moments and pressure

• mass is the amount of matter in an object, measured in kilograms (kg)
• weight is the amount of force acting on an object due to gravity, measured in newtons (N)
• weight (N) = mass (kg) x gravitational field strength (N/kg)
• the centre of mass is a point in a body or system at which the whole mass is considered to be concentrated
• the centre of gravity is the point from which a body or system may be considered to act in uniform gravity.
• If you suspend an object then release it, it will sooner or later come to rest with its centre of mass directly below the point of suspension. This object is then in equillibrium
• the moment is the turning effect of the force
• moment (Nm) = Force (N) x distance from pivot (m)
• the line along which a force is acting on an object is called the line of action
• increasing distance from the pivot will increase the moment
• if the turning forces on an system are balanced then they must be the same both clockwise and anti-clockwise
• a lever is a rigid bar that pivots around a fulcrum (pivot). It transmits an input force (the effort) through the lever to a lead.
• the effort needed to life a load depends on:

  • the size of the load
  • the distance between the fulcrum and the load
  • the distance between the effort and the load
• the turning effect of a lever is called the moment or turque
• a lever is a force multiplier
• examples of levers include scissors and doors
• gears are force multipliers
• Gears are a rotating wheel with teeth (cogs) that interlock with another wheel. Used to transmit and often multiply a rotational force. Hence they act in a similar way to each other.
• gears are used to:

  • reverse direction of rotation
  • increase speed of rotation
  • move the rotational moment of another axis
  • keep rotation of the axis synchronised
• a wheel with more teeth (but the same size) turns slower
• a wheel 2x bigger will turn at 1/2 the speed but the moment of the turning speed will have twice the momentum.
• ratio of moment = ratio of teeth : ratio of radii
• pressure (N/m^2) = force (N) / area (m^2)
• every time the particles in a fluid hit the side of the container, they exert a force at the right angles to the container. this is called pressure
• pressure is always exerted at right angles
• pressure increases with depth. this is because water at the bottom of the container is pushed on by the weight of the water further up, which causes it to be higher under pressure
• liquid exerts lateral pressure on the walls of the container
• mass = density x volume
• weight = mass x gravitational field strength x height
• to increase pressure:

  • use a more dense liquid
  • deeper fluid
  • higher gravitational field strength
• pressure = height x density x gravititational field strength
• Archimedes principle - The buoyant force/the size of the upthrust acting on an object is equal to the weight of the fluid displaced.