# FIU Modeling Workshop – Day 8

We began today with a Newton’s 3rd Law demonstration:
Same track set up, with Force sensors attached to the top of each car.  The twist for this demonstration is to use the magnets to apply to force between the two cars and not the direct contact.  There are a couple of things to note for this demonstration.  Have one car against the stopper and start with the second car “far away” from the first car.  Zero both probes, and make sure you reverse the direction for one of them so they both have positive in the same direction of the track.  Have the magnets inside the car so the same pole faces outward and thus repel the cars.  Push the force probe of the second car (not the car itself)
From there, were picked up on the lab with which we finished yesterday.  Before starting the experiment we briefly discussed the merit of breaking the lab groups into different types of investigations, in which we would have 3 different trials: “A” would look at keeping the mass of the cart constant, but adding mass to the hanger; “B” would keep the hanger constant, and add mass to the car; “C” would move mass from the car to the hanger, keeping the mass of the system constant.  In an effort to save time, we have everyone do option “C” but I may or may not look at all the groups (maybe in my honors class?)
Also showing a way to move through the whiteboard process more quickly (as needed by time constraints or if class is not productive in meetings), Jon walked us through a “Circle the wagons” meeting.  In this format, all the groups show their white boards, and the teacher leads the group to try to draw conclusions in looking at all the results at once.
{As we were getting started, Jon also mentioned that when you are “normal” whiteboard meeting after a lab, in subsequent labs, start with a different group each time and change the order you call the groups forward.}
During the meeting, we had a great discussion on whether you should explain/guide to the students before starting the lab that they will need to plot Force vs acceleration so that the slope is mass, or wait until the end.
{My thought is to wait until the end, have all the groups manipulate their graphs, as teacher does it on projected screen}
At this point, Jon showed us a quick follow up demo/lab (used vernier “Lab 9 – Newton’s 2nd Law”)
Jon taped an accelerometer to the force probe (Jon uses Velcro tape at his school).  Then you just click the record data button, and then push the cart back and forth.  Viola, data showing \$F \propto a\$

From there, we started individual work on Unit V worksheet 1 (#’s 1-4) and worksheet 2 (#’s 1-3)
As we got started, we briefly discussed strategies for word problems (w/ forces).  A summary of what we said was:
• Have students sketch what is happening and identify the system with dotted circle/box
• Get the words out of the word problem
• Create a Free Body Diagram
• Next to FBD, draw an arrow showing the direction of acceleration
• That will be “+” direction for the problem
• This convention will aid circular motion problems later in the year
Notes from whiteboard session:
•  Wkst 2: #2 is a great problem since a given number isn’t use in the calculation, but rather for analysis at the end.
• Wkst 2: #3 mass not given, so students need to determine it from the Weight
• Chris- Make sure units are included in the calculation not just at the end
• Possibly change wording of problem since the normal force changes not F­­­w
Jon then went on to describe how he helps his students understand “elevator” problems.  If you are standing on a bathroom scale, and you want to increase your “weight” you can pull on the bottom of the counter and squeeze the scale.  This is the same effect as when the elevator is accelerating upwards.  On the contrary, if you want to lose “weight,” you can push on the top of the counter and push you body off the scale.  This same effect occurs when the elevator accelerates downward.
From there, we Jon showed us some fun demos
1. Have student kneel w/elbows touching knees & hands “praying”.  Put chapstick at tip if fingers.  Then have student place hands behind his/her back. They then need to try to knock over the chapstick by touching their nose to it.  Due to differences in center of mass, girls should be able to do this, while boys usually can’t.
2. Have student stand facing the wall, with toes touching the base of the wall.  Have student take 3 steps (toe to back of heel) away from the wall.  Bend at the waist \$90^o\$ with their forehead touching the wall.  Place a small chair (or other “small” mass) in their hands and tell them to stand up.  Again, boys will struggle,  girls will tend to be successful.
3. Have one student (biggest student) sit all the way back into a chair with his/her feet flat on the floor.  Have a second  student (smallest) stand in front of first student and push into the first student’s forehead.  Tell first student, without moving their feet, to stand up.  At the same time, the second student pushes on the forehead of the first, preventing him/her from standing up.
4. Have student stand with right shoulder and outside of right foot touching the wall.  Then tell the student to lift his/her left foot.
Friction Lab
We then moved on to a lab on Friction. We used a friction block and force probe.  The basic procedure was to the block at constant speed with different masses resting on top of the block.  We used the vernier file “Lab 12a Static Kinetic Fric.”  A sketch of the graph produced looked basically like this:
The max force represents the static friction force, and when the force is basically horizontal (red line) then the friction force equals the measured force.

To speed things up, each group given different normal force (“zero mass” was mass of block plus 250 g) and needed to get good data (slope of oscillating data was as close to horizontal as possible).  Find the average value of the force using the statistics button.
Unit V Feedback
The Good:
• We felt we were becoming comfortable using computer based equipment
• Continuation of the sequence of showing 3rd law  (adding non-contact interaction)
• Low tech demo’s/labs
• Modified atwood machine has a lot of physics baggage we’ll need to use as a paradigm
Suggestions:

• When it comes to multiple representations of event
• FBD, motion maps, graphs, equations
• Only let students say verbal description of the event– Carbon dioxide not “See” “Oh” “Two”

Unit VI:

Our introductory/paradigm demo was Jon and Chris tossing a ball back and forth.  Jon then asked questions like: “Once it leaves my hand, where will it go?””Does the ball have a choice as to where it goes after it leaves my hand?”

One thing that come to mind during this demo was the following video from Veritasium.com:

What do you notice?
What can you measure?
{at this point, Jon showed us the equipment that we would be using}
{Jon build hold that converts dynamic cart w/ spring into ball launcher}
Here’s a rough sketch:

Where the blue shape is the dynamics cart with the spring plunger extended, the silver circle is the ball to be launched, and the brown shape is the holder Jon built out of wood.  He also cut/routed a groove for the ball to roll in on the top of the “shelf.”
After being shown equipment What can you manipulate?
{If you don’t have time to build this and have students tape it, use videos in loggerpro}
Open logger pro
Click Insert – Movie
Click “expand menu” in bottom right corner
Click scale icon (looks like a ruler)
Make sure you have scale (meter stick) in the movie
Click and trace standard length in screen & define length
Click on track (find name) button and click on specific point on object
Continue clicking on the same spot of the object (vernier advances to next frame)
Jon and Chris then tried to show us the classic Monkey-Blow gun demo using the Pasco equipment:

Since this is quite expensive, Jon explained how he made a “homemade version” of this:

Materials:
Electric conduit (1/2 inch? Metal)
Nail with cone of paper hot glued in
Electromagnet
Wire
12 V power source (3 or 6 V should also work)
Target – Balloon with brass mass inside, washer stretching the opening
Stuffed animal with metal screw in its head

He attaches the Electromagnet to the ceiling in the back of his room and runs the ingoing and outgoing wires above is ceiling (drop-down I’m guessing) to the front of his room.  He uses the conduit as the blow gun and makes darts by gluing cones of paper to the head of the nail.  Have the two wires run up the side of the conduit and each extent the bare wires beyond the opening of the conduit.  Bend the wires so they touch in the middle of the opening.  As the dart shoots out, it will separate the wires, breaking the connection. Here’s his sketch:

(click to embiggen)