Self-Assessment Day
Today.
1. Problem-solving: are you getting it (if not, why not)?
• Lecture-heavy class, but problem solving skills are
critical!
2. Clicker dry run.
3. Standardized “Pre-test” of conceptual understanding.
• For (your) self-assessment and (my) teaching calibration.
• WILL COUNT AS A CLICKER GRADE.
TODAY HAS THREE PURPOSES.
There are a lot of you and I can’t give you all the one-on-one time I’d love to. Today I’m going to try to get YOU to self-assess where YOU are getting stuck, and how to
get over it while also reinforcing problem-solving procedure. Clicker dry run. No graded ones today because I want to test out the system. PLEASE SET YOUR CLICKER FREQUENCY. This room is ALWAYS AA. BRING
CLICKERS EVERY CLASS IN FUTURE.
We’re also going to do a QUIZ of conceptual understanding.
Basic equations of motion.
(with constant acceleration)
Velocity as a function of time
and acceleration.
v = vo + at
Δx = vot + at
1
2
v 2 = vo 2 + 2aΔx
2
Displacement as a function
of time and acceleration.
Velocity as a function of
displacement and accel.
I realize I rushed through these a little too fast with equations of motion last week so wanted to hammer these in again in a simple way.
Last week we went through the definitions of displacement/velocity/acceleration.
BUT THESE THREE are the more important ones you will typically be using in most (real-life and in physics class) problems.
Except for problems that explicitly ask for average velocity/accel, these equations will be the ones you use most. Basic equations of motion.
(with constant acceleration)
? of time
n
Velocity as a function
e
h
w
t,
s
a
and acceleration.
f
ow
v = vo + at
Δx = vot + at
1
2
v 2 = vo 2 + 2aΔx
H
2
Displacement as a function
of time and acceleration.
Velocity as a function of
displacement and accel.
I realize I rushed through these a little too fast with equations of motion last week so wanted to hammer these in again in a simple way.
Last week we went through the definitions of displacement/velocity/acceleration.
BUT THESE THREE are the more important ones you will typically be using in most (real-life and in physics class) problems.
Except for problems that explicitly ask for average velocity/accel, these equations will be the ones you use most. Basic equations of motion.
(with constant acceleration)
? of time
n
Velocity as a function
e
h
w
t,
s
a
and acceleration.
f
ow
v = vo + at
Δx = vot + at
1
2
v 2 = vo 2 + 2aΔx
H
2
Displacement as
an?function
e
h
w
,
e
r
of time
and
acceleration.
e
h
W
Velocity as a function of
displacement and accel.
I realize I rushed through these a little too fast with equations of motion last week so wanted to hammer these in again in a simple way.
Last week we went through the definitions of displacement/velocity/acceleration.
BUT THESE THREE are the more important ones you will typically be using in most (real-life and in physics class) problems.
Except for problems that explicitly ask for average velocity/accel, these equations will be the ones you use most. Basic equations of motion.
(with constant acceleration)
? of time
n
Velocity as a function
e
h
w
t,
s
a
and acceleration.
f
ow
v = vo + at
Δx = vot + at
1
2
2
2
v = vo + 2aΔx
H
2
Displacement as
an?function
e
h
w
,
e
r
of time
and
acceleration.
e
h
W
?of
e
r
Velocity as a function
e
h
w
,
t
s
a
displacement
w f and accel.
Ho
I realize I rushed through these a little too fast with equations of motion last week so wanted to hammer these in again in a simple way.
Last week we went through the definitions of displacement/velocity/acceleration.
BUT THESE THREE are the more important ones you will typically be using in most (real-life and in physics class) problems.
Except for problems that explicitly ask for average velocity/accel, these equations will be the ones you use most. On average, a thick piece of plywood exerts a negative
acceleration of 2.0 x 106 m/s2 on any incident bullet. Let’s
say a bullet hits perpendicularly to a wall made of 2.0 inch
thick plywood at 2,000 mph.
Will it be generally dangerous when it comes out?
How long will the bullet take to get out through the wall?
1 mile/hour = 0.447 m/s
1 inch = 0.025 m
v = vo + at Δx = vo t + 12 at 2 v 2 = vo 2 + 2aΔx
I wrote this problem as an example of something that encompasses nearly all the things we’ve talked about so far… units, dimensional analysis, scientific notation,
velocities, and accelerations.
Think about this problem and get started on it for 3 minutes, and then I will ask some survey questions about it, and go through it.
NOW WE WILL DO AN INTERACTIVE SOLUTION TO THIS PROBLEM TO SEE WHERE YOU GOT STUCK and HOW TO OVERCOME!
Self-assessment
problem solving
[UNGRADED].
How far did you get?
