Torque and Static Equilibrium Notes

Mechanical Equilibrium

• System/object is in equilibrium when net force ($F_{net}$) acting upon it = $0.0 N$
  • Dynamic Equilibrium: System/object is in motion (velocity $≠ 0 m/s$, acceleration = $0 m/s^2$)
  • Static Equilibrium: System/object is not in motion (velocity = $0 m/s$)

Net Force and Torque

• Net force = $0 N$
  • Along x-axis = $0 N$
  • Along y-axis = $0 N$
  • (Along z-axis = $0 N$)
• Net torque = $0 N.m$
  • Sum of clockwise + anti-clockwise = $0 N.m$
  • Note: Clockwise torque/moment is negative, while anti-clockwise is positive.
• Consider forces and torques along a plane (two dimensions).

Gravitational Force and Centre of Gravity

• Gravitational force: Force that objects with mass exert upon each other, always attractive.
• Gravitational force is distributed over the volume of a structure.
• Centre of gravity (cg): A single point at which the total force of gravity can be considered to act. It can be within or outside the body.
• Location of cg can be different from that of cm for the same body.