How to Identify & Label the Forces Acting on a System

• Understanding the Action of a Force:

  • A force is a vector quantity
  • Force shown with arrows
  • Diagram gives visualisation of these forces
  • Has both Magnitude & Direction
  • Direction is important recall Law 2 refers to Net Force
  • Net Force = Sum of All Forces (all acting forces)
  • Know many forces acting on object moving net force shows movement
• Understanding the Action of a Force:

  • Action & reaction same size (magnitude) & direction is static as such of these forces = 0 (static)
  • If push book to cause it to slide, extra vector/force applied (hz movement), object now moving → add up all force, net force = 5N to the left
• Understanding the Action of a Force:

  • A force is a vector quantity
  • Law 2 states “The magnitude of acceleration or deceleration is proportional to the net force acting on the object”
  • Net Forces must be external to the system, thus we also need to define the system
  • & separate system from everything else
• Free Body Diagrams:

  • Free Body Diagrams help us understand the potential action of all forces acting on an object
• Free Body Diagrams:

  • If want to know what is causing the movement then we need to separate the system of interest
  • Identify system of interest then separate it from everything else
• Free Body Diagrams:

  • Boxing: system = face or the fist
  • Fist applying force to face
• Free Body Diagrams:

  • Rugby tackle: both not in contact with the ground, no forces outside acting, one athlete compared to another
  • Player being tackled
• Free Body Diagrams:

  • High Jump: horizontal to vertical
  • Athletes foot with ground
• Free Body Diagrams Steps:

  1. Identify the system
  2. Seperate the system of interest
  3. Identify the CoM of the object
  4. Identify the external forces
  5. optimal... remove the picture leaving the boxes
  6. Find the resultant of the forces
• Free Body Diagrams Step 1:

  • Identify the system
  • Boxing = fist with face
  • Rugby tackle = one person with another
  • HJ = foot with ground
• Free Body Diagrams Step 3:

  • Identify the CoM of the object
  • When talk about net forces only act on CoM of the object
  • Put whole object mass to a point
  • Apply forces to that point in space
  • Need one point to apply force to
• Free Body Diagrams Step 4:

  • Identify the external forces
  • Always some that will be the same; applied force will have reaction force
  • Always be a force of gravity acting downward
  • 3 things that will always be happening:

  1. Action/active force
  2. Reaction force
  3. Gravitational force
• Free Body Diagrams Step 4:

  • FA = Action Force
  • FRf = Reaction Force
  • Fg = Gravitational Force
  • Torques = Gravitational force
  • HJ: more complex components
  • An angle to go up & over the bar (green arrow) will also have a reaction
  • Fnet applied = Action Force
  • Fnet action = Reaction Force
  • FGRF is something referred to as the Normal force
  • Fnet = net force
• Free Body Diagrams Step 4.b (a):

  • optional... remove the picture leaving the boxes
  • Shows force acting on each of these situations
  • HJ: hz component, vt component, net force applied & reaction
  • FGFR = vertical
  • Ff = friction force = horizontal
  • Helps identify forces acting on that mass in that particular situation
  • Then need to get the sum of these vectors to get the outcome
• Free Body Diagrams Step 5:

  • Find the resultant of the forces
  • To identify the action of the Net Force we need to find the resultant of the forces
  • Net force = sum of forces
  • Because all forces are equal & opposite here, we will only use the reaction forces
  • Find Fnet using Pythagoras’ Theorem
  • Tells how much force athlete is applying