Forces as Vectors

• Force Pairs:

  • When there is an interaction between two objects, a force is exerted on each object
  • This is known as a force pair
• Force pairs are described by Newton's Third law:

  • Examples of these force pairs include:
  • The downwards force due to the weight of a laptop resting on a desk, the desk exerts a normal force back up on the laptop
  • The force exerted by a basketball player throwing a basketball is opposed by the reaction force of the basketball on the player
  • The tension force exerted along a cable to a suspended object is opposed by the force of the weight of the object
• • Force pairs can be represented by arrows in vector diagrams
• The force pairs present with respect to a rock being pushed by a person:

  
• The force pairs present with respect to a rock being pushed by a person:

  • The applied force that the person exerts on the rock is opposed by a reaction force from the rock
  • The weight of the rock on the ground is opposed by a normal force
  • The weight of the person is also opposed by a normal force
  • The force applied by the person driving their feet into the ground is opposed by friction
• Forces as Vectors:

  Net Force

  • Vector diagrams include arrows in a particular direction which represent the different forces on an object
  • The size of the arrow corresponds to the size of the force
• Net, or resultant, forces can be calculated by adding or subtracting all of the forces acting on the object

  • Forces working in opposite directions are subtracted from each other
  • Forces working in the same direction are added together
• • If the forces acting in opposite directions are equal in size, then there will be no resultant force – the forces are said to be balanced.
• Resultant forces on an object: