Pedagogue Padawan 180

One of several Newton’s 3rd Law activities that students explored today was this setup of 2, 1-kg masses and 3 spring scales connected by string:

Students were instructed to draw free body diagrams for each mass and each scale. They then were asked to predict the reading of each scale. Students that trusted their FBDs made the correct prediction. Students who didn’t most frequently predicted that the scales would each read 20 N. Some predicted that the scales would each read 7 N (20 N split three ways). A great activity to reinforce how powerful a tool free-body diagrams are if they are trusted!

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Today I very carefully introduced students to Newton’s 3rd Law with an interactive demonstration. The equipment setup is two force sensors connected by a string resting on the table. I grip a force sensor in each hand and pull them apart; at first, while keeping them at rest. Students draw system schema and FBDs which we then compare and discuss. Students then predict the relationship between the forces in the two FBDs. I then display the data from the force sensors which clearly indicates the equal magnitude of force on each sensor. We discuss which forces are the same and why. We then repeat the activity for the case of the two sensors moving with constant velocity. We then repeat the activity for the case of the two sensors moving with increasing velocity in one direction. Finally, I demonstrate that no matter how the two sensors move, the force on each by the string has the same magnitude.

The key ideas of this lesson:

• paired forces are always equal in magnitude
• paired forces are the interaction between two objects as captured in the system schema
• paired forces are never found in the same FBD
• constant-velocity motion may result in balanced forces, but these forces are not paired forces

In years past I’ve had students work through these activities in small groups. I have found that carefully leading the whole class through these activities with time to make predictions and time to discuss is much more effective.

System Schema:

FBDs for sensors and table (for changing velocity case) :

Force Sensor Data:

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Several students are analyzing last year’s robot, Sally, from the FIRST Robotics Competition to quantify her performance and uncover the secret behind her exceptional pushing ability. They placed a force plate against a wall and drove full power while against the force plate. Sally provided about 550 N of force!

After working through some of the introduction activities for the balanced forces model, we started whiteboarding the packet problems yesterday. We had a bit of a rough start, but a great discussion, as we struggled with the idea that constant velocity motion is due to balanced forces. I also realized that students had some really good questions about friction that we haven’t yet investigated. Next year, I may add a friction investigation before we start tackling these problems.

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Today in Honors Physics, we explored inertia and Newton’s First Law with seven activity stations and two demos (handout). Some of the activities are familiar (seatbelt Barbie, spin the human, nuts about hoops & bottles), some provide a shared experience for a later discussion of impulse (toilet paper, chopping blocks), and some blow their minds and lead to a great discussion (hitting the stake, floating fishing bobber). One demo is the classic clearing the table and the other is less familiar but equally impressive in which eggs are dropped into cups of water. (Blog post with photos and videos of some of the activities.)

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Today I tried something new for the activity to model the relationship between mass and weight. We didn’t have time; so, instead of having groups perform the experiment, I led the class through it. To make it more engaging, I used a LabQuest 2 from Vernier to gather the data. I then shared the data over an ad-hoc wireless network to iPads distributed to each student. This allowed the students to see the data as it was collected and to perform the analysis on their own. The whole process will work better once the LabQuest 2 is configured on the network, but I was pleased that the LabQuest 2 could handle a room full of iPads and with the overall efficiency of the process.

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In AP Physics B, we started fluids today. I ended class with a demonstration of Heron’s Fountain (without referring to it by name) and challenged the students to examine it and then post an explanation for how it works.

Today we started the Balanced Forces Particle Model unit in Honors Physics. Of all the forces introduced today, we focused most on the mysterious normal force. We bridged from the foam (and spring) to the whiteboard plank to the lab table:

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Today, students in AP Physics B were hired by Physics Toys, Inc.:

Engineers at Physics Toys, Inc. have designed a new battery-powered airplane toy. They have hired you to determine the necessary tensile strength of the string connecting the airplane to the bracket. Physics Toys, Inc. has a variety of equipment, please ask if you need something.

Lots of possible assumptions, measurement techniques, and analysis!

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Today was the lab practicum for the constant-velocity particle model unit. Students had to determine the velocity of two different buggies and then, given a separation of 2 meters, determine where they would collide. I tend to get excited and lose focus; so, I filmed the actual collisions. Students liked watching the slow-motion replay of their buggies’ collision.

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