GCSE P3 Chapter 2

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  • A moment is the turning effect of a force
  • Moment(Nm) = force(N) X distance(m)
  • The distance is the perpendicular distance from line of action of the force to the pivot
  • To increase the moment the force and/or the perpendicular distance must increase
  • Using this principle it is easier to understand why it is much easier to undo a wheel-nut using a spanner than your hand
  • The use of a lever makes jobs easier
  • The force that is being moved is the load
  • The force applied is the effort
  • The lever acts as a force multiplier so that only a small effort is needed to move a large load
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Centre of mass

  • The centre of mass is the point at which the mass of an object is said to be concentrated
  • Any object that is freely suspended will rotate from side to side until it comes to rest at its point of equilibrium. Its centre of mass will be directly below the suspension point
  • The centre of mass of an irregular 2D shape can be found by:
  • The shape is suspended freely in a clamp stand
  • Because it is freely suspended it will move from side to side
  • When it comes to rest it will be in equilibrium so its centre of mass will be directly below the point of suspension
  • Hang a plumbline (piece of string with a small weight on the end) from the same pin
  • Mark the position of the plumbline against the sheet
  • Repeat the procedure hanging the sheet from a different point
  • The position of equilbibium is where the two lines cross
  • Sometimes the centre of mass can lie outside the object
  • For symmetrical objects the centre of mass lies where the lines of symmetry meet
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Moments in balance

  • When an object is in equilibrium it is balanced and not moving
  • The total clockwise moments are equal to the total anticlockwise moments
  • Principles of moments states that for an object in equilbrium the total clockwise moments and the total anticlockwise moments are equal
  • Seesaws and balance scales are examples of principles of moments
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  • The line of action of an object acts through the centre of mass
  • If the line of action lies outside the base of support of the object, there is resultant moment and the object is likely to topple
  • The wider the base and the lower the centre of mass the further an object has to tilt before the line of action lies outside the base and therefore the more stable it is
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  • Liquids are virtually incompressible and the pressure is transmitted equally in all directions so a force exerted at one point on a liquid will be equally transmitted to other points in the liquid
  • Hydraulic pressure systems make use of these properties of liquids to create a very large force to move a very large load using only a small effort bcause act as force multipliers
  • Pressure(Nm2) = force(N) / cross sectional area(m2)
  • A force is applied to a piston which has a small cross sectional area
  • This creates a high pressure (the smaller the area the large the pressure for a given force)
  • This pressure is then transmitted to other parts of the liquid in the other piston
  • The second piston has a much larger cross sectional area
  • This creates a very large force (force = pressure X area so the larger the pressure and area the larger the force generated)
  • The force exerted by the hydraulic pressure system depends on: force exerted on system, area of cylinder on which this force acts, area of cylinder that exerts the force
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Circular motion

  • When an object moves in a circle it is constantly changing direction and therefore its velocity is changing causing it continually accelerate
  • This acceleration is called centripetal acceleration 
  • An object only accelerates when a resultant force acts on it
  • The force acting on an object moving in a circle is called centripetal force and always acts towards the centre of the circle
  • If the centripetal force stops acting the object will continue to move in a straight line at a tangent to the circle
  • Centripetal force is not a force in its own right because it is always provided by another force such as gravitational force, tension or electric force
  • The centripetal force needed to make an object move in a circle increases as:
    • Mass of object increases
    • Speed of object increases
    • Radius of circle decreases
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The pendulum

  • A simple pendulum consists of a mass, called a bob, suspended on the end of a string
  • A pendulum moves to and fro along the same line which is an example of an oscillating motion
  • When the bob is displaced to one side and let go, the pendulum oscillates back and forth through its position of equilibrium. When it comes to rest it will be in its postion of equilibrium
  • Amplitude = from position of equilibrium to highest point on either side
  • Time period = time taken for one complete cycle
  • (from point of equilibrium up to its highest point on one side through equilibrium up to highest point on other side back to position of equilibrium or the time taken from highest point on one side back to highest point)
  • Time period depends only upon length of string
  • Frequency = number of complete cycles of oscillation per second
  • Time period = 1 / frequency
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