Physics 106P: Lecture 1 Notes


Just-in-Time Teaching

(in a Large Lecture Setting)

(Mats Selen, UIUC Department of Physics)

Q: Is conceptual understanding as important as problem solving skills?

A: YES, of course !

Q: What can we do to help develop this skill??

Modify lecture structure: “ACTs”

“Pre-Flights” before every lecture

 Implementation

 Results

Page 1

Setting the stage:

Physics 101 overview:

 Algebra-based intro physics.

 About 400 “pre-med” students per semester

 Covers mechanics, heat & thermo, waves & sound

 Very fast paced, lots of work, 5 credit hours.

Weekly structure:

 Two 1-hour lectures

» Active learning

 One 2-hour discussion section

» includes weekly quiz

 One 3-hour laboratory section

» Hands-on “discovery”

 Web based homework

» Great system…another talk

Page 2

Why we should stress concepts:

Physics 101 Midterm Exam 1, Spring 2000

On this exam, the conceptual problems were worth about 30% of the total.

Page 3

Pre-Flights !!

Students are asked to answer a set of conceptual questions

(on the Web) prior to every lecture (and discussion, and lab).

The main structure is:

 Students read about material in text.

 Students answer pre-flight questions on material prior to lecture.

» Physics 101 PF’s due at 6am, lecture starts at 1pm.

» Graded on participation, not correctness.

 Instructor uses pre-flight responses to guide lecture preparation.

» Stress difficult material

 Pre-flights are reviewed during lecture, often presented again a ACTs, and often capped off with a demo.

With careful preparation, the pre-flights can form the

“backbone” of the lecture.

Page 4

What the students see on the web:

What I typed in a simple text file:

Page 5

Simple setup on our NT server:

Text (& pictures) for PF 2 in here

Page 6

The instructors interface to the student responses (also on web): Statistics:

Free response:

Page 7

Lecture 2, Pre-Flights 1&2

If the average velocity of a car during a trip along a straight road is positive, is it possible for the instantaneous velocity at some time during the trip to be negative?

1 - Yes

2 - No correct


As long as the net distance traveled over the given time was positive, the average velocity will be positiveregardless of whether the car went in reverse at any point during that time.

I could have forgotten something at home and had to turn around, but eventually I reached my destination away from my starting pt.


Velocity cannot be negative in reality.

0% 20% 40% 60% 80%

Page 8

Lecture 6, Pre-Flight Questions 7&8

Two identical boxes, each having a weight W, are tied to the ends of a string hung over a pulley (see picture). What is the tension T in the string? [see text 4.10]

1. T=0

2. T=W

3. T=2W correct

0% 20%




40% 60%

Page 9

Students see their own answers

Two identical boxes, each having a weight W, are tied to the ends of a string hung over a pulley (see picture). What is the tension T in the string? [see text 4.10]

1. T=0

2. T=W

3. T=2W

Due to Newton's second and third laws, the rope itself is massless, so any force transmitted across it is done so without the diminishing of any magnitude. As each box has an equal weight, the tension T must be zero, as each box's force cancels the other's out.

The force applied to the rope is transmitted to the other side. This example would be just like a person hoisting up a box, pulling on the rope with a force of

W. In this case, the tension would just be W.

The string has the tension of two weights..

The tension on the string would be the same if you hung the two boxes on the same end. making it equal to 2W.

Page 10

Lecture 20, Preflight 1

Suppose you float a large ice-cube in a glass of water, and that after you place the ice in the glass the level of the water is at the very brim.

When the ice melts, the level of the water in the glass will:

1. Go up, causing the water to spill out of the glass.

2. Go down.

3. Stay the same.






20% 40% 60%

Page 11

Nice Tools:

We can “filter” on responses based on other questions !!

Page 12

Lecture 20, Preflight 2

Which weighs more:

1. A large bathtub filled to the brim with water.

2. A large bathtub filled to the brim with water with a battle-ship floating in it.

3. They will weigh the same.

CORRECT Tub of water



Students who got

Preflight #1 right.


Tub of water + ship

0% 20% 40% 60% 80%




Students who got

Preflight #1 wrong

0% 20% 40% 60%

Page 13

Lecture 12, Preflight 2 & 3



What physical quantities are conserved in the above collision?

1. Only momentum is conserved

2. Only total mechanical energy is conserved

3. Both are conserved

4. Neither is conserved


Since there are no external forces acting on the cars, linear momentum is conserved. However, since there is an inelastic collision between the cars, energy is lost.






60% momentum of the system is not conserved because the cars slow down upon impact. however, energy is never created nor destroyed, only transferred.

Momentum is conserved because the net external force acting on the isolated system is zero

Mechanical energy is conserved because the net work done by external non-conservative forces is zero.

it is an inelastic collision, so momentum and energy are lost

Page 14

Sometimes we are surprised...

Answers to question 1

25% 25% 27%

59% 61% 57%


10% 8%

6% 5%


0% 20% 40%

All Students

60% 0% 20% 40% 60% 80%

Students with answers to Preflight 2 right

0% 20% 40% 60%

Students with answers to Preflight 2 wrong

Explanation: Question was actually quite “abstract” since student could not visualize where the energy went

Page 15

Students have fun with answers...

Shown is a yummy doughnut. Where would you expect the center of mass of this breakfast of champions to be located?

(Explain your reasoning Homer).

In the center. Assuming a perfectly symmetrical donut, all the mass is equidistant from the center.

Until someone takes a bite. (Doh)

CORRECT you're not getting my answer unless i get sprinkles.....suckers ! unfortunately, i think the center of mass of this perfectly symmetrical donut would be the center of the donut which does not seem to exist; so, i'll just say homer ate it.

I think it would be in a the middle of the dough in a circular pattern. Kind of like the onion in an onion ring.

UMMMMM..... Onion rings!!!!


Page 16

Some students thoughts on doing ~50 pre-flights

Page 17


Are they worth the work ?

Well…what is the work?

 Question preparation.

» Carefully chosen questions can be the backbone of the lecture.

» Takes some time to get it right.

 Last minute “on the fly” lecture preparation.

» Con: Takes me all morning on the day of lecture.

» Pro: Takes me all morning on the day of lecture.


 Students seem to like it.

» They know I take the time to read their responses.

» They have a stake in the lectures.

 I love it !!!!

» The most fun I have had teaching.

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