Unit
Conversions
Basic
Units
Derived Units
Worked Examples
Approximate Conversions to Memorize
Getting the Feel of Conversions
Online Conversion Calculator
Basic
Units
In mechanics there are only
three basic units: length, mass and time. US Customary units are based on
feet (or inches), pounds, and seconds (or hours). MKS, or SI units are based
on meters (m), kilograms (kg) , and seconds (s). Pounds
are used for both mass (a basic quantity, sometimes written as lbm) and
force (a derived quantity, sometimes written as lbf). Under earth gravity
1 lbm weighs 1 lbf.
|
Quantity
|
U.S.
Customary Units
|
MKS
Unit Equivalent
|
|
Mass
|
1
lbm
2.204
lbm
|
0.4536
kg
1
kg
|
|
Length
|
1
in.
39.37
in.
1
ft.
3.281
ft.
|
0.0254
m
1
m
0.3048
m
1
m
|
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Derived
Units
Derived units include force,
velocity, stress or pressure, energy, power, volume and others. Here are just
a few that you will need for the torsion lab. You can find further examples
in standard mechanics textbooks.
|
Quantity
|
U.S.
Customary Units
|
MKS
Unit Equivalent
|
|
Force
|
1
lbf
0.2248
lbf
|
4.448
N
1
N
|
|
Stress
or Pressure
|
1
lb/in2 = 1 psi
0.0001450
psi
1000
lb/in2 = 1 ksi
0.1450
ksi
|
6895.
N/m2 = 6895. Pa
1
Pa
6
895 000 Pa = 6.895 MPa
1
MPa
|
|
Torque
|
1
lbf-in
8.850
lbf-in
|
0.1130
N-m
1
N-m
|
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Worked
Examples
|
Elastic Modulus
|
|
The
elastic modulus, E, of 2024 aluminum alloy is approximately 
. What is E in MKS units?
Using
the basic units and expressing the unit MKS values in terms of their US
equivalents:
This
example could also be done using the derived units,
|
|
Weight
|
|
What
is the weight, in lbf of a circular PMMA (plexiglass) cylinder,
15 mm diameter and 130 mm long?
You need to know the
density of PMMA. It is approximately 1.2 g/cm3.
Mass=volume
x density= 
Mass
in US units= 
Weight
in US units equals the mass in US units, so the weight is 0.068 lbf.
|
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Approximate
Conversions to Memorize
You can approximate most
unit conversions in your head if you can recall a few basic results. Here are
a few numbers you should know:
1
kg approx.2 lbm
1
kg mass weighs about 10 N, thus 2 lbf approx.
10 N, or 1 lbf approx. 5
N.
1
inch approx. 25 mm
1
m approx. 40 inches
1
ksi approx. 7 MPa
1 N-m approx. 10 lbf-in
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Getting
the Feel of Conversions
Knowing the approximate
value of important engineering constants and material properties will also help
you become more comfortable with working with both US and MKS units. Here are
some commonly used quantities:
|
Quantity
|
Value in U.S. Customary Units
|
Value in MKS Units
|
|
Elastic
Modulus of Aluminum
|
|
|
|
Elastic
Modulus of Steel
|
|
|
|
Elastic
Modulus of PMMA
|
420
ksi
|
2.9
GPa
|
|
Yield
stress of low strength steel
|
|
345 MPa
|
|
Yield
stress of very high strength steel
|
|
|
|
A
pencil diameter
|
0.3
in
|
7.6
mm=.0076 m
|
|
Paper
clip diameter
|
.032 in (or 32/1000 in)
|
.8 mm
|
|
Weight
of a small car
|
2250
lbf
|
10
000 N = 10 kN
|
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