FRICTION
➢ This is the force which opposes motion of the body.
VISCOCITY
➢ This is the friction between solids and fluids (gases or liquids) or between fluids.
CAUSES OF FRICTION
Friction is caused by:
✓ Molecular adhesion
✓ Surface roughness
✓ Deformations
❖ All solid materials have some degree of surface roughness. Even surfaces that look smooth can be rough at the microscopic level. They have many ridges and grooves.
LAWS OF FRICTION
The following are some important laws regarding frictional force.
1. Frictional force (Ff) is directly proportional to the normal force (R) between the two surfaces in contact.
That is:
Ff α R
Ff = μR
Where by μ = Coefficient of friction which depends on the type of materials in contact.
R = Normal force/normal reaction
Ff = Frictional force
2. Frictional force is independent of the surface area of contact.
3. Frictional force is independent of the speed once an object has been set in motion.
4. Frictional force depends on the nature (roughness) of the surfaces in contact.
NORMAL FORCE AND FRICTION
NORMAL FORCE
➢ This is the net force compressing two parallel surfaces together.
Its direction is perpendicular to the surfaces. For a mass resting on a horizontal surface, the only component of thenormal force is the force due to gravity.
TYPES OF FRICTION
i) Static friction
ii) Dynamic friction/kinetic friction
i) STATIC FRICTION (FS)
➢ This is the type of friction which occurs when two objects are not moving relative to each other.
❖ Static friction acts because the body tends to move when a force is applied on it.
❖ The initial force to get an object moving is applied mainly to overcome static friction.
❖ Static friction is in most cases higher than the dynamic friction.
o Coefficient of static friction is denoted by μS.
ii) DYNAMIC FRICTION/KINETIC FRICTION (FK)
➢ This is the type of friction which occurs when two objects are moving relative to each other and rub
against each other.
