Friction (Cambridge International IGCSE Physics)

Friction in solids

Spec point — 1.5.6 · Describe solid friction as the force between two surfaces that may impede motion and produce heating

• Solid friction is the force between two surfaces that may impede motion and cause heating

  • Frictional forces oppose the motion of an object and can slow down the motion of the object

• When friction occurs, energy is transferred by heating

  • Friction causes an increase in temperature of the object and its surroundings
  • The work done against frictional forces transfers energy from the kinetic store of the object to the internal (thermal) stores of the object and its surroundings, causing the rise in temperature

• Friction in solids is caused by imperfections in the surfaces of the objects moving over one another

The interface between the ground and the sledge is bumpy, causing the frictional force

• Friction between solids can be reduced by:

  • lubricating the surfaces
  • smoothing the surfaces

Friction in fluids

Spec point — 1.5.7 · Know that friction (drag) acts on an object moving through a liquid Know that friction (drag) acts on an object moving through a gas (e.g. air resistance)

Friction in fluids

• Gases and liquids are known as fluids

  • Fluids are different to solids because the particles in fluids are free to move around

• Friction acts on objects moving through gases and liquids as the particles collide with the object

  • This type of friction is called drag

Drag in liquids

• When objects fall through a liquid due to the force of weight, the liquid exerts an upwards frictional (drag) force on the object as it falls

• Drag forces oppose the motion of an object

  • They act to slow it down

• Drag increases as the speed of the object increases

• When the drag force becomes equal to the force of weight, the object stops accelerating and reaches a terminal velocity

Drag in gases

• Air resistance is a type of friction that slows the motion of an object moving through air
• Air particles collide with the object as it moves through the air
• As a result, air resistance increases the temperature of the object and the air particles due to heating

The return module of a rocket heats up due to the work done against air resistance as it travels a distance through the atmosphere

• Air resistance is covered in greater depth in the revision note Free fall

• Friction in fluids can be reduced by:

  • streamlining the shape of the object moving through the fluid

Exam Hint Ensure that you name the frictional force acting on an object moving through a fluid as drag or air resistance depending on the fluid. Simply stating "upward force" or "resistance" will not gain the mark.

In addition, note that drag forces increase with velocity, rather than with time.