Clockwise or counterclockwise rotation about a fulcrum

  • Thread starter invadertak
  • Start date
  • #1

Homework Statement

The diagram shows a uniform plank of length 5m and weight 225N that rests horizontally on two supports, with 1.1 m hanging over the right support. To what distance x can a person who weighs 450 N walk on the overhanging part of the plank before it just begins to tip?


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Homework Equations

[tex]\Sigma[/tex] T = 0
[tex]\Sigma[/tex] F = 0

The Attempt at a Solution

Take upward direction as positive and clockwise direction of motion as positive
Name the leftmost fulcrum L and the right fulcrum R
Fy = FL + FR - FWplank - FWPerson
Fy = 0
Thus FL + FR - 225N - 450 N = 0
Thus FL = 675N - + FR

This is where I get stuck. My equation kept boiling down to rubbish and when I checked the web for a solution, I found out I need to make the torque produced by Force L a negative number with respect to fulcrum R. However, won't the upward force of L produce a clockwise rotation? The rotation at R caused by force L is the only place that I seem to be messing up as other equations are exactly as the memo indicated. But I have taken clockwise to be positive (as the memo also indicated) and yet the memo says that my torque at L is negative.

Answers and Replies

  • #2
Sorry-one correction- FL=675N - FR