Forces in balance


8.1 Vectors and scalars

  • Displacement is distance in a certain direction
  • Vectors are physical quantities that have magnitude and direction, e.g. acceleration, force, 
  • Scalars are physical quantities that have magnitude but no direction, e.g. mass, temp
  • The size of a quantity is its magnitude

8.2 Forces between objects

  • Forces (N) can change the shape of an object, or change its motion or its state of rest
  • A force is a push/pull that acts on an object because of its interaction w/ another object 
  • Contact forces are forces that must touch to interact, e.g friction, air resistance, tension. They occur when an object's supported by or strikes another object
  • Non-contact forces are forces that don't need to touch to affect each other, e.g. magnetic force, electrostatic forces, force of gravity
  • Newton's 3rd law states when 2 objects interact w/ each other, they exert equal and opposite forces on each other

8.3 Resultant forces

  • The resultant force is a single force that has the same effect as all the forces acting on an object
  • Newton's first law of motion states that an object at rest will remain at rest unless acted on by an unbalanced force and an object in motion will continue in motion w/ the same speed and direction, unless acted on by an unbalanced force.
  • If the Rf isn't 0, the movement of the object depends on the size and direction of the Rf
  • A free-body force diagram shows forces acting on an object, w/out any other objects drawn. The forces are represented by arrows pointing in the direction of the force

8.4 Moments at work

  • The moment of a force is a measure of the turning effect of the force on an object. 
  • The turning effect can be increased by:
    • increasing the size of the force
    • using a spanner w/ a longer handle


  • A crowbar is a lever used to raise one edge of a heavy object. The weight is the load, the force the person applies is the effort, the point the crowbar turns is the pivot. The crowbar enables only a fraction of the effort needed to life the object. The lever's a force multiplier, as the effort moves a bigger load
  • moment, M (N/m) = force, F (N) x distance from pivot, d (m)
  • T investigate the turning effect…


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