Chapter 2 Review of Forces and Moments - Brown University
Chapter 2 Review of Forces and Moments - Brown University
Chapter 2 Review of Forces and Moments - Brown University
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US units have a frightfully confusing way <strong>of</strong> representing mass – <strong>of</strong>ten the mass <strong>of</strong> an object is reported<br />
as weight, in lb or lbm (the latter is an abbreviation for pound mass). The weight <strong>of</strong> an object in lb is not<br />
mass at all – it’s actually the gravitational force acting on the mass. Therefore, the mass <strong>of</strong> an object in<br />
slugs must be computed from its weight in pounds using the formula<br />
W (lb)<br />
m(slugs)<br />
=<br />
2<br />
g(ft/s )<br />
where g=32.1740 ft/s 2 is the acceleration due to gravity.<br />
A force <strong>of</strong> 1 lb(f) causes a mass <strong>of</strong> 1 lb(m) to accelerate at 32.1740 ft/s 2<br />
The conversion factors from lb to N are<br />
1 lb = 4.448 N<br />
1 N = 0.2248 lb<br />
(www.onlineconversion.com is a h<strong>and</strong>y resource, as long as you can tolerate all the hideous<br />
advertisements…)<br />
As a rough guide, a force <strong>of</strong> 1N is about equal to the weight <strong>of</strong> a medium sized apple. A few typical force<br />
magnitudes (from `The Sizesaurus’, by Stephen Strauss, Avon Books, NY, 1997) are listed in the table<br />
below<br />
Force Newtons Pounds Force<br />
Gravitational Pull <strong>of</strong> the Sun on Earth 22<br />
3.5×<br />
10<br />
21<br />
7.9×<br />
10<br />
Gravitational Pull <strong>of</strong> the Earth on the Moon 20<br />
2×<br />
10<br />
19<br />
4.5×<br />
10<br />
Thrust <strong>of</strong> a Saturn V rocket engine 7<br />
3.3×<br />
10<br />
6<br />
7.4×<br />
10<br />
Thrust <strong>of</strong> a large jet engine 5<br />
7.7×<br />
10<br />
5<br />
1.7×<br />
10<br />
Pull <strong>of</strong> a large locomotive 5<br />
5×<br />
10<br />
5<br />
1.1×<br />
10<br />
Force between two protons in a nucleus 4<br />
10<br />
3<br />
10<br />
Gravitational pull <strong>of</strong> the earth on a person 2<br />
7.3×<br />
10<br />
2<br />
1.6×<br />
10<br />
Maximum force exerted upwards by a forearm 2<br />
2.7× 10 60<br />
Gravitational pull <strong>of</strong> the earth on a 5 cent coin 2<br />
5.1×<br />
10 − 1.1×<br />
10 −2<br />
Force between an electron <strong>and</strong> the nucleus <strong>of</strong> a Hydrogen atom 6<br />
8×<br />
10 − 1.8×<br />
10 −8