A. I panicked and froze.
B. The units/scientific notation confused me.
C. Wait… which equation should I use?
E. Got an answer (or was doing fine until I just ran out of time).
There are a LOT of you.
I can guarantee you will all find very different things intuitive.
What I want YOU to do is identify where you got stuck and recognize what to do about it.
This set of questions is for your self-assessment.
Units WILL be on the exam! You just need to practice them.
Self-assessment
problem solving
[UNGRADED].
How far did you get?
A. I panicked and froze.
DEEP BREATH. Remember procedure. Draw
a picture. Write your variables. What objects
and informations are important?
B. The units/scientific notation confused me.
Units: Book Examples 1.4, 1.5, 1.7, 1.8
Sci notation: Read book Appendix A.2
C. Wait… which equation should I use?
PT #3: What variables do I know? What am I trying to solve for?
Which equation has these in it?
E. Got an answer (or was doing fine until I just ran out of time).
There are a LOT of you.
I can guarantee you will all find very different things intuitive.
What I want YOU to do is identify where you got stuck and recognize what to do about it.
This set of questions is for your self-assessment.
Units WILL be on the exam! You just need to practice them.
On average, a thick piece of plywood exerts a negative
acceleration of 2.0 x 106 m/s2 on any incident bullet. Let’s
say a bullet hits perpendicularly to a wall made of 2.0 inch
thick plywood at 2,000 mph.
Will it be generally dangerous when it comes out?
How long will the bullet take to get out through the wall?
Self-assessment problem solving [UNGRADED].
What is the sign of the acceleration vector?
A. Negative.
B. Positive.
C. There will be no acceleration (zero).
D. I don’t know.
Correct: (A)
WHAT’S IMPORTANT? DRAW THE DIAGRAM.
LABEL AXES.
On average, a thick piece of plywood exerts a negative
acceleration of 2.0 x 106 m/s2 on any incident bullet. Let’s
say a bullet hits perpendicularly to a wall made of 2.0 inch
thick plywood at 2,000 mph.
Will it be generally dangerous when it comes out?
How long will the bullet take to get out through the wall?
Self-assessment problem solving [UNGRADED].
What is the sign of the acceleration vector?
A. Negative.
Lesson: DRAW A PICTURE
B. Positive.
AND (labelled, signed) AXES!
C. There will be no acceleration (zero).
D. I don’t
Makeknow.
sure your knowns have
the correct UNITS and SIGNS!
Correct: (A)
WHAT’S IMPORTANT? DRAW THE DIAGRAM.
LABEL AXES.
On average, a thick piece of plywood exerts a negative
acceleration of 2.0 x 106 m/s2 on any incident bullet. Let’s
say a bullet hits perpendicularly to a wall made of 2.0 inch
thick plywood at 2,000 mph.
Will it be generally dangerous when it comes out?
How long will the bullet take to get out through the wall?
Self-assessment problem solving [UNGRADED].
What information do I know?
A. Just a
B. x0, v0, v
C. Δx, v0, a
D. x0, v0, a, t
E. I don’t know.
(C)
I really want to drill this into your heads. If I woke you in the middle of the night and asked you to solve a problem, I want to see variables WRITTEN DOWN!
On average, a thick piece of plywood exerts a negative
acceleration of 2.0 x 106 m/s2 on any incident bullet. Let’s
say a bullet hits perpendicularly to a wall made of 2.0 inch
thick plywood at 2,000 mph.
Will it be generally dangerous when it comes out?
How long will the bullet take to get out through the wall?
Self-assessment problem solving [UNGRADED].
What information do I know?
A. Just
a
Lesson:
WRITE
YOUR KNOWNS
B.INx0VARIABLE
, v0, v
FORM!
C. Δx, v0, a
D. x‘em
0, v0, a, t
Write
down. For real, just
know.do it.
E. I don’t
actually
(C)
I really want to drill this into your heads. If I woke you in the middle of the night and asked you to solve a problem, I want to see variables WRITTEN DOWN!
On average, a thick piece of plywood exerts a negative
acceleration of 2.0 x 106 m/s2 on any incident bullet. Let’s
say a bullet hits perpendicularly to a wall made of 2.0 inch
thick plywood at 2,000 mph.
Will it be generally dangerous when it comes out?
How long will the bullet take to get out through the wall?
Self-assessment problem solving [UNGRADED].
What variable do I need to determine?
A. Final velocity, v.
B. Final time, t.
C. Both final time and displacement, x.
D. Both final velocity and final time.
E. There is no spoon.
(D) and (E)
With or without spoon, you need a plan. Read the problem carefully and THINK ABOUT WHAT IT’S ASKING. Here we want to know IF IT’S DANGEROUS. When I read
that I think that means, is the bullet going really fast when it comes out?
THEN: Need to figure out the time from one end to the other.
