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Assignment__2

# Assignment__2 - Assignment#2 PROBLEM 2.2 PROBLEM STATEMENT...

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Assignment #2 1/7/2005 PROBLEM 2.2 PROBLEM STATEMENT: The official weight of a baseball is 5 oz (0.3125 lbf). What is the kinetic energy of a 90 mph fastball in ft-lbf, kJ, and Btu? DIAGRAM DEFINING SYSTEM AND PROCESS: v = 90 mph W = 0.3125 lbf GIVEN: W = 0.3125 lbf, v = 90 mph ASSUME: Standard gravitational acceleration, g=32.17 ft/s 2 FIND: Kinetic energy (in ft - lbf, kJ, and Btu) GOVERNING RELATIONS: Kinetic energy, . Weight QUANTITATIVE SOLUTION: Mass of the ball: The kinetic energy is: a. b.

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c. DISCUSSION OF RESULTS: Since it was assumed that the gravitational acceleration was standard (32.17 ft/s 2 ), the mass in lbm has the same numerical value as the weight in lbf. If g were not standard, this would not be the case, since the value of g c (a units conversion constant) is always 32.17 in USCS units, irrespective of the gravitational force. PROBLEM 2.4 PROBLEM STATEMENT: Just before reentry from a 250 mile high orbit, the space shuttle weighs approximately 250,000 lbf and has a velocity of 17,300 mph. At this altitude the gravitational acceleration is 29 ft/s 2 . What is the total kinetic plus potential energy of the shuttle in ft-lbf and in Btu? GIVEN: h = 250 mile, W = 250000 lbf, v = 17300 mph, g = 29 ft/s 2 FIND: Total kinetic and potential energy (ft - lbf and Btu) GOVERNING RELATION: , QUANTITATIVE SOLUTION: The kinetic energy of the space shuttle is computed as follows: The potential energy is: Thus, the total energy of the shuttle is
which is also equivalent to DISCUSSION OF RESULTS: This is clearly a situation in which kinetic and potential energy are not negligible! When converted to internal energy during reentry, the air temperature and shuttle skin rise to very high temperatures. PROBLEM 2.5 PROBLEM STATEMENT: A 10 kg mass is moved from the floor to a position on a staircase 10 m above the floor. The new position is reached by the following paths: (a) The mass is lifted directly upwards by a frictionless pulley in a block- and-tackle system. (b) The mass is moved on a frictionless dolly to an elevator, which is also frictionless. The elevator moves to the height necessary, and then the dolly is used to move the mass back to the final position.

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