3.2 Equilibrium of forces and 3.3 Static Equilibrium

Equilibrium

• an object is said to be in equilibrium (AKA mechanical equilibrium) when the 2 conditions are met:

  1. sum of the forces acting on it is 0 N - translational equilibrium → sum of upwards forces = sum of downwards forces
  2. sum of the moments (torques) acting on it is 0 N m - rotational equilibrium → sum of clockwise moments = sum of anticlockwise moments

Stability

• stable equilibrium: if the object is displaced, it experiences a net force or torque in the opposite direction to the displacing force = restoring force/torque
• centre of mass is raised, then returns
• unstable equilibrium: object is displaced, it experiences a net force or torque in same direction as displacing force
• centre of mass is lowered, and continues
• neutral equilibrium: if the object is displaced, its equilibrium is unaffected
• height of centre of mass is unchanged