On average, a thick piece of plywood exerts a negative
acceleration of 2.0 x 106 m/s2 on any incident bullet. Let’s
say a bullet hits perpendicularly to a wall made of 2.0 inch
thick plywood at 2,000 mph.
Will it be generally dangerous when it comes out?
How long will the bullet take to get out through the wall?
Self-assessment problem solving [UNGRADED].
What variable do I need to determine?
A. Final
velocity, v.problems need to be
Lesson:
Sometimes
B. down
Final time,
t.
broken
into multiple
smaller problems!
C. Both final time and displacement, x.
D. Both
final and
velocity
anda final
Notice
make
plan!time.
E. There is no spoon.
(D) and (E)
With or without spoon, you need a plan. Read the problem carefully and THINK ABOUT WHAT IT’S ASKING. Here we want to know IF IT’S DANGEROUS. When I read
that I think that means, is the bullet going really fast when it comes out?
THEN: Need to figure out the time from one end to the other.
On average, a thick piece of plywood exerts a negative
acceleration of 2.0 x 106 m/s2 on any incident bullet. Let’s
say a bullet hits perpendicularly to a wall made of 2.0 inch
thick plywood at 2,000 mph.
Will it be generally dangerous when it comes out?
How long will the bullet take to get out through the wall?
Self-assessment problem solving [UNGRADED].
Which equation can I use?
A. v = vo + at
B. Δx = vot + 12 at 2
C. v 2 = vo 2 + 2aΔx
D. None of the above.
Which equations have all knowns, one unknown?
Equation A does not; we don’t know how fast OR when.
Equation B is ok; we don’t know when (t), but we know everything else.
Equation C is ok; we don’t know how fast (v), but we know everything else.
So correct answers are (B) and/or (C)! Incidentally, if you solve C, you then know how fast and when, so can solve A.
Or (C) *then* (A)!
On average, a thick piece of plywood exerts a negative
acceleration of 2.0 x 106 m/s2 on any incident bullet. Let’s
say a bullet hits perpendicularly to a wall made of 2.0 inch
thick plywood at 2,000 mph.
Will it be generally dangerous when it comes out?
How long will the bullet take to get out through the wall?
Self-assessment problem solving [UNGRADED].
Which equation can I use?
A. v = vo + at
B. Δx = vot + 12 at 2
C. v 2 = vo 2 + 2aΔx
D. None of the above.
One equation with one unknown is solvable!
Which equations have all knowns, one unknown?
Equation A does not; we don’t know how fast OR when.
Equation B is ok; we don’t know when (t), but we know everything else.
Equation C is ok; we don’t know how fast (v), but we know everything else.
So correct answers are (B) and/or (C)! Incidentally, if you solve C, you then know how fast and when, so can solve A.
Or (C) *then* (A)!
On average, a thick piece of plywood exerts a negative
acceleration of 2.0 x 106 m/s2 on any incident bullet. Let’s
say a bullet hits perpendicularly to a wall made of 2.0 inch
thick plywood at 2,000 mph.
Will it be generally dangerous when it comes out?
How long will the bullet take to get out through the wall?
Self-assessment problem solving [UNGRADED].
Which equation can I use?
How fast, when?
A. v = vo + at
Where, when?
B. Δx = vot + 12 at 2
C. v 2 = vo 2 + 2aΔx How fast, where?
D. None of the above.
One equation with one unknown is solvable!
Which equations have all knowns, one unknown?
Equation A does not; we don’t know how fast OR when.
Equation B is ok; we don’t know when (t), but we know everything else.
Equation C is ok; we don’t know how fast (v), but we know everything else.
So correct answers are (B) and/or (C)! Incidentally, if you solve C, you then know how fast and when, so can solve A.
Or (C) *then* (A)!
Work it through again on
your own!
• Answers:
• v = 772 m/s ~ 1727 mph
• t = 6.1 x 10-5 s = 61 μs!
* solutions may have fewer sig figs.
http://sarahspolaor.faculty.wvu.edu/home/physics-101
These slides and notes will be online.
Feel free to work it through again on your own time with these slides as a guide (they’ll be online)
Pre-test
• For (your) self-assessment and (my)
teaching calibration.
• There are 30 questions. You will have the
rest of class.
• WILL COUNT AS A CLICKER GRADE. You
will get a score of at least 90 just for
answering all questions. Every 0.33 points
above that is one that you got correct.
• Please stay until *at least* 12:20.
To best teach you, I need to know what you know now. Today we take a well-established quiz that is designed to tell me that!
To encourage you to take this seriously, it will count as a clicker grade. I’ll try to load it up to eCampus this week.
Problem Solving Pro-tips
1. Draw a picture!
2. Use and label your reference frame.
3. List what you KNOW and DON’T KNOW in
variable form.
4. Practice helps you pick best formulas!
REMINDER!