IMPORTANCE OF UNITS

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Chemistry in Action
THE IMPORTANCE OF
UNITS
In December 1998, NASA launched the 125-million
dollar Mars Climate Orbiter, intended as the red
planet’s first weather satellite. After a 416-million
mi journey, the spacecraft was supposed to go into
Mars’ orbit on September 23, 1999. Instead, it
entered Mars’ atmosphere about 100 km (62 mi)
lower than planned and was destroyed by heat.
The mission controllers said the loss of the
spacecraft was due to the failure to convert
English measurement units into metric
units in the navigation software. Engineers
at Lockheed Martin Corporation who built the
spacecraft specified its thrust in pounds, which is
an English unit. Scientists at NASA’s Jet
Propulsion Laboratory, on the other hand, had
assumed that thrust data they received were
expressed in metric units, as newtons. Normally,
pound is the unit for mass. Expressed as a unit for
force, however, 1 lb is the force due to gravitational
attraction on an object of that mass. To carry out the conversion between pound and newton,
we start with 1 lb = 0.4536 kg and from Newton’s second law of motion,
force = mass x acceleration
= 0.4536 kg x 9.81 m/s2
= 4.45 kg m/s2
=4.45 N
because 1 newton (N) = 1 kg m/s2. Therefore, instead of converting one pound of force to 4.45
N, the scientists treated it as 1 N. The considerably smaller engine thrust expressed in newtons
resulted in a lower orbit and the ultimate destruction of the spacecraft. Commenting on the
failure of the Mars mission, one scientist said: “This is going to be the cautionary tale
that will be embedded into introduction to the metric system in elementary
school, high school, and college science courses till the end of time.”
SOURCE: Chemistry 10th edition by Raymond Chang
Chemistry for Engineers
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