Physics - P3.2 - Using Physics to Make Things Work

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P3.2.1 - Moments

  • Moment: turning effect of a force
  • M = F X d
    • M - moment of force - Nm
    • F - force - N
    • d - perpndicular distance from line of action of force to pivot - m
  • Increase force/distance to pivot = increased moment
  • Easier to use spanner by pushing on end of long one than short one: smaller moment
  • Use levers to make jobs easier - force we're moving: load, force applied to lever: effort - lever acts as force multiplier: force we apply can be less than load
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P3.2.2 - Centre of Mass

  • Object is made of many particles but mass can be thought of as concentrated at a single point: centre of mass
  • Any object freely suspended comes to rest with centre of mass directly below point of suspension: in equilibrium
  • Finding centre of mass: suspend from pin held by clamp - feely suspended: able to turn -> comes to rest: hang plumbline from same pin -> mark plumbline position on object -> hang object with pin with different point and repeat -> centre of mass: where lines marked from plumbline meet
  • Position of centre of mass depends on shape of object - can lie outside object
  • Symmetrical object: centre of mass along axis of symmetry - more than one axis: where lines cross
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P3.2.3 - Moments In Balance

  • Object in equilibrium: balanced, not turning - take moments at about any point and find that: total clockwise moment = total anticlockwise moment
  • Examples of the principle of moments can be seen in sew-saws and balance scales
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P3.2.4 - Stability

  • Line of action of weight of object acts through centre of mass - if line of action of weight lies outside base of object: resultant moment: object topples over
  • Wider base = lower centre of mass: has to tilt further before line of action of weight lies outside base: stability can be increased by making base wider and lowering centre of mass
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P3.2.5 - Hydraulics

  • P = F / A
    • P - pressure - pascals, Pa or N/m²
    • F - force - N
    • A - cross-sectional area at right angles to direction of force - m²
  • Liquids virtually incompressable and pressure in liquid is transmitted equally in all directions: force exerted at one point on liquid transmitted to other points in liquid - used in hydraulic pressure systems
  • Force exerted by hydraulic pressure system depends on: force exerted on system, area of cylinder force acts on, area of cylinder that exerts force
  • Use of different cross-sectional area on effort and load sides of hydraulic system allows system to be used as force multiplier: small effort used to move large load
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P3.2.6 - Circular Motion

  • Object constantly changes direction when moving in a circle: continuously changing velocity: it's accelerating - called centripetal acceleration
  • Object only accelerates when force acts on it - called centripetal force: always acts towards centre of circle - centripetal force stops working: object continues to move in straight line at a tangent to circle
  • Centripetal force is needed to make object perform circular motion - it's increased by: mass of object increasing, speed of object increasing, radius of circle decreasing
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P3.2.7 - The Pendulum

  • Pendulum moves to and fro along same line - example of oscilating motion
  • Simple pendulum consists of mass (bob) suspended on end of string - oscilates through equilibrium position when bob is displaced to one side - equilibrium position is position of pendulum when stops moving
  • Amplitude: distance from equilibrium position to highest point
  • Time period of oscilation: time taken for one complete cycle - depends on pendulum length: increased length = increased time period
  • Frequency of oscilations: number of complete cycles of oscilations/second
  • T = 1 / f
    • T - time period - s
    • f - frequency - Hz
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