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Day 3 and Day 4 of Forces:

March 3, 2017

Day 1: What are forces? What is evidence that a force is happening? What do forces do (qualitatively)…

Day 2: Representing forces (FBDs ) and N2nd Law Lab (Collect data with Half-atwoods)

Day 3:  Discussion of N2nd Law Lab (whoa we needed it!), and Horizontal Dynamics Problems

  • We spent a lot of time discussing the Lab from time! Boy was that needed. Students were struggling with the a vs. F graph, what it was even about. Partly this was because we spent all our time in kinematics look at __ vs. time graphs. Also, each point on this graph was a different experiment, from which data came from two other graphs (velocity vs. time and force vs. time)! We spent time re-talking about the slope, etc, and eventually worked toward determining the mass of our cart.
  • Problem 1 was finding distance and top speed for a race car, given the total forward amount of force and total resistive amount of force. We had some clicker questions about identifying what forces act on the car before solving it. I set up the problem, qualitatively, and then students were assigned to work a specific numerical problem from here:  [Note / Edit: Next time, for clicker questions, I would have buggies out riding along a path of brushes, to help examine and look at forces…]
  • For Problem 2, which they worked on their own from the start, was a cart on track being pulled both ways by half-atwood setups with different masses.The force by one string was measured with a force probe, and the motion of the cart was measured with a motion detector. They had to predict the reading on the other force sensor. They had to identify forces, draw free-body diagrams, determine the value of a single force that when acted on the object causes the same acceleration, and then determine the unknown force.  After working that demo problem as a group, individual students were assigned to work an individual problem from here:

Day 4:  Mass/Weight Lab, and Riding Elevator Problem

Students started day with a quick mass/weight lab (50-500 grams) using known masses and vernier sensor. We talked about what their was notice about our graph, and students offered idea like, “it looks linear”… “it looks like the amount of force is about 10x the mass”… another student said 100g gives about 1N of force. They had to use their noticing to predict what the scale would read with 2.0 kg, and we tested that idea. We then tested 1.0 kg, and saw the reading more precisely as 9.8 N/kg. We talked a bit about the formalism of the book’s equation, w = mg, and how g as 9.8 N/kg was totally different than 9.8 m/s/s… a strange coincidence.

We then talked briefly about defining g in this case as gravitational field strength, and weight forces more generally , and a bit about what a scale actually reads, and what are the proper conditions necessary for a scale to “read” the weight.

Then students, were asked to work an elevator problem.

1st: students were asked to draw multiple representations for an elevator ride, with 5 stages. At rest, speeding up, cruising upward, slowing down, at rest again.

Students had to make pictorial representation with Brian identified as the object of interest (standing on a scale), and identify contact and long-range forces acting on Brian. Then a motion diagram with clearly dashes (—) to delineate stages, how v change and the a vector clearly labeled within each stage.

Students then had to make stacked velocity and position vs. time graphs (with —) used to delineate the stages, and then draw a FBD (showing individual force magnitudes) and Fnet. Then use that to help them make a Fnet vs time graph, and a F(reading on scale) vs. time graph.

Once, students had an answer, I made them discuss with another group that was nearing completion. Those groups got the vernier scale and Lab Quest Mini, to go take data and compare to their graph. I talked with each group about assumptions about constant acceleration (and how the graph they see may not be exactly constant force).

If students graphs matched their qualitative predictions, they were then given data from a run I did that morning, and asked to find

  • my mass
  • the elevator’s top speed
  • total distance traveled.

Students were asked to measure the height of one floor in our building and use this to assess their answer. Students did a great job and most groups had a lot of fun.


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