Day 106: Why I Teach Free Fall AFTER Forces

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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.

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About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

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