EXERCISE THERAPY
DF2021/P
Friction and Lubrication
Friction
Friction occurs when one body slides in relation to another; the frictional force (FF) is a shear force
occurring between two adjacent surfaces which are in contact. The shear resistance is due to the
uneven nature of the surfaces of materials and the consequent tearing resulting during motion. As
the pulling force is applied to the body, the frictional force will match its value (Newton's Third
Law – 'to every action there is an equal and opposite reaction') prior to movement; once movement
occurs the frictional force will be of a constant value, determined by:
(1) The nature of the materials involved
(2) The value of the normal load pressing the two surfaces together.
Static friction is the term used for the frictional force occurring prior to movement of the body
taking place.
Dynamic friction is the level of frictional force observed during movement.
Types of Friction
(1) Dry: the surfaces of the materials are dry and clean.
(2) Boundary: the surfaces of the material are coated with contaminants, e.g. grease, dust and other
debris.
(3) Viscous (fluid): fluid is present between the surfaces.
Coeficiency of friction
Is the ratio of the force needed to overcome the friction to the force holding the two surfaces
together.
The higher the coeficiency of friction the more stable the object.
The friction force acts in a direction parallel to the area of contact, and opposes the motion or the
tendency to move. The friction force depends on two things: – The normal force (Rn) – The nature
of the surfaces involved (µ).
Limiting Value of Static Friction
As more force is applied, the friction force increase. The friction force will continue to increase
until the instant immediately prior to the initiation of movement.
Advantages of friction

Prevents slipping and injuries. E.g when walking.

Helps in rolling movement. E.g wheel chairs/cars.

Helps fix things. E.g nails on boards or walls.

Helps in smooth sliding movements. E.g writing.

Helps things or animals move in air. E.g parachutes or birds.

Helps produce heat.

Helps in stopping movement.

Keeps positions of objects. E.g furniture and standing bodies.
Application of friction in Physiotherapy

Use of rubber on walking aids to prevent falls.

Use of brakes in wheelchairs.

Use of rolling wheels in wheelchairs (rubber wheels).

Use of gym carpets/blankets to prevent slipping/falls.
Lubrication
Friction is important in the prevention of slipping. However, there are times when the therapist
wishes to reduce its value in order to facilitate an action. Lubrication is the use of an additional
material to separate surfaces. Lubricants act to even out the surfaces. Lubricants come in many
forms; for example, talcum powder to lubricate the skin as it slides over re-education boards, fabric
for use in blanket pulls across the gym floor and oil in massage.
Advantages of lubrication

Provide coating.

Protects equipment from erosion, wear and tear.

Helps cool temperature.

Improves efficiency of machinery.

Prevents breakdown and failure of equipment.

Helps in facilitating sliding movements.
Application of Lubrication in Physiotherapy

Use of powder on sliding boards to make exercises easy in weak patients.

Use of powder and oils in massage.

Use of gel in ultrasound application.

Use of grease/oil in machines e.g pulleys, treadmills, ergometers.