3.2 forces in action

• what is the equation for net force?

  net force = mass x acceleration

  f = ma
• what is the unit of force?

  newtons (N)
• how do you work out the weight of an object?

  weight = mass x gravitational field strength

  w = mg
• what is resultant force?

  vector sum of all forces acting on an object
• what is tension? 


  force within a stretched cable or rope
• what is the normal contact force? 


  the force that arises when two objects rest against each-other acting in a 90 degree angle to the plane of contact
• what is upthrust? 


  the upwards buoyancy force acting on an object when it falls through a fluid
• what is friction? 


  force that arises when two surfaces rub against each-other
• what is drag? 


  frictional force experienced by an object ftravelling through a fluid
• what factors effect drag for an object travelling through air?

  speed of the object

  shape and texture

  density of fluid

  cross sectional area
• how to determine centre of mass?

  A freely suspended object will come to rest with its centre of mass vertically below its suspension point, so a plumb line can be used to determine the centre of mass of an object. The object is suspended at a point. A plump bob is suspended from a string directly in front of the object, and a straight line along the string is drawn on to the object. The object is then suspended from another point, and another line is drawn. The intersection of these lines is the centre of mass of the object.
• what is the relationship between speed an drag for an object falling through air?

  drag is directly proportional to the square of the speed
• when does terminal velocity occur?

  when the drag force on an object is equal and opposite to its weight
• what is the moment of a force?

  force x perpendicular distance from the pivot point
• what is the SI unit for moments of force? 


  Nm
• what is true for an object in equilibrium?

  when an object is at rest the net force acting on the object is zero and the net moment is zero
• what is the principle of moments?

  for a body in rotational equilibrium the sum of anticlockwise moments is equal to the sum of clockwise moments.
• what is a couple?

  two equal and opposite forces that when applied to an object parallel to each other but in different lines of action cause a rotation but no translation.
• what is the torque of a couple?

  force x distance between the couple
• what are the conditions for a triangle of forces?

  arrows are drawn end to end and it is a closed shape with a net force of 0
• what is density? 


  density = mass/volume
• what is the units for density?

  kgm^-3
• how to determine density?

  measure mass using measuring scales

  measure volume using water and measuring cylinder or eureka can
• what is pressure? 


  normal contact force per unit area

  p = F/A
• what are the units for pressure?

  Pa - Nm^-2
• what is the equation for pressure in a liquid?

  pressure = height x density x gravitational field strength
• what is the equation for upthrust?

  upthrust = Axρg

  A = cross sectional area

  ρ = density

  x = height of object
• what is archimedes' principle?

  the upthrust exerted on an object wether partially of fully submerged in a fluid is equal to the weight of the fluid it displaces