I teach the opening of the force unit by focusing on the concepts of inertia, net force (balanced verses unbalanced) and free-body diagrams. The math is simple addition and subtraction. It is important to establish this important foundation before introducing the details of F=ma, friction and forces in 2-dimensions. The main goals of the unit are listed in the unit overview, which I give to the students so that they can see a where we are going in the next few weeks.
The goal of this lesson is to challenge students mental models of how objects move and develop new models to explain what they observe (SP2). Students also construct viable arguments that describe their observations in this activity (MP3).
In this lesson I use several toys that allow students to explore the concept of inertia.
Students explore the idea of inertia when I give them a hover puck. They make observations about its motion and how it does not require persistent force in order to remain in motion. Because this type of motion is uncommon, I have learned that a battery powered hover puck is a required tool in my classroom and is a bargain for $20 on Amazon.
I also give the students a fun way to explore the concept of an object at rest stays at rest. Students stack nuts on top of an embriodery hoop and they have to get them into a bottle on which everything is balanced. The technique is like yanking a table cloth out from under plates. This activity is so popular that it is common for students to stay after school to try and get the high score for the year.
Working in groups of 3-4, students explore the concept of inertia with four different activities. I hand out the Inertia Stations worksheet and spend 5 minutes to explain what they are doing today.
Vocabulary definition: I expect the student to use the text resources that are available in the classroom to define some basic vocabulary. The goal here is to get students to understand that inertia is a latin word for "laziness" and is used because objects tend to keep doing whatever they were doing. I want students to know that it is a property of matter, not a force that keeps object moving, which I have found is a common misconception.
Hoop-nuts Challenge: I set-up the hoop-nuts challenge (pictured as the lesson image) and tell them that the challenge here is to get the nut into the bottle, touching only the hoop. But I don't spoil the fun of showing them how to do it! Once the students figure out how to get one nut into the bottle, they are free to try more nuts and I point to the Hall-of-Fame wall where past winners names are posted. The goal of this activity is to reinforce the idea that an object at rest stays at rest unless acted on by unbalanced forces. The nut stays in place until its support (the hoop) is removed. The secret here is to quickly grab the hoop from the inside of the ring with a quick horizontal motion. Here is a video that shows a stack of about 15 nuts going in.
Socrative Questions: I tell students to use their phones or a computer to answer 10 questions on Socrative (SOC #: 13694452). The goal is to get students thinking about the nature of forces and how it relates to motion and we will later explore the common misconceptions.
Hover Puck Activity: I tell the students that this is the most important of the activities and that when the hover puck gets to them, they must stop what they are doing and complete that activity as the puck must make it around to all groups. The goal of this activity is for students to develop a new mental model of motion. I have learned over the years that it is very difficult for students to let go of the mental model of constant velocity requiring constant application of force because this has been their life long experience. When the students see that the puck travels across the room with a single initial push, it helps me convince them that constant velocity does not require constant force. The problem is friction is so prevalent, working to resist the motion of an object, this is why constant motion requires constant force most of the time. I let students come to this realization on their own and we discuss it in the closure.
Some of these activities are so engaging that students will spend all of their time on just one, so I inform them that I expect all four activities should be completed before the end of the period and I collect their work. I find that the remaining 40 minutes is enough time for the groups to work through all the activities. I circulate the classroom making sure students are on task and answering questions.
With 5 minutes left in the period, I have students return supplies and then go back to their desks. I call on students to report what they learned about the motion of the puck when it was turned on. Students explain their observations and I ask who had similar observations. I focus on comments along the lines of "with the puck on, once I gave it a shove, it just kept going". I want it to be clear in students minds that once the hover puck is in motion, it does not require the application of force to keep it going.
I then give out their homework which is the front side of Inertia Homework Worksheet. Students use the online resources force and free-body diagrams (FBD), and Newton's 1st Law to complete this sheet. These links are posted on our classroom site and also sent out with Remind.com.
The concept of "object in motion remains in motion..." is a difficult concept for students to accept, so I want this to be the last thought as they leave class. With about 1 minute left, I instruct the students to get their things and line up along the hall outside the room. I hold up the hover puck and say that this object has inertia and so it tends to keep doing what ever it is doing. I set it down and with a gentle nudge, send down the entire length of the 100 foot hallway.