Resultant Forces

• Resultant Force


  The overall force on a point or object when multiple forces act on it, which can add together or subtract from each other until there's the equivalent of just one force acting in a single direction
• Free Body Diagrams


  1. Show all the forces acting on an object

  2. Describe all the forces acting on an isolated object or system (i.e. every force acting on the object or system but none of the forces the object or system exerts on the rest of the world)

  3. The sizes of the arrows show the relative magnitudes of the forces and the directions show the directions of the forces acting on the object
• If there are a number of forces acting at a single point, you can replace them with a single force (so long as the single force has the same effect as all the original forces together)
• Resultant Force


  The single force that has the same effect as all the original forces together
• If the forces all act along the same line (they're all parallel), the overall effect is found by adding those going in the same direction and subtracting any going in the opposite direction
• The resultant force is 200 N to the left
• Work Done


  When a force moves an object through a distance, energy is transferred and work is done on the object
• To make something move (or keep it moving if there are frictional forces), a force must be applied
• The thing applying the force needs a source of energy (like fuel or food)
• The force does 'work' to move the object and energy is transferred from one store to another
• Whether energy is transferred 'usefully' (e.g. lifting a load) or is 'wasted' (e.g. overcoming friction), you can still say that 'work is done'. 'Work done' and 'energy transferred' are the same thing
• One joule of work is done when a force of one newton causes an object to move a distance of one metre
• Using Scale Drawings to Find Resultant Forces

  1. Draw all the forces acting on an object, to scale, tip-to-tail

  2. Draw a straight line from the start of the first force to the end of the last force - this is the resultant force

  3. Measure the length of the resultant force on the diagram to find the magnitude and the angle to find the direction of the force
• Scale Drawing of Forces

  • Driving force of 4 N north
  • Wind force of 3 N east
• The resultant force is 5 N on a bearing of 037°
• Equilibrium


  If all of the forces acting on an object combine to give a resultant force of zero, the object is in equilibrium
• Checking if an Object is in Equilibrium

  1. Draw the forces acting on the object to scale in a tip-to-tail manner

  2. If the tip of the last force you draw ends where the tail of the first force you drew begins, the object is in equilibrium
• Splitting a Force into Components

  1. Draw the force on a scale grid

  2. Draw a horizontal arrow from the bottom end of the force and a vertical arrow to the top end of the force to form a right angled triangle

  3. Measure the length of each arrow and convert the lengths to N using the scale
• Splitting a 10 N Force at 53° Above Horizontal

  • Horizontal component = 6 N
  • Vertical component = 8 N