4.2.2—That, when there is no resultant force and no resultant torque, a system is
- Syllabus
- 9702–2028–2029
- Objective
- 4.2.2
- Level
- AS
A rigid body is in complete equilibrium only when both resultant force and resultant torque are zero
Static equilibrium requires ΣF=0 and Στ=0. The body has no linear acceleration and no angular acceleration in the chosen frame.
Check force balance in each independent direction, then check moments about a convenient point. Satisfying one condition alone is insufficient.
A beam can have clockwise and anticlockwise moments balanced yet still accelerate sideways if a horizontal resultant force remains.
Zero resultant force does not prevent rotation, and zero resultant torque does not prevent translation.
ConceptA-Level CAIE Physics AS