Extension 1: Measuring Force and Distance





Overview

Students found in Lesson 8 that the closer the input (Force Probe) is to the fixed pivot, the more the input buckles.  In Lesson 9 this qualitative understanding becomes a relationship in numbers.  The force on the lever arm (measured in number of jumbo paper clips) times the distance of the force from the fixed pivot is always the same, as long as the lever arm is pulled down the same amount.  Students’ data support the law of the lever.




Materials

For the class  Chart paper and markers  for recording data

For each pair of students

  • Force Machine from Lesson 8
  • 12” link (not numbered) and stand
  • Post-it ™ or small piece of masking tape
  • 25 jumbo paper clips

Procedure

  1. The Force Machine and its variables:  Review Experiment A of Lesson 8. Here are the major points This video shows an analysis of the variables in the Force Machine.
  • Outcome

The two variables explored in Lesson 8 are the force exerted on the Force Probe that causes it to buckle, and the distance of the Force Probe from the fixed pivot, which students varied by pushing down at different points on the lever arm.

  1. Experiment D:  Students compare forces at two distances, 4” and 8”, from the fixed pivot. Why does each pair of force – distance combinations have an equal effect?  First, introduce the set-up for Experiment D  and let students play with it. Then introduce the experiment. See video.  Here are suggestions for working with students on Experiment D and a video on Experiment D data  and their patterns.
  • Outcome

For a given number of clips at 4” from the fixed pivot, the rubber band will be stretched the same amount by half that number of clips at 8” from the fixed pivot.

  1. Experiment E:   Students compare the number of paper clips needed at 3”, 4”, 6”, and 8” from the fixed pivot, each of which has the same effect as 12 clips at 2” from the fixed pivot.  Here is an introduction to the  set-up for Experiment E  and a video on doing Experiment E.  Collect  data from all students in a class data table. Consider differences in the results, possible reasons for these differences, and the likely correct results. Then look for patterns in the data.  Here is a video on Experiment E data.
  • Outcome

The relationship between force and its distance from the fixed pivot is a relationship found by multiplying force x distance. When the force on the rubber band is kept the same, then the force, in paper clips, times the distance of the force to the fixed pivot is always the same. In Experiment E, the number of paper clips x input arm = 24 paper clip – inches.

  1. Science notebooks: Here are suggestions for students' entries in their science notebooks.   

Troubleshooting

The lever arm does not move freely: too much friction. Measurement problems Procedural problems

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