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Teaching Physics Blog

AP Physics:Free-Fall Motion Inquiry Activity

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Learning Objectives
• Understand and apply the principle of free-fall motion in a uniform gravitational field.

Assessment Type
This inquiry activity is a wonderful launch point for a discussion on free-fall motion and particularly the non-negligible role that air resistance play in free-fall acceleration. The activity can also be used as a formative assessment of students' abilities to design an experiment, generate and test a hypothesis, and make careful observations.

Assignment Details
  1. Provide students with just a few basic supplies: manilla folders, empty film canisters, and pennies.
  2. Ask students to collect data using these simple tools to answer the following two questions:
    a) Does mass impact the rate at which objects fall?
    b) Does shape impact the rate at which objects fall?
  3. Allow students to experiment, provide as much autonomy as possible and try to accommodate student requests for other resources (rulers, timers, tape, etc) as they carry out their experiments.
  4. After students have experimented for some time, ask them to share their conclusions by facilitating a class discussion. Any statement made by a student or group should be confirmed by a simple experiment shown to the class.
  5. Through the process of testing one another's conclusions, work with the students to generate a statement describing the conditions under which mass and shape seem to impact the rate at which objects fall, being sure that everyone agrees to the language used.
  6. As a final test of their conclusions, choose two new objects that the students have not experimented with -- say a styrofoam cup and a plastic cup. Ask the students to discuss which would hit the ground first when dropped from the same height, and under what conditions the opposite result could be achieved.

Instructor's Notes
  • Physics instructors often teach students that mass does not impact an object's free-fall rate. While this is true in the absence of air resistance, it is not true under normal circumstances. This activity can be used to demonstrate why Galileo's understanding of free-fall motion took thousands of years to emerge.
  • The joy and challenge of this activity is the open nature of the experimentation. Allow students to pursue experimental dead ends and even to make incorrect conclusions from their experiments. Through the class discussion, students will confront their misconceptions and be forced to reconcile them with the experimental evidence.
  • This activity can be used as an introduction to free-fall motion, before the simplifying assumption of ignoring air resistance is made. It could also be used after free-fall motion has been studied in detail and as an introduction to friction and/or terminal velocity.

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