3.2 - Forces in Action

• Archimedes' Principle:

  The upwards force acting on an object submerged in a fluid, is equal to the weight of the fluid it displaces.
• Centre of Gravity:


  The single point through which the object's weight can be said to act.
• Centre of Mass: 


  The single point through which all the mass of an object can be said to act.
• A couple 


  A pair of moments which cause the same direction of rotation
• Density:

  The mass per unit volume of a material.
• Drag:

  The frictional force that an object experiences when moving through a fluid
• Equilibrium:

  For an object to be equilibrium, both the resultant force and resultant moment acting on the object must be equal to zero.
• Friction:

  The resistive force produced when there is relative movement between two surfaces.
• Moment of Force:

  The product of a force and the perpendicular distance from the pivot.
• Newton: 


  The unit of force.
• Newton: 


  The unit of force.
• Newton's Second Law:

  The sum of the forces (resultant) acting on an object is equal to the rate of change of momentum of the object.
• Normal Contact Force:

  The reaction force between an object and surface
• Pressure:

  The force that a surface experiences per unit area. It is measured in Pascals (Pa)
• Principle of Moments:

  For an object to be in equilibrium, the sum of the clockwise moments acting about a point must be equal to the sum of the anticlock wise moments acting about the point.
• Tension:

  The result of two forces acting on an object in opposite, outwards directions.
• Terminal Velocity:

  The maximum velocity of an object that occurs when the resistive and driving forces acting on the object are equal to each other.
• Terminal Velocity:

  The maximum velocity of an object that occurs when the resistive and driving forces acting on the object are equal to each other.
• Upthrust:

  The upwards force that a fluid applies on an object
• Weight:

  The product of an object's mass and he gravitational field strength at its location
• Why does moment of a force and torque of a couple have the same unit Nm

  Both are calculated by a force multiplied by a distance
• Torque of a couple

  For 2 rotating forces the product of one force and the perpendicular distance between them
• Torque of a couple

  For 2 rotating forces the product of one force and the perpendicular distance between them
• Torque

  A moment of a couple
• When is an object in equilibrium

  Resultant force of 0N Principle of moments is met
• Free Body Diagram 


  A diagram showing all the forces acting on an object. lt is a good starting point to any mechanics problem.