The ComPADRE Collections

Resolving Force Vectors: Interactive Demonstration

Marsha M. Hobbs
Jackson Preparatory School
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project


Suspended Block

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.

Learning Goals

The primary goals of this activity are to develop a conceptual understanding of equilibrium and to be able to apply Newton's second law to specific situations. The questions are designed to guide students from the concrete demonstration presented to the abstract mathematical relationships involved. Students will develop an appropriate free body diagram and then use the free body diagram to create the correct, two-dimensional mathematical equations.

Context for Use

Educational level: Introductory physics
Setting: Classroom
Time required: 20 minutes
Special equipment: Block, pulleys, masses
Pre-requisite knowledge: Weight, Tension, F=ma, Trigonometry

Description and Teaching Materials

The teacher should set up the demonstration at the front of the room prior to class, leaving an inclined plane beneath the suspended block initially. See notes in the file suspended block for details.
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 . 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

One of the critical components of this activity is development of the free body diagrams. Students often see free body diagrams presented, but have difficulty in recreating them individually. The first step is for students to draw a good, accurate free body diagram.

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.


Worksheets can be taken up at the end of class for teacher review and grading or the second problem can be given for homework.

References and Resources

More information about this demonstration can be found on PIRA websites. The DCS number for the suspended block is 1J30.10 It works well to Google this number for multiple references.