Projects: LOOPS : Topic 6 Separation of Forces
This page last changed on Jul 07, 2008 by kbell.
Standard 8PC2.d. Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction.
Classroom discussion. Start with the most common situation of all: an object (a book, say) sitting on a table. Ask students to identify all the forces acting on the book. Common answers: gravity, friction, upward force of table. In each case, ask what direction does the force point, and how large is the force (compared to other forces). There will be differences of opinion, presumably, among the students. Try to bring these out and encourage the students to argue with each other and to try to convince each other. Introduce a MW model of a table and book in gravity where it is possible to see the sagging of the table and to eventually break the table. This directly responds to the standard phrase "elastic forces due to tension or compression in matter." The MW model can include a switch to turn off gravity. Since the book will now fly up, this gives another way that the force of the table on the book is manifest. We may be able to make a MW model that shows the forces on the book, including gravity and the net force of the table.
Next, push on the book and make it slide at a constant speed across the table. Again, ask the students to identify all the forces acting on the book, and their directions and magnitudes. This introduces friction. A probe demo can show that friction causes heating. (Embed a penny in Styrofoam and push this across more Styrofoam--most of the heat then goes into the penny.) Again, demonstrating a molecular model of friction might help show the molecular basis of frictional heating.
Investigations. Using the simulation environment, students draw force vectors that represent their mental model of the forces acting on the book in the two situations: stationary and moving at constant speed. In each case, the activity forces them to take a snapshot of the arrows before they can run the experiment (to encourage them to think about their own mental model before committing to it). After they run the model the activity asks them whether they are pleased with the result or whether they want to change the input parameters and try again.
Assessments: With the computer-based investigations: do they ever get it "right" (i.e., keeping the book from moving in the first place, keeping it from accelerating in the second) and if so, how many trials does it take them and do they converge on the right answer (e..g, if the book is accelerating in the direction of the external force, do they know to increase the frictional force to reduce the acceleration?
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