Statics, Materials and Motion
Statics & Moments
14 min · 2 worked examplesFree
Why a structure stays still, how to find the force in a support, and the lever arm that decides everything.
After this lesson you can
- Apply both equilibrium conditions, not just forces
- Take moments about a sensible point
- Find the reactions at supports
Equilibrium
A body that is not accelerating is in equilibrium, and that gives two conditions rather than one.
The forces must balance: the vector sum is zero, which in components means ΣFx = 0 and ΣFy = 0.
And the turning effects must balance: ΣM = 0 about any point.
Both are required. Two equal and opposite forces that are not in line sum to zero yet still spin the object, that is a couple, and it is exactly how a motor produces torque. Checking only the forces is the most common mistake in a first statics problem, and it produces answers that satisfy the arithmetic while the object rotates away.
In the full lesson
- Equilibrium
- Moments
- Reactions and centre of mass
Free with an account. Read it, work the examples, answer the questions, then build it in the simulator.