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Question
A child sits on the end of a playground see-saw. Which of the following values is the most appropriate estimate of the torque created by the child?
  1. $6 \textrm{ N}\cdot\textrm{m}$
  2. $60 \textrm{ N}\cdot\textrm{m}$
  3. $600 \textrm{ N}\cdot\textrm{m}$
  4. $6000 \textrm{ N}\cdot\textrm{m}$
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Solution Video

OpenStax College Physics for AP® Courses Solution, Chapter 9, Problem 4 (Test Prep for AP® Courses) (1:01)

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This is College Physics Answers with Shaun Dychko. A child sits on the end of a playground see-saw and we have to guess which of these estimates of the torque are correct? So we have to make just assumptions about what the mass of the child is and what their distance is from the pivot point? Suppose the child is 30 kilograms, which is about 64 lbs or something and this maybe is about 2 meters and so the torque would be this distance of 2 meters times 30 kilograms times 10 newtons per kilogram; we can substitute 10 for g because you know, we are just dealing with estimates here so 10 is close enough to 9.8 and that makes our calculations a little easier and then times by sin of 90 because the force is straight down— this is the force of gravity and it's perpendicular to the lever-arm. So 600 newton meters looks like a reasonable estimate for the torque and that is answer (c).