Using cps in a physics class by Charles Chiu

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Using cps in a physics classby Charles Chiu

• Some topics in physics may be covered more
  effectively
• when we intersperse our lecture with Q/A session.
  We
• demonstrate this approach in teaching Fma using
  the
• elevator example. We will use CPS to conduct Q/A
  sessions
• to encourage large class participation.
• I. Startup question Introduce the lesson
• II. Explain on the basic concepts
• III. Followup question reiterate the concepts
  discussions.
• IV. Further question Reinforce and assess class
  learning.

19th Annual Faculty Teaching Seminar, Jan 2004,
UTC 4.104
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I. Introduce the lesson Fma.
Appeal to daily experience Consider the
experience in riding an elevator. There is
weight changes which depend on the motion of
elevator.
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Express familiar situation in more concise terms
of the lesson

• Weight Wmg. g is gravitational acceleration on
  earth, g?10m/sec2 downward.
• Units For a man with mass 50 kg, his weight is
  W mg 50x10 N 50 kg-wt.
  Units 1kg-wt10N. On a bathroom scale,
  it would reads 50kg-wt.  
• Weight change Put a bathroom scale between the
  man and the elevator floor. The apparent weight
  is given by the scale reading S.

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States of motion the corresponding scale
readings
Case-1 Moving with a constant acceleration to
have the velocity going from 0 to 10m/s. Let
scale reading be S1. Case-2 Moving at a constant
velocity 10m/s, scale S2. Case-3 Moving at
another const. velocity 20m/s, scale S3.

•  

velocity (m/s)
S1 S2 S3
20
10
time
0
case-1 agt0 case-2a0 case-3a0
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Ready for the startup-question How do scale
readings depend on the motion?

• A The higher the speed, the heavier the weight.
• S3gtS2gtS1gtW.
• B Initial acceleration gives a heavier feeling.
  Then the higher the speed, the lighter weight.
• S1gtW, S3ltS2.
• C There is the initial heavier feeling. But when
  motion stays at a constant speed, the
  scale-reading reverts back to W. S1gtW,
  S2W, S3W.
• D Apparent weight is only a "psychological"
  concept. Three scale readings should all be
  identical to original weight.
• S1W, S2W and S3W.

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II. Basic Concepts Visualize goings on using
free-body diagram

• Newton's second law of motion When a net force F
  is applied to an object with m, the object will
  move with an acceleration a.
• Free body diagram. The sketch describes the
  forces forces a involved.
• The object is given by a dot labeled by m.
• Direction of a is indicated by an arrow.
• Forces on m are indicated by arrows, with F
  F1F2 along direction of a.
• Example When a is up, the length of S-arrow is
  longer than that of mg-arrow.

Example case-1
S
a
m
mg
S-mgma SWma.
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Solution to the startup question

• Freebody diagram gt SWma.
• Case 1 There is upward acceleration, i.e. a gt0
  (acceleration), S1gtW.
• Case 2 a0. This leads to S2W.
• Case 3, a0. It leads to S3W.
• So "C S1gtW, S2W, S3W is the correct choice.

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Essential steps in using Fma

• Sketch a free body diagram with F1, F2,, m, a.
• Write down the equation ? F1 ? F2 ma.
• Each force along a has positive sign.
• Each force opposite to a has negative sign.
• Solve for the specific force in question.

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III. A followup question (Full hints
Multi-level questions)
Multilevel questions The Extra gives better
students the opportunity to move ahead, while
waiting for others to complete the question.
Solution For the case a0.1g, Smg-ma0.9mg.
Hint Freebody Diag.
mg S ma
Class feedback Followup explanations and
discussions.
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IV. A further question

• What would be the scale reading after the rope
• which suspends the elevator is broken?
• Hint After the rope is broken, the elevator
• will be accelerated downward with ag.
• With less hint. Further reinforce and assess
• the learning of the lesson. More discussions if
  needed.
•  

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Summary

• I. Startup question (Q/A)
• It relates a familiar situation to the
  lesson.
• Expect the lesson will provide insights
  to the answer.
• II. Explanation on the basic concepts
• III. Follow-up question (Q/A)
• With full hints to reiterate key points
  discussed.
• Use two-level questions to involve the
  whole class.
• Class feed back more explanations.
• IV. A further question (Q/A)
• With less hints to reinforce the
  learning of the lesson.
• Objective assessment and more
  discussions.