Prob 1.2
8/18/04
PROBLEM 1.2
PROBLEM STATEMENT:
If an object weighs 170 lbf on Earth, find its mass in
lbm and kg and its weight in Newtons.
GIVEN:
The object’s weight in English units (170 lbf).
FIND:
The mass in lbm and kg and the weight in Newtons.
ASSUMPTIONS:
g= 32.2 ft/s
2
= 9.8 m/s
2
GOVERNING RELATIONS:
.
c
c
22
mg
W
g
lbm ft
kg m
g3
2
2
1
sl
b
f
sN
=
⋅⋅
==
PROPERTY DATA:
Not applicable
QUANTITATIVE SOLUTION:
.
.
2
c
2
c
Wg
mg
170lbf 32 2lbm ft s
lbf
Wm
1
7
0
gg
32 2ft s
⋅
=⇒
=
=
=
l
b
m
.
./
..
.
2
2
c
170lbm
m7
7
1
1
k
g
2 2046lbm kg
mg
77 11kg 9 8m s
W7
g
1kg m s
N
⋅
=
5
5
7
N
DISCUSSION OF RESULTS:
This problem illustrates the importance of using the units conversion factor g
c
.
Note that, even though g
c
has the numerical value of 1 in SI units, the units of
weight would not be correct if one just wrote W=mg. In English units, g
c
has a
numerical value of 32.2, but its units are not those of g (m/s
2
). Always make a
clear distinction between g and g
c
and their units.
PROBLEM 1.4
PROBLEM STATEMENT:
A spacecraft of dry weight 50,000 lbf leaves Earth with
180,000 lbf of fuel on board flies to a planet where the acceleration due to
gravity is 12 ft/s
2
. During the flight to the planet, 2/3 of the fuel is consumed.
How much thrust does the rocket need to insure liftoff from the planet?
DIAGRAM DEFINING SYSTEM AND PROCESS:
f
∆
W = 120,000 lbf
W
r
= 50,000 lbf
W
f
= 180,000 lbf
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View Full DocumentProb 1.2
8/18/04
GIVEN:
W
r
= 50,000 lbf, W
f
= 180,000 lbf, a=12 ft/s
2
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 Spring '07
 Schmidt
 1Kg, 1 ft, ft/s, 9.8m

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