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Lesson 3 Moments – Turning forces.ppt

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Lesson 3 Moments – Turning forces.ppt

  1. 1. Turning effect of a force The turning effect of a force is called the moment of the force The moment is calculated by multiplying the force by the distance from the pivot
  2. 2. Example 1 The parallel forces W and P are acting upwards through A and B respectively. Let W be greater than P. Their resultant (W + P) acts upwards through the point C such that P x y =W x X. Since W is greater than P, the point C will be nearer to B than to A.
  3. 3. Example 1 The parallel forces W and P are acting in opposite directions through A and B respectively. Let W be greater than P. Their resultant (W - P) acts through the point C on AB produced such that P x y =W x X.
  4. 4. Moments of forces The moment of a force is a measure of the turning effect of the force about a point The moment depends on the following: •The magnitude of the force •The length of the lever upon which the force acts (the lever being the perpendicular distance between the line of action of the force and the point about which the moment is being taken)
  5. 5. Resultant moment: When two or more forces are acting about a point their combined effect can be represented by one imaginary moment called the ‘Resultant Moment’. The process of finding the resultant moment is referred to as the ‘Resolution of the Component Moments’. The magnitude of the moment: is the product of the force and the length of the lever. Thus, if the force is measured in Newtons and the length of the lever in metres, the moment found will be expressed in Newton-metres (Nm).
  6. 6. Resolution of moments: To calculate the resultant moment about a point find: • The sum of the moments to produce rotation in a clockwise direction about the point. •The sum of the moments to produce rotation in an anticlockwise direction. •Take the lesser of these two moments from the greater and the difference will be the magnitude of the resultant. •The direction in which it acts will be that of the greater of the two component moments.
  7. 7. Weight & Mass Weight = Mass x Acceleration Mass is the fundamental measure of the quantity of matter in a body and is expressed in KG and TON Weight is the force exerted on a body by the Earth’s gravitational force and is measured in Newton (N) and kilo – Newton (KN)
  8. 8. MOMENTS OF MASS Since: •The Force Of Earth’s Gravity is Constant= 9.81 m/s2 Therefore: •The Weight Of Bodies Is Proportional To Their Mass •The Resultant Moment Of two Or More Weights About A Point Could Be Expressed In Terms Of Their Mass Moments
  9. 9. 3 m 0.5 1 m 0.5 1 m 10 kg 30 kg Moments are taken about O, the middle of the plank. Clockwise moment = 30 X 0.5 = 15 kg m Anti-clockwise moment = 10 X 1.0 = 10 kg m Resultant moment 5 kg m clockwise
  10. 10. What happens when you try to open a door with one finger? Where do you push? Hi, my name is Mr Stick hinges
  11. 11. Do you push near the hinges? I can’t do it!
  12. 12. Do you push far from the hinges? That’s easier!
  13. 13. The turning effect of a force depends on two things; The size of the force Obviously!
  14. 14. The turning effect of a force depends on two things; The distance from the pivot (axis of rotation) Not quite so obvious! Axis of rotation
  15. 15. Turning effect of a force – moment of a force Moment (Nm) = Force (N) x distance from pivot (m) Note the unit is Nm, not N/m!
  16. 16. A simple example! nut spanner (wrench) 50 N 0.15 m
  17. 17. A simple example! nut spanner (wrench) 50 N 0.15 m Moment = Force x distance from pivot Moment = 50 N x 0.15 m Moment = 7.5 Nm
  18. 18. What do you do if the nut won’t move and you can’t push harder?! nut spanner (wrench) 50 N 0.15 m
  19. 19. Get a longer spanner! nut spanner (wrench) 50 N 0.25 m Moment = Force x distance from pivot Moment = 50 N x 0.25 m Moment = 12.5 Nm
  20. 20. More than one force Take an uneven see-saw for an example Do you think we’ll be safe in this power point? It’s not looking good! pivot
  21. 21. If the see-saw is balanced, what must be the weight of the dog on the left? pivot 1.2 m 2.2 m 110 N ? N
  22. 22. The force on the left is trying to turn the see- saw anticlockwise about the pivot pivot 1.2 m 2.2 m 110 N ? N
  23. 23. The force on the right is trying to turn the see-saw clockwise about the pivot pivot 1.2 m 2.2 m 110 N ? N
  24. 24. If the see-saw balances, the turning effect anticlockwise must equal the turning effect clockwise pivot 1.2 m 2.2 m 110 N ? N Anticlockwise moment clockwise moment =
  25. 25. Anticlockwise moment = clockwise moment ? X 1.2 = 110 x 2.2 ? X 1.2 = 242 ? = 242/1.2 ? = 201.7 N pivot 1.2 m 2.2 m 110 N ? N Anticlockwise moment clockwise moment =
  26. 26. Rotational Equilibrium When there is no resultant moment on an object (when anticlockwise moments = clockwise moments) we say the object is in rotational equilibrium. YouTube - Alan Partridge's Apache office
  27. 27. Cont.. Whew! We seemed to have survived the power point pivot
  28. 28. Don’t speak too soon! pivot
  29. 29. pivot BO
  30. 30. pivot BOMB
  31. 31. pivot BOMB
  32. 32. pivot
  33. 33. pivot Nice one!
  34. 34. pivot You’ll have to do some questions now, or he’ll kill me too!

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