Datasheet

Chapter 1: Mechanical Adjustments · Page 23
Scales that show both ounces and grams are not uncommon, likewise with scales that
show both kilograms and pounds. They all appear to work because they are being used
on the Earth's surface. When astronauts took equipment from the earth to the moon, the
equipment weighed less because gravity on the moon isn't as strong. However, each
piece of equipment still had the same mass (total protons neutrons and electrons). Since
scales that measure mass are actually measuring mass based on earth-weight and calling
it mass, those scales would incorrectly tell you the object had less mass on the moon.
Understanding the difference between weight (force) and mass isn't just required for
space travel. There are lots of equations that involve force, mass, and acceleration that go
into the design of all things mechanical. The designs of bridges, generators, airplanes,
cars, and missiles all depend on the correct use of force and mass in a variety of
equations. If an engineer tries to use a force value where a mass value was actually
required, his/her design won't work right. Table 1-1 shows a list of forces, masses, and
accelerations in three different systems - SI, cgs, and British Engineering.
Table 1-1: Units of Force, Mass, and Acceleration
System of Units Force Mass Acceleration
System International (SI)
Newton
(N)
kilogram
(kg)
meter per second squared
(m/s
2)
cgs dyne
gram
(g)
centimeter per second squared
(cm/s
2
)
British Engineering
pound
(lb)
slug
foot per second squared
(ft/s
2
)
The force exerted on an object is equal to its mass multiplied by the rate at which it
accelerates. That's (F)orce = (m)ass × (a)cceleration:
amF ×=
Newton's second law of motion states that the acceleration of an object is directly
proportional the applied force and inversely proportional to its mass.
m
F
a =
To get from this to F=m×a, put the terms on opposite sides of the = sign, then multiply both
sides by m.