GCSE Physics (AQA)

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Using Principle of Moments

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Forces Moments, levers and gears

Using Principle of Moments

3:18 Moments
Spec 4.5.4
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  • A balanced system uses the principle of moments to understand forces.
  • The principle of moments states a system is balanced when total clockwise moment equals total anti-clockwise moment.
  • Equation: Total clockwise moment = Total anti-clockwise moment.
  • Example: Anti-clockwise moment = F1 x D1; Clockwise moment = F2 x D2.
  • Principle of moments: F1 x D1 = F2 x D2.
  • In exams, identify when to use the principle of moments.
  • Example problem: Two children on a seesaw, Kai (300 N) at 0.8 m from pivot, Jamal (400 N) distance unknown.
  • Step 1: Sketch seesaw and forces.
  • Jamal needs to be closer to the pivot to balance the seesaw.
  • Step 2: Calculate Kai's moment: 300 N x 0.8 m = 240 Nm.
  • Step 3: Jamal's moment equals Kai's moment (240 Nm).
  • Step 4: Rearrange moment equation to solve for distance: Moment/Force = Distance.
  • Step 5: Calculate distance: 240 Nm / 400 N = 0.6 m.

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