# Day 108: Intro to Projectile Motion and Video Analysis

*College-Prep Physics*: This is our first time doing video analysis and I was only able to get the computer lab for 1 class period. So I made the activity very structured. We used a pre-made video from Live Photo Physics and I gave detailed instructions on the mechanics of video analysis using Logger Pro.

You can download the instructions here: Projectile Motion Video Analysis DIRECTIONS 2014

NOTE: I can’t seem to find the video on the Live Photo Physics website, so I’ve upload it here: Ball_Toss_12.mov If anyone knows the original location of the video, please let me know.

# Day 106: Why I Teach Free Fall AFTER Forces

*College-Prep Physics:* On whiteboards, students were asked to predict the model for the force and motion of a dropped ball soccer ball (m = 0.4 kg):

- Draw a motion diagram, a position-time graph, a velocity-time graph, and a force diagram.
- Determine the ball’s acceleration.

Then, as a class, we tested our predictions using Logger Pro and a motion detector (red graph above).

Next, I asked them to determine the acceleration for a ball with twice the mass. Most groups immediately said it would still be -9.8 m/s/s. Some referred to prior knowledge that all objects fall at the same rate. Others referred to the math they just did on their whiteboards — doubling the mass doubles both the gravitational force on the ball and its mass, so the acceleration will remain the same.

Gravitational acceleration = 9.8 m/s/s for all objects. Why? Why not? *(Sometimes, always, never)*

Show hammer and feather dropped on moon.

Now predict the model for the force and motion of a tossed ball rising and falling — just after leaving hand to just before catch.

- Motion diagram, position-time graph, velocity-time graph, force diagram.
- Determine the ball’s acceleration.

What’s the same as before? What’s different than before? Why?

Then, as a class, we tested our predictions using Logger Pro and a motion detector (blue graph above).

Because students had already done forces, Newton’s Laws, and dynamics, these free fall scenarios were just natural extensions.

# Day 105: Real-World Problem Solving (Dynamics)

*College-Prep Physics: *

**DO ANY TWO **(in order of complexity)

1. Determine the starting and stopping acceleration of the elevator:

2. Determine the coefficient of kinetic friction between any two surfaces using only a meter stick, stopwatch, and a balance.

3. Determine the force of the slap shot:

Students were required to use the dynamics problem solving template. Sample work for each problem below:

##UBFPM

# Day 102: A Visual Model for Polarization of Conductors

*AP Physics C: *This works best on an overhead projector(!). Take 2 pieces of colored cellophane or glass filters that are the same size. The blue represents the mobile sea of negatively charged electrons. The red represents the stationary positively charge atomic cores (nuclei).

Then place the electron sea on top of the atomic cores to represent a neutral conductor. Notice it’s black.

The electron sea shift slightly in the presence of a charged object (slide the blue filter), causing one side to become positively charged and the other side negatively charged.

I like this because it shows that the conductor as a whole is still neutral. It also refutes the misconception that lots of electrons all move to one side. In reality, it’s just a slight shift in the sea of electrons.