• The effects of forces on an object often depend on the type of force acting

  • The push force (thrust) of an engine can cause a car to speed up, whilst the force exerted by the brakes (friction) can cause it to slow down

  • The gravitational pull of the Sun on a comet causes the comet to change direction

  • When two opposing forces push on each end of a spring, the spring changes shape (it compresses)

What is a resultant force?

• A resultant force is a single force that describes all of the forces operating on a body

• When multiple forces act on one object, the forces can be combined to produce one net force that describes the combined action of all of the forces

• This single resultant force determines:

  • The direction in which the object will move as a result of all of the forces

  • The magnitude of the net force experienced by the object

Balanced and unbalanced forces

• The forces acting on an object can be described as balanced or unbalanced

  
  

• Forces are balanced if multiple forces act in opposing directions with an equal magnitude in each direction

  • The effects of the forces then cancel out

  • There is no resultant force in that plane of direction

• Forces are unbalanced if the effects of the forces acting in each plane do not cancel out

  • There is a resultant force in one or more planes of direction

• A book is a rest on a table

  • The gravitational pull of the Earth on the book (weight) acts in a downward direction

  • The push force of the table on the book (normal contact force) acts in the upward direction

  • The forces are equal in magnitude and opposite in direction

  • The forces are therefore balanced

  • There is no resultant force acting on the book