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Learning Introductory Physics with Activities

Paul J. Emigh, Rebecka Tumblin, Kathryn Hadley, Danielle Skinner

Section 5.2 Changing Systems

In physics, you will often encounter situations where you can choose which System you will analyze. While your choice of system cannot change any physically observable quantity, it can often have a big impact on how your analysis proceeds and what it can reveal about the situation.

Exercises Activity: Point of View

1.

The figure below shows a green mentor perched on the shoulders of his orange trainee. An external Force (not shown) acts on the trainee, causing him and his mentor to move upwards together at constant speed.
Figure 5.2.1.
(a) Perspective 1: Separate Systems.
Start by considering the mentor and his trainee as separate systems. Sketch a free-body diagram for each system. Explicitly identify any forces (across both diagrams) that have equal magnitude, and give a reason for each.
(b) Perspective 2: A Single System.
Now consider a single system consisting of both the mentor and his trainee. Sketch a free-body diagram for each system.
(c) Changing Your Perspective.
Examine your free-body diagrams from the two perspectives closely. Identify at least one force that appears on diagrams from multiple perspectives. Explain why this force appears in both perspectives.
Identify at least one force that appears only on a diagram from one perspective. Explain why this force does not appear in both perspectives.
Choosing a system is a tool for you to analyze part of the natural world: you can always choose any collection of objects as your system of interest.
  • Different systems will have different forces!
  • Things that are internal do not act forces on the system.
  • Things that are external can act forces on the system.
  • The system you choose cannot change physical predictions about what happens to the world.
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