# hw1 - Prob 1.2 PROBLEM 1.2 PROBLEM STATEMENT If an object...

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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 lift-off 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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Prob 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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hw1 - Prob 1.2 PROBLEM 1.2 PROBLEM STATEMENT If an object...

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