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I'm in the Charlottesville airport ready for my trip to see the Portland Thorns. At school, physics fight preparation starts Monday.
Friday was my last class with the AP Physics 1 section. They will be mentoring conceptual students in preparation for the physics fight tournament on May 21.
The discussion in Friday's class turned to physics teaching, We talked about meeting their mentees where they are, not where mentors want them to be. We talked about mentoring not being about the glory of the mentor, but about support for the students being mentored.
As a practical example, I gave this algebra-based section a 30-second explanation of free-fall with air resistance, including the differential equation for velocity and how that results in an exponential change from acceleration of g to terminal speed. As their mouths hung open, I asked the class: how would you have felt if I had started with this presentation on the first day of class? It's fully correct, after all... and are you ready to explain free-fall with air resistance? Why not? I've explained it to you...
They got my point in a visceral way. I saw the students nodding, I saw the figurative light bulb go on over their heads. Not that I won't have to help out some mentors who talk at their group at way too high a level! But at least now I'll be able to say "remember what we talked about?"
FUNDAMENTALS CHECK #11
101. Planet A and Planet B orbit one another. Planet A is four times the mass of planet B. What is the ratio of the net force on planet A to the net force on planet B?
102. Under what conditions is the angular momentum of a system conserved?
103. The unbalanced force on an object is in the direction of ________.
104. A 1 kg block falls from a table to the floor 1 m below; air resistance is negligible. Consider the block-earth system. How much work was done by external forces on the block during its fall?
105. A person stands on a scale in an elevator. Give all ways the elevator could be moving if the scale reading is greater than the person's weight.
106. A block slides along the floor to the right, slowing down due to friction. What is the Newton's third law force pair to the force of friction on the block?
107. A block slides along the floor to the right, slowing down due to friction. What is the direction of the block's acceleration?
108. A block attached to a string oscillates as a pendulum in simple harmonic motion. Describe the direction of the unbalanced force on the block throughout its motion.
109. How do you determine speed from a position-time graph?
110. A meterstick of mass m hangs horizontally. How much torque is provided by the earth about the meterstick's center of mass?
My answers to fundamentals check 11:
ReplyDelete101. 1:1. (Newton's Third Law.)
102. When no net external torque acts on the system.
103. acceleration.
104. zero. (The only force on the block is its weight, that is, the force of the earth on the block. This is internal to the block-earth system. No external forces act on the system at all, let alone do work on the system.)
105. moving up and speeding up; or, moving down and slowing down. (Unbalanced force is up, which means acceleration is up. When an object speeds up, its acceleration is in the direction of motion; when an object slows down, its acceleration is opposite the direction of motion.)
106. the force of the block on the floor. (Friction is the force of the floor on the block, so that pairs with the force of the block on the floor.)
107. left. (When an object slows down, its acceleration is opposite the direction of motion.)
108. There will be a component of unbalanced force acting along the curved path, directed toward the equilibrium position. This is the "restoring force" for simple harmonic motion, which is zero at the equilibrium position and gets larger as the distance from equilibrium gets larger. There will also be a component of unbalanced force toward the center of the circular path. This is the "centripetal force" for an object in circular motion. This force, equal to mv^2/r, will be zero on the edges where the block has no speed for an instant; and will be maximum at the equilibrium position where speed is greatest.
109. from the slope.
110. zero. (The force of the earth acts at the meterstick's center of mass, so has no lever arm about that position.)