Virtual Exploration of Forces on an Inclined Plane
Lesson 7 of 11
Objective: Students will collect data, propose definitions, and test their proposals about forces on object on an inclined plane.
I am, again, out of the classroom today, this time for a district-wide discussion of NGSS standards. Having known about this ahead of time, I have created a plan where my students can move forward without me. They are working on a simulation of the motion of objects on a ramp (or "inclined plane").
The goal for today, based on the previous class, is to expand student thinking about Newton's Second Law of Motion (NGSS Performance Expectation HS-PS2-1). They are familiar with the process of drawing free-body diagrams that lead to net force equations. The complicating aspect of an inclined plane, introduced here, challenges student thinking about the conventional separation of forces into horizontal and vertical components. The weight of an object does get resolved into orthogonal components, with the angle of the plane determining the rotation of those forces. Students collect data, use their Newtonian Analysis skills, and attempt to induce the relationships between an object's weight and its normal and parallel forces. In so doing they develop and use models (Science & Engineering Practice #2), they plan and carry out an investigation (#3), they analyze and interpret data (#4) and they use mathematics and computational thinking (#5).
My substitute reads the instructions to the class to prepare them for the day's work. In addition, he provides students with a set of notes that are very specific to the simulation (one of the University of Colorado's PhETs) featured in today's lesson. My goal is to provide just enough scaffolding so that students can be successful without giving away the answers that are to be induced. As such, the tips are just related to the operation of the simulation, not to the content.
The goals for the day are for students to collect enough data to propose a function for the "parallel force" (the portion of an object's weight that is directed down the ramp), then test the proposal by choosing a novel situation and predicting the outcome. A similar process is used to find a function for the "normal force" (the portion of an objects' weight that is directed towards the surface of the ramp).
Students begin the work today by accessing the video link that is referenced in the notes handout. They know about this link from our previous class but, today, they take the time to view it. In this seven-minute video, I demonstrate some of the controls of the simulation that are not immediately obvious to a new user. In addition, I alert students to some of the pitfalls inherent in the simulation. For example, it's imperative that the motion down the ramp start with an initial velocity of zero; failure to do so will result in a false analysis. After viewing the video, students have the background necessary to complete the simulation.
Inclined Plane Exploration
Students begin the investigation by accessing the website, running the code to initiate the simulation, and following the instructions on the handout. They work in small teams and begin to collect data.
The overall strategy is to use the option of a frictionless surface to induce a rule for the parallel force (Fp) - that portion of an object's weight that is directed downhill on a ramp. With a frictionless surface the ONLY force acting in the direction of the plane is the parallel force, hence it is the net force. The mass and acceleration can be used to determine this force and, if students gather enough data, they can suggest a mathematical formula for the parallel force.
Having completed this task, students tackle the more difficult task of determining the normal force. In this portion of the investigation, students need to use their parallel force formula from the first part along with the resulting mass and acceleration to isolate the frictional force. From there, my students can use an equation previously developed (Ff = u * Fn) to determine the normal force (Fn).
The activity is self-paced and student teams move from one activity to the second when ready. Students create spreadsheets for their data and many share their files with me. Both activities are challenging and students use the remainder of class to gather data, propose formulas, and test their proposals.