I return to the classroom today needing to do a bit of clean up: the work done in the previous class was complex, messy, and not entirely successful. Today I provide another opportunity for students to complete the investigation of forces on ramps, but begin with a few minutes of advice and strategy.
The overall goal for the day is to expand student thinking about Newton's Second Law of Motion (NGSS Performance Expectation HS-PS2-1). Students 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).
To begin the day, I share some hints and tips about the best use of the simulation. There are a wide variety of controls and many ways to use the simulation. Indeed, the versatility of the simulation is one reason why the previous lesson went poorly - there are many opportunities for students to make unnecessary adjustments to the settings, each of which can interfere with the desired results.
After that, I lead my students through a short but valuable exercise to ensure that all students are looking at the forces correctly. I begin with a slide on the Smartboard that has room for free-body diagrams - one for the investigation of the parallel force and one for the investigation of the normal force. We talk about the frictionless case that is used in the first investigation and create a free-body diagram whose ONLY force is the parallel force. Clearly, if students can select a mass and measure the resulting acceleration then, by Newton's Second Law, they can find the net force and, hence, the parallel force.
We repeat the process for the normal force. Determining the normal force is much less direct. The free-body diagram includes the parallel force and the frictional force. One must, therefore, have a formula for the parallel force to proceed. Students can find the mass and acceleration using the simulation, then set it equal to the difference of the parallel and frictional forces. Once the value of the frictional force is found, we can use a previous relationship (Ff = u * Fn) to establish the normal (Fn) force.
Having complete free-body diagrams provides the right scaffolding for students as they try to finish up the investigation today. The remainder of class time is dedicated to the completion of this task.
Students return to the investigation by accessing the website and following the instructions on a handout received in a previous class. They work in small teams and collect the virtual data. Most teams have a good data set for the first task of the investigation. The bulk of the time today, therefore, is spent on the second task: inducing a rule for the normal force (the portion of an object's weight, in this case, that is directed toward the surface).
The overall strategy, for the simulation, 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 are used to determine this force and, with enough data, students 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).
Students make better progress today; minor issues that caused setbacks in the previous class are dealt with quickly today, either by the students themselves or in consultation with me. The increased familiarity with the website and the clarification of procedures leads to a more successful session of data-collecting. Given the loss of time earlier in this investigation, I allow students to work right up until the bell without any interruptions.