as some of the things that I say. During the lecture, I point out that forces in the opposite direction will be negative and I said that this is a really important point. I follow that up with "Did you notice I said 'this is really important?'  That means you should write down what I said and circle it or highlight it somehow."

I hand out the Net Force Worksheet and students complete this worksheet independently to actively practice the concepts they just heard about.  Most of them finish in less than 10 minutes.  There are always a few students who have questions or struggle, so I circulate the room and provide support as students work.  I'm also checking to ensure that they took good notes.  If they don't have good, detailed notes, I'll take notice and let them know that I expect better notes in the future.

This worksheet is not very challenging, so I don't collect it.  10 minutes after handing out the worksheet, I display the solutions on the overhead so students can check that their understand is clear.  The total time for this section is less than 25 minutes.


Forces Scattegories and Types of Forces

20 minutes

Knowing that students do not come to me as blank slates, I use this final activity to activate their prior knowledge about different types of forces. I have them play a game called called Force Scattegories. Displaying the Types of Forces Powerpoint on the overhead, I tell the students to work in groups of 3-4 and have one person in their group write down as many forces as the group can think of. I keep this fast paced and let them know they have only one minute!

At the end of one minute, I tell students to stop writing. Then each group reads their list out loud one at a time. If any other group has that force in their list, then they all must cross it off their list. The group that has the most forces on their list that are not crossed off win the competition.  I have a box full of inexpensive trinkets called "the prize box".  Each member of the winning group gets one item out of the box.  Students love going to the prize box and this gives many of them added incentive to work hard.  I use the prize box only once every couple of weeks.

Next, I project the list of forces what we'll be using frequently in the next couple of weeks.  Students have the remaining time in the period to look up and define those forces.  They do this in their notebooks.

  • Gravity
  • Friction
  • Normal
  • Air resistance/drag
  • Tension
  • Applied

To find the definitions, I have the students use an online resource called the physics classroom.  The other option is to have students use a classroom set of textbooks or to print out the definitions from the physics classroom website and have those available for students.


5 minutes

It is important that I know what the students know and understand.  For this reason, students take the same Socrative - Inertia and Net Force quiz that was given at the beginning of the period.  This is an exit quiz which I will use as a grade and to determine if there are any common gaps in students understanding.