54
Recalling “Newton’s law of gravitational force,” force in freely falling body on
earth is mathematically represented by following formula:
F
G
m m
R
m G
m
R
gravity
e
e
=
=
1
2
1
2
Substituting values of m e and R,
m e = mass of earth = 5.98 × 10
24
kg
R = radius of earth = 6.38 × 10
6
m
F gravity = 9.81 (m/s
2
) × m 1
The value “9.81 m/s
2
” is the acceleration due to gravity usually denoted by
abbreviation “g.”
Newton’s second law is not valid when small particles (e.g., particles settling in
wastewater treatment works) move at a low velocity and in non-turbulent (laminar)
condition. In such condition, Stokes’ law shall be used, which has the following form:
F
R v
d = 6πµ
Where:
F d is the drag force (Stokes’ drag) between the particle and fluid
μ is the dynamic viscosity
R is the radius of the spherical object
v is the flow velocity relative to the object
Stokes’ law makes the following assumptions:
• Laminar flow
• Spherical particles
• Homogeneous (uniform in composition) material
• Smooth surfaces
• Particles do not interfere with each other
Movements of all particulate matter or “particulates,” which include dust, mist or
fume, are governed by laws of physics. Such particles affect health when a person
breathes them in. The movement of particles in lungs depends on the size, shape,
and density of the particulate material. Apart from the physical properties, the
impact on health also depends on the chemical and toxic properties of the material.
Particles are deposited in the lungs by one or combination of the following four
different ways: interception, impaction, sedimentation, and diffusion.
• Interception: Deposition when a particle travels close to a surface of the airway
passages and an edge of the particle touches the surface.
• Impaction: Deposition of particles interfacing an obstacle due to inertia.
• Sedimentation: Deposition due to gravitational forces.
2 Fundamentals of Physics for Environmental and Medical Professionals
Recalling “Newton’s law of gravitational force,” force in freely falling body on
earth is mathematically represented by following formula:
F
G
m m
R
m G
m
R
gravity
e
e
=
=
1
2
1
2
Substituting values of m e and R,
m e = mass of earth = 5.98 × 10
24
kg
R = radius of earth = 6.38 × 10
6
m
F gravity = 9.81 (m/s
2
) × m 1
The value “9.81 m/s
2
” is the acceleration due to gravity usually denoted by
abbreviation “g.”
Newton’s second law is not valid when small particles (e.g., particles settling in
wastewater treatment works) move at a low velocity and in non-turbulent (laminar)
condition. In such condition, Stokes’ law shall be used, which has the following form:
F
R v
d = 6πµ
Where:
F d is the drag force (Stokes’ drag) between the particle and fluid
μ is the dynamic viscosity
R is the radius of the spherical object
v is the flow velocity relative to the object
Stokes’ law makes the following assumptions:
• Laminar flow
• Spherical particles
• Homogeneous (uniform in composition) material
• Smooth surfaces
• Particles do not interfere with each other
Movements of all particulate matter or “particulates,” which include dust, mist or
fume, are governed by laws of physics. Such particles affect health when a person
breathes them in. The movement of particles in lungs depends on the size, shape,
and density of the particulate material. Apart from the physical properties, the
impact on health also depends on the chemical and toxic properties of the material.
Particles are deposited in the lungs by one or combination of the following four
different ways: interception, impaction, sedimentation, and diffusion.
• Interception: Deposition when a particle travels close to a surface of the airway
passages and an edge of the particle touches the surface.
• Impaction: Deposition of particles interfacing an obstacle due to inertia.
• Sedimentation: Deposition due to gravitational forces.
2 Fundamentals of Physics for Environmental and Medical Professionals