Statics- equilibrium of rigid bodies

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SUMMARY

The discussion focuses on determining the vertical reaction forces at points A, B, and C for a hinged plywood sheet under static equilibrium. The plywood measures 1×1.2 meters and has a mass of 18 kg, with forces acting only in the vertical direction. The key insight is that when taking moments about point A, only the reaction force at support B is considered due to the nature of the forces being aligned along the y-axis. Understanding the concept of taking moments about an axis rather than a point is crucial for solving similar 3-dimensional static equilibrium problems.

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  • Understanding of static equilibrium principles
  • Familiarity with Free Body Diagrams (F.B.D.)
  • Knowledge of moment calculations in rigid body mechanics
  • Basic concepts of forces acting in the vertical direction
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  • Explore moment calculations about different axes in 3-dimensional problems
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Homework Statement



An opening in a floor is covered by a 1×1.2-m sheet of plywood of mass 18 kg. The sheet is hinged at A and B and is maintained in a position slightly above the floor by a small block C. Determine the vertical component of the reaction (a) at A, (b) at B, (c) at C.

Homework Equations


The Attempt at a Solution



I know how to approach this problem, however, I am just a bit confused on how to set up my F.B.D. i attached the correct f.b.d for this problem . My question is, how come if we take the moment about point A, we only have a reaction force in the j direction at support B?
 

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The plywood is hinged at A and B and the only loads imposed are the weight and the reaction due to the block at C. All of these forces are in the direction of the y-axis. It doesn't matter about which point you take moments, it's just the nature of the forces for this particular problem.
 
You take moments about an axis, not a point. That is the key to solving 3-dimensional problems like this.
 

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