AQA A Level Maths: Mechanics复习笔记4.1.2 Using Moments - Equilibrium

What happens with moments if a rigid body is in equilibrium?

• If a rigid body is in equilibrium then:
  • The resultant force is 0 N
  • The resultant moment about any point is 0 N m

   

• The sum of the magnitudes of the clockwise moments equals the sum of the magnitudes of the anti-clockwise moments

How do I solve problems involving moments and equilibrium?

• Step 1: Identify all forces and perpendicular distances from the pivot
• Step 2: Use the fact that the resultant force is 0 N to form an equation
• Step 3: Use the fact that the resultant moment about any point is 0 N m to form equations
  • If there is an unknown force you can choose a pivot that is on the line of action of that force – this will mean that the moment of that force is 0 N m

   

• Step 4: Solve the equations

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