Resultant Forces (Edexcel IGCSE Physics: Double Science)

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

Adding forces to find the resultant force

• Resultant forces can be calculated by adding 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

Diagram showing the resultant forces on three different objects

• Imagine the forces on the boxes as two people pushing on either side
  • In the first scenario, the two people are evenly matched - the box doesn't move
  • In the second scenario, the two people are pushing on the same side of the box, it moves to the right with their combined strength
  • In the third scenario, the two people are pushing against each other and are not evenly matched, so there is a resultant force to the left

• Friction is defined as:

A force which opposes the motion of an object

• Frictional forces always act in the opposite direction to the object's motion
• Friction occurs when two (or more) surfaces rub against each other
  • At a molecular level, both surfaces contain imperfections - i.e. they are not perfectly smooth
  • These imperfections tend to push against each other

Friction acting between the surface of a sledge and the surface of the snow

Friction is a force which opposes an objects motion, acting in the opposite direction to the motion of the object