Resolving Force Vectors: Interactive Demonstration
In this physics class students will investigate equilibrium, as defined as a situation in which there is no net force acting on an object. The class will focus on the "suspended block" demonstration which will be used to capture the attention of the students, followed a guided development of a mathematical model of the demonstration and the theory of equilibrium. As the teacher presents the demonstration, students will answer some basic questions about the demonstration, share their ideas and answers with a peer and then participate in class discussion. An applet can be used to reinforce the discussion. A sample worksheet for this is provided.
Context for Use
Time required: 20 minutes
Special equipment: Block, pulleys, masses
Pre-requisite knowledge: Weight, Tension, F=ma, Trigonometry
Description and Teaching Materials
After a brief introduction of previously covered material on equilibrium, the teacher should present the suspended block demonstration. The teacher should tell students the mass of the cart, and the masses hanging over the pulleys. The initial question for the students should be: "Is the block sitting on the inclined plane?"
The teacher should hand out the worksheet and guide students to answer questions 1 and 2, discuss the answer with a neighbor and then with the class (Think, pair, share).
After students identify all the forces and discuss their reasoning for whether the block is sitting on the inclined plane, the teacher should remove the inclined plane. The block should not move at all if the demonstration is set up correctly. The class should then complete the sheet slowly, stopping for discussion at the end of each step.
The discussions can be supplemented with an applet such as http://www.walter-fendt.de/ph14e/inclplane.htm . This is a file describing the demonstration theory and set-up. (Microsoft Word 36kB Jul29 07) This is a sample worksheet for guiding student discussion. (Microsoft Word 32kB Jul31 07)
Teaching Notes and Tips
Students also have difficulty breaking the vectors from a free body diagram into components. This lesson forces students to slow down and develop a systematic approach to the vector analysis.