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Learning Introductory Physics with Activities

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

Section 3.6 Force Analysis

Activity 3.6.1. Warmup.

Observe the motion of the aircar in the video below.
Carry out the steps below for what you observed in the video.
  1. Identify a system and choose a reference frame
  2. Identify external forces and draw a free-body diagram.
  3. Identify the acceleration
  4. Use the Laws of Motion to make claims about the forces
  5. Check that your claims make sense
The five steps above make up a general strategy that can be used to approach many physical situations, This strategy is typically known as the “Forces” approach to solving problems, or Force Analysis.

Exercises Practice Activities

Activity 3.6.2. Metacognition.

Why do you think Force Analysis might be useful for situations like the one above?

Activity 3.6.3. The Aircar.

Consider what changes you could make to the aircar that might result in the following motions: at rest, moving with constant speed, speeding up, and slowing down. For each, use Force Analysis to understand what is happening.

Activity 3.6.4. The Bag of Groceries.

Consider the three situations below. For each, use Force Analysis to understand what is happening.
  1. You hold a bag of groceries in your hand.
  2. You lower the bag of groceries; the bag moves downward faster and faster.
  3. You lift the bag of groceries; the bag moves upward at constant speed

Activity 3.6.5. Moving a Strange Box.

Mike and Lucas are attempting to move a box, which does not move.

(a)

Use Force Analysis to understand what is happening:
  1. Identify a system and choose a reference frame
  2. Identify external forces and draw a free-body diagram
  3. Identify the acceleration
  4. Use the Laws of Motion to make claims about the forces
  5. Check that your claims make sense

(b)

El now pushes the box with a mental force, and it finally begins to speed up.
Figure 3.6.3. Three people trying to move a box.
  1. Modify your free-body diagram
  2. Indicate the direction of the acceleration
  3. Write a symbolic expression for the magnitude of the acceleration in terms of the other forces

Activity 3.6.6. Deducing Forces.

In each case below, three forces act on a \(0.5 \mathrm{~kg}\) object. Two of the forces are shown on the free-body diagrams, but the third is missing.
Figure 3.6.4. Three cases, each showing two of the three forces acting on an object.
Draw and label the missing force vector for each case for the three motions below.
  • Motion 1: \(a = 2 \mathrm{~m/s^2}\) to the right
  • Motion 2: \(a = 3 \mathrm{~m/s^2}\) downward
  • Motion 3: constant speed

Activity 3.6.7. The Book on the Ramp.

A physics textbook is on a tilted, frictionless table, supported by a string.
Figure 3.6.5. A book on a ramp connected to a string.
Use Force Analysis to describe the forces acting on the book.
  1. Identify a system and choose a reference frame (what reference frame do you think will make approaching this situation easiest?)
  2. Identify external forces and draw a free-body diagram
  3. Identify the acceleration
  4. Use the Laws of Motion to make claims about the forces (assume you know the magnitude of the gravitational force is \(F^g\text{.}\))
  5. Check that your claims make sense (suppose the table is slanted so that it becomes steeper: does the magnitude of each force increase, decrease, or stay the same?)

References References

[1]
  
The “Bag of Groceries” and “Deducing Forces” activities adapted from Active Learning Guide for College Physics by Etkina, Gentile, and Van Heuvelen.
[2]
  
The “Moving a Strange Box” activities loosely adapted from Tutorials in Introductory Physics.
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