Balanced and Unbalanced Forces
Lesson 2 of 11
Objective: SWBAT determine which direction an object will move when multiple forces are acting on it.
The Why Behind Teaching This:
This unit covers standard 5-PS2-1: Support an argument that the gravitational force exerted by Earth on objects is directed down. During the unit, students will investigate a variety of objects to see that the force of gravity is constant on Earth and pulls things down towards its center. We will also be investigating a variety of ways to overcome gravity.
Several of the lessons in this unit are engineering design projects requiring students to follow the steps of the engineering design process to construct a project. These projects address standard 3-5-ETS1-1: Define a simple design problem reflecting a need or a want that includes specified criteria for success and constraints on materials, time, or cost. It also addresses engineering standard 3-5-ETS1-2: Generate and compare multiple possible solutions to a problem based on how well each is likely to meet the criteria and constraints of the problem. There are also several experiments in the unit which address standard 3-5-ETS1-3: Plan and carry out fair tests in which variables are controlled and failure points are considered to identify aspects of a model or prototype that can be improved.
This specific lesson addresses standard 5-PS2-1 by introducing the concept that forces act in pairs and the motion of an object is dependent on which force is stronger. While learning about gravitational force pulling things down towards the center of the Earth, students often wonder why everything doesn't move straight down towards the center of the Earth. By building an understanding that other forces impact the motion of objects, they will then understand how the force of gravity is sometimes overcome by other forces.
The goal of this lesson is for students to be able to calculate the net force acting on an object and describe the motion of the object due to the forces acting on it.
Students will demonstrate an understanding of this concept by correctly calculating net force, and direction of motion, during a game and on an exit ticket.
Preparing for Lesson:
- A heavy table in the classroom for demonstration
- science notebooks
- a copy of the net force notebook page for each student (net force notebook page - answer key)
- A laminated copy of the Net Force Game Board for each pair of students (enough copies will need to be made for each student to have one in the Explore section).
- A game token for each pair of students (one per student in the next section).
- A copy of the game cards for each set, Net Force Game Cards Set A, Net Force Game Cards Set B, Net Force Game Cards Set C (I laminate these to use each year).
- 12 index cards per group
- game board for each student
- game token for each student
- A copy of the net force exit ticket for each student
How Forces Work Together and Against Each Other
I begin today's lesson before students even realize the lesson has begun. I try to move a table in the room to a new location but can't seem to get it to move (I am acting like I am pushing but not really pushing). I ask for help from a student in the class. I choose a small student who may struggle pushing it on her own. I again act like I am helping to push it, but I am not really pushing. The table moves but I act like it does not move far enough. I ask another student if they can come up and help by pulling the table from the other end. This time I push with them and the table moves easily. You can see in the Table Moving Video that the purpose was to use both pushing and pulling forces that work together to lead into our discussion about net force.
I make the connection between today's lesson and the previous lesson, Types of Forces, through our discussion about the movement of the table. I ask students the following questions:
- How did we get the table to move to a new location? You applied a force to it by pushing and pulling on it.
- Was our applied force the only force acting on the table? No gravity was pulling the table down too.
- Why couldn't I move the table on my own? You weren't strong enough.
- What force was acting against my push, stopping the table from moving? Friction
- What was the table rubbing against that was causing friction? The legs of the table are rubbing against the floor.
- How is this friction related to the force of gravity acting on the table? The gravity is what is pulling it against the floor to produce the friction.
- When the other student came to help me, were our forces acting together or against each other? Together
- When were our forces strong enough to overcome the force of friction? When there were two people pushing and one person pulling.
If students are not able to give me the answers above, I lead them to the answers with further questioning. This conversation reviews some of the information they learned in the introductory lesson about forces, while also adding in new concepts.
Adding Notes in the Science Notebook
I provide each student with a copy of the net force notebook page for students to glue into their science notebook. I place my notebook on the overhead with a copy of the page glued in as well. Having my notebook projected up on the overhead provides a visual for students who struggle with spelling and focusing. I typed the notes so that there are only a few blanks to fill in during our discussion. I provide notes in this format to save time with copying. There are also diagrams in the notes that I want to be the same for each student so having the notes page makes this possible. The answers that we will be filling in through our discussion can be found on the net force notebook page - answer key.
I begin by reading the definition of net force: the total amount of force acting on an object. I refer back to the movement of the table at the beginning of the lesson and ask how many forces were acting on the table. This requires more discussion because students forget about gravity and friction and tell me three forces from the three people pushing/pulling on the table. After additional questioning they are able to tell me 5. I explain that the net force acting on that table would be the total amount of force I was applying, student A was applying, student B was applying, the force being applied by gravity, and the force applied by the friction from rubbing the floor.
I ask students if the forces are acting together such as when I was pushing with student A, would we add those forces together, or subtract them. Students tell me add them and we fill this in on the notebook page. I read the next line and we fill in forces acting against each other are subtracted.
I ask students what they think would happen if two forces acting against each other are equal. I give the example of arm wrestling because this is something they have experience with. They tell me that the hands don't move and no one wins. I ask what would happen if I was arm wrestling someone and the force I apply is greater. They tell me that I would push the other persons arm down and I would win. We relate this to net force by reading the next two sentences and filling in the information. I remind them of the tool we used at the beginning of the year called a balance. I get it out and set it on the table. I ask them what will happen if I place an object on one side. That side will go down. I ask them what would happen if I place a heavier object on the other side. That side of the balance will go down and the other side will go up. I do this to show them the motion. I ask them what would happen if I take that object off, and place an object that weighs the same as the first on it (such as a marker on each side). They would balance in the middle.
