Modeling Unit 5

Unit V: Constant Force Particle Model“Atwood Machine” with vernier track
From there, we started the next unit.  Here’s what each end of the track looked like (the middle is just a track)


Jon changed the first question slightly:
What factors will effect the motion? (between letting go of the cart and hanging mass hitting ground)

What factor effects the cart’s acceleration?
Hanging mass
Mass of car
Friction: {adjust tilt of track until cart rolls at constant speed}
(pasco hanger approx. applies force to balance friction)
Mass of pulley
Mass of the earth/gravity
Angle of the track – ?
Starting speed -?
{Chris showed us a quicker way of working through the process by guiding us to eliminate the factors mentioned that cannot be adjusted (Mass of Earth) or that could be removed with creative lab design.  The last two options we left open, that depending on your class, you may or may not want to divide and conquer.}
Purpose: What is the graphical & mathematical relationship that exist between the mass of the cart and the force that is accelerating it.
Before getting started, we talked about multiple variations to this experiment

  • Keeping the mass of the hanger while adding mass to the car
  • Using photogate(s) above the track instead of motion detector
    • Variation of this option is to attach picket fence to cart the cart and use vernier program
  • Using kinematic equations and measure total time with stopwatch for total distance measured
  • Having the students predict the mass of the system from data, and then, after showing prediction to the teacher, measuring the mass and comparing results to predictions {I like this!}
Attach right angle to cart
Pulley at end of track
Motion detector
Standard masses

Next up were some demonstrations of  Newton’s 3rd Law:
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
The Bad:
  • Modified atwood machine has a lot of physics baggage we’ll need to use as a paradigm


  • 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”

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