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Moments: A Guide to Rotational Forces and Equilibrium

Learn about moments, the measure of turning effect in physics, and how they relate to stability and equilibrium. Discover how to calculate moments, increase their size, and apply the principle of moments in various real-life scenarios.

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Moments: A Guide to Rotational Forces and Equilibrium

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  1. Moments

  2. Introduction For stability or equilibrium we have already seen that: For rotational forces this means that the effect of the forces rotating the object clockwise must equal the forces rotating the object in an anticlockwise direction. A MOMENT is the measure of the turning effect of a force.

  3. What is a Moment? A moment is defined as: MOMENT (M) = force (f) x perpendicular distance from a pivot (d) So the units of a moment are: Newton metres or Nm

  4. There are two ways of increasing the size of a moment…….. • Increase the size of the force • Increase the distance from the pivot This can be seen in several everyday situations: • A long spanner undoes a nut easier then a short one • Its hard to open a door by pushing near the hinge • a large weight can be lifted using a large lever • A crane has a limited working radius

  5. Increasing the moment….

  6. For equilibrium (balance): The sum of the clockwise moments must be equal to the sum of the anticlockwise moments (moments may be taken about any point on a structure) This is known as ‘The principle of Moments’

  7. For Example…..

  8. Example 2 Unknown weight y=? yx0.1 +5x0.4=0.1y+2 Clockwise moments about fulcrum= • Anticlockwise moments about fulcrum= 20x0.3=6 • For Balance clockwise moments = Anticlockwise moments • 0.1y+2.0=6 0.1y=4 y=40N

  9. Example 3 • What are the sizes of the reactions X and Y at the trestles? Hints: • Take moments about any point • Total Upwards forces must equal total downwards forces • Moments through the point chosen as fulcrum=0 • Answers: Y=240N X=360N

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