A balance is something they have worked with before and is a great visual for the concept of balanced and unbalanced forces. Bringing this tool out for demonstrating is a great time to review the tool and how it works while not giving up instructional time on the new concept.
Next, we complete the practice problems together calculating net force and determining what direction the box will move. I do the first two with them, then I give them a few seconds to complete each of the last two independently so I can check for understanding. I award those who get it correct by themselves before I go over the answer with a ticket (our classroom incentive).
Practicing Net Force With a Game
I have students close their notebooks while I pass out a Net Force Game Board and token to each set of elbow partners. Having them work with a partner allows students who may still be struggling to have a peer to ask for help. They will be doing this independently later in the lesson. Having more then 2 people working together is too many because the only task they have is to calculate the net force and move the token. If too many students work together, there would be a lot of students not doing anything and thus not gaining from the activity.
I instruct students to place their token on the start square. The I explain how the game works: I will show a card with a diagram of forces acting on a box. With your partner, you will need to calculate the net force acting on the object and determine how it will move. You will move your game token in the direction the box will move, one square for every 100 Newtons of force.
I place a game board on the overhead with a token at start just as the students have set up at their desks. I show the first card (A1) in Net Force Game Cards Set A and demonstrate how to move the token based on the diagram. I place the second card (A2) on the overhead and circulate to see where students move their token. I want to be sure they understand the concept before moving on. The first game I am spending time to correct movement and explain each one if students get it wrong. I go through the cards in order as the final card in the set gives the answer on where they token should end up.
After the first practice game where I am correcting their mistakes and explaining each error, I let them play without my corrections. I place each card from Net Force Game Cards Set B on the overhead one at a time, allowing a few seconds for calculations and movement. Before I place the last card from the set on the overhead, I circulate to see where tokens ended up. All tokens should end up in the same location so this gives me an idea of which groups are struggling. I show the final card and award groups that are on that square with tickets (our classroom incentive).
We do one more game using Net Force Game Cards Set C. I place one card at a time on the overhead. While students are calculating and moving, I focus my attention on those groups who were not successful in round 2 of the game. I want to catch their mistakes and point them out as we go so that I can correct their errors. Before placing the final card on the overhead, I circulate to check the location of their tokens. I place the final card on the overhead, and award those who are correct with tickets again.
Even though there are 12 cards in each set, each game only takes about 3 minutes to complete. The forces are all in increments of 100 so that they can do the addition and subtraction in their heads which makes it easier. As you can see in the Net Force Game Video, groups quickly do the math in their head and move their game piece. Completing this game allows students to practice calculating net force, adding when forces work together and subtracting when they work against each other, and requires them to demonstrate an understanding that the object will move in the direction of the stronger force. They are applying the new information they just learned from the notebook page in a fun way.
The first time we play several groups did not end up in the correct location at the end. The second game, only 3 groups out of 10 did not end up in the correct place. The last game, 1 group did not end up in the right square. It was the same groups missing so this gave me a good idea of which students were struggling. This was a quick formative assessment that provides me with information regarding which students need more of my assistance during the independent activity.
Students Create Game Cards
After playing the game three times, I pass out 12 index cards to each set of partners. I explain to students that they will now create game cards to use for the game. They must create cards like the ones used in the games played, track where the token is moving throughout their game, and indicate on their final game card where the token should end up.
By having groups create a set of cards, they are practicing calculating net force and relating it to movement by moving their token. They will also have to decide what forces will get the token to move in a certain way because sometimes their token will end up at the edge of the board and they will have to determine what direction and amount of force will have to come next to get it where they want it to go. This is a higher level thinking activity because it requires a lot of planning and adjusting. You can see in the video of students creating net force game cards that these two students make an error and have to self adjust. The card they create would move the token off the board so they have to figure out how to change it so the move can work. Having two students work together instead of doing it individually provides for self correction of errors and for peer help instead of needing my assistance.
While groups create cards, I have the opportunity to circulate and listen to conversations to ensure that proper vocabulary is being used. It also allows time for me to call over the students who struggled during the game and work with them individually. One on one help will help to ensure that they walk away from the lesson meeting the goal.
Groups Lead the Game
After about 10 minutes of creating game cards, I pass out a game board to each student who does not already have one in front of them. Students will work independently on this part of the lesson so that I can assess their understanding individually.
I call one group up at a time to lead the game. The group places one card at a time on the overhead allowing for a few seconds of time to calculate net force and determine motion. I am playing along on a game card in the back of the room so that I can check the location they have determined as the final landing spot on their last card. If I do not end up in the same location as them, that tells me that they made an error when planning and are not yet proficient in mastering the objective.
I allow the group to award students who end up in the correct location with tickets. They enjoy passing out tickets and "being the teacher" for this activity. All groups have an opportunity to present their game to the class while I play along, and monitor the students playing at their seats.
As I collect games boards, I pass out a net force exit ticket to each student. This is a short formative assessment that allows me to quickly and easily see which students have met the goal and which need a little more practice on the skill. I was very please to see that all students present for the lesson met the goal. The only error that was made was that many students, over half, put that there will be no movement when the two forces are equal for the answer to number one. That is correct, but I was really looking for them to use the proper science term, balanced. I will reinforce the proper science vocabulary in future lessons so that I know students understand when forces are equal they are balanced.