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A2L Item 027

Meta-Information
Description: 
Qualitative consideration of the net force on a vertical spring toy.
Goal: 
Contrast internal, external forces and net force.
Level: 
High School
Level: 
Undergraduate
Keywords: 
Dynamics
Keywords: 
Forces
Keywords: 
Mechanics
Keywords: 
Reasoning
Keywords: 
Springs
Source: 
UMPERG

A toy is made from two blocks and a spring as shown at right. When the spring is compressed and suddenly released, the toy will jump off the table surface. Which of the following is true about the net force on the toy just after it is released?

  1. The net force is zero.
  2. The net force points up.
  3. The net force points down.
  4. The direction of the net force cannot be determined.
Commentary

Answer

(2); This question seems difficult but it is available to beginning students. Students can analyze the problem considering the entire toy as a single system or decompose into the separate masses. Viewed as a single system, since the center of mass accelerates up, the net force must point up. Free body diagrams for each mass individually would show no net force on the bottom mass (because the normal force assumes a value necessary to balance gravity and spring force) and a large net force on the upper mass (spring force exceeds gravity). If sketched to scale, the two can be added showing that the net force derives from the normal force on the lower block.

Background

This question is intended to have students distinguish between internal and external forces. The question also can be approached in a variety of ways.

Questions to Reveal Student Reasoning

Can the toy ever leave the surface? Would there be a net force if it did leave the surface?