Statics: Forces, Equilibrium, and Free-Body Diagrams
Forces in balance
Equilibrium conditions
A rigid body is in static equilibrium when the net force and net moment are both zero: These conditions let engineers determine unknown reactions in supports, beams, and trusses.
Free-body diagrams matter
A free-body diagram isolates one object and shows every external force and moment acting on it. If the diagram is wrong, the calculation will usually be wrong too.
Moments and leverage
The moment of a force about a point is , where is the perpendicular distance from the point to the force's line of action. Moments explain why longer wrenches, levers, and beams can amplify or resist rotation.
How to solve a statics problem
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Step 1: Draw a clean free-body diagram.
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Step 2: Choose axes and a moment point wisely.
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Step 3: Write equilibrium equations.
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Step 4: Solve the unknown reactions or forces.
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Step 5: Check units, signs, and physical reasonableness.
What condition is required for a body in static equilibrium?
Zero net force is required; for full rigid-body equilibrium, net moment must also be zero.
Correct answer: The sum of external forces must be zero
What quantity measures the turning effect of a force?
Torque is also commonly used, especially in rotational mechanics.
Correct answer: moment
Why do engineers often take moments about a support point?
Taking moments about a support point can cancel unknown reactions at that point.
Correct answer: To simplify equations by removing some unknown reactions