476
Appendix B International System of Units
Therefore,
1 m/s = 1.94 knots :;:::j 2.0 knots and 1 km/hour = 0.54 knot.
(B.7)
Velocity is a vector quantity; hence to define velocity completely, the magnitude
of velocity as well as the velocity direction are needed.
B.3.3 Acceleration
Acceleration, a, is the rate at which velocity changes with time; it is expressed
in m/s2. In particular, the specific notation of 9 is reserved for gravitational
acceleration, related to the attraction of any body by the Earth. In particular,
mean gravitational acceleration 9 = 9.806 m/s 2 :;:::j 9.81 m/s2, and varies within
about 1 % with position on the Earth's surface as:
9 = 9.806 - 0.026 cos 2cp,
(B.8)
in which cp is the latitude.
B.3.4 Discharge
Discharge is the rate at which a volume of water passes a section over some
unit of time. In the SI system it is expressed in m 3 /s. Discharge is normally
symbolized by Q.
B.3.5 Force
Force is described in terms of its effects. By Newton's second law of motion,
force is proportional to the product of mass and acceleration. In the SI system,
the unit of force is the newton (N), which is the force necessary to accelerate
one kilogram of mass at an acceleration of one metre per second squared, i. e.:
force (N) = mass (kg) x acceleration (m/s 2 ).
(B.9)
One newton is a very small force. For example, a small apple with a mass of
0.11 kg experiences a gravitational force on Earth of about one newton. Here
we must stress the difference between the mass of a body and its weight. Mass
is an expression of the amount of matter in something, a steel bar or a bucket of
water. Therefore, the mass does not change with the changing of gravitational
acceleration. On the other hand, weight is a gravitational force depending on
gravitational acceleration. Hence, the mass of an astronaut's body walking on
the Moon is the same as on the Earth, however, his weight is approximately 6
times smaller, as the gravitational acceleration on the Moon is about six times
smaller than that of Earth.
It should be noted that kilograms are frequently used incorrectly in terms
of weight. This is a result of the metric systems which expressed weight as
Appendix B International System of Units
Therefore,
1 m/s = 1.94 knots :;:::j 2.0 knots and 1 km/hour = 0.54 knot.
(B.7)
Velocity is a vector quantity; hence to define velocity completely, the magnitude
of velocity as well as the velocity direction are needed.
B.3.3 Acceleration
Acceleration, a, is the rate at which velocity changes with time; it is expressed
in m/s2. In particular, the specific notation of 9 is reserved for gravitational
acceleration, related to the attraction of any body by the Earth. In particular,
mean gravitational acceleration 9 = 9.806 m/s 2 :;:::j 9.81 m/s2, and varies within
about 1 % with position on the Earth's surface as:
9 = 9.806 - 0.026 cos 2cp,
(B.8)
in which cp is the latitude.
B.3.4 Discharge
Discharge is the rate at which a volume of water passes a section over some
unit of time. In the SI system it is expressed in m 3 /s. Discharge is normally
symbolized by Q.
B.3.5 Force
Force is described in terms of its effects. By Newton's second law of motion,
force is proportional to the product of mass and acceleration. In the SI system,
the unit of force is the newton (N), which is the force necessary to accelerate
one kilogram of mass at an acceleration of one metre per second squared, i. e.:
force (N) = mass (kg) x acceleration (m/s 2 ).
(B.9)
One newton is a very small force. For example, a small apple with a mass of
0.11 kg experiences a gravitational force on Earth of about one newton. Here
we must stress the difference between the mass of a body and its weight. Mass
is an expression of the amount of matter in something, a steel bar or a bucket of
water. Therefore, the mass does not change with the changing of gravitational
acceleration. On the other hand, weight is a gravitational force depending on
gravitational acceleration. Hence, the mass of an astronaut's body walking on
the Moon is the same as on the Earth, however, his weight is approximately 6
times smaller, as the gravitational acceleration on the Moon is about six times
smaller than that of Earth.
It should be noted that kilograms are frequently used incorrectly in terms
of weight. This is a result of the metric systems which expressed weight as