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Kosky, Balmer, Keat and Wise: Exploring Engineering, Fourth Edition, Solution Manual, Chapter 4
Solution Manual Chapter 4: Energy
Pay attention to the inferred number of significant figures in your answers! Conversion factors:
1.00 J = 0.738 ft lbf; 1.00 kg = 2.20 lbm and gc = 32.2 lbm·ft/lbf·s2.
4.1. Determine the translational kinetic energy of the automobile in Example 4.1 if its speed is
reduced to 55. miles per hour.
Need: TKE of vehicle of mass 1.0 × 103 kg at 55. mph
4.2. Determine the translational kinetic energy in Engineering English units of the automobile in
Example 4.1 if its mass is increased to 4.00 × 103 lbm.
Need: TKE of vehicle of mass 4.00 × 103 lbm at 55. mph
Know: 55. mph = 81. ft/s (e.g., from convert.exe); gc = 32.2 lbm ft/lbf s2
4.3. Determine the translational kinetic energy of the atmosphere in Example 4.2 if the average air
speed increases to 15. m/s.
Need: TKE of terrestrial wind in J if wind = 15. m/s cp. 10.m/s
Know: At 10. m/s, TKE = 1.7 1020 J
Kosky, Balmer, Keat and Wise: Exploring Engineering, Fourth Edition, Solution Manual, Chapter 4
4. Repeat the calculation of Example 4.5 in SI units. Check that your answers agree with the
solution in Example 4.5 using the appropriate conversion factors.
Need: GPE of anvil of mass 100. kg located 1,000. m high.
100. kg and the cliff is 1000. meters high?
Need: GPE of anvil of mass 100. kg located 1,000. m high.
4.6. Determine the gravitational potential energy (GPE) of an 8.00 × 103 kg truck 30. m above the
ground. (A: 2.4 × 106 J to two significant figures, since h is known only to two significant
figures.)
Need: GPE = ____ J
Know: Mass of truck = 8.00 103 kg; height above datum is 30. m.
4.7. A spring at ground level—that is, at height = 0.00 m—shoots a 0.80 kg ball upward with an
initial kinetic energy of 245 J. Assume that all the initial TKE is converted to GPE, how high does
the ball rise (neglecting air resistance)?
Need: Maximum height = ____ m.
Know: Mass of ball = 0.80 kg; kinetic energy = 245 J.
4.8. Chunks of Earth orbital debris can have speeds of 2.3 × 104 miles per hour. Determine the
translational kinetic energy (TKE) of a 2.0 × 103 lbm chunk of this material in SI units. (A: 4.8 ×
1010 J to two significant figures.)
Need: TKE = _____ J
Know: m = 2.0 103 lbm = 9.07 102 kg; v = 2.3 104 mph = 2.3 104/3600. 1609
[miles/hr][ hr/s][ m/mile] = 1.03 104 m/s.
4.9. An airplane with a mass of 1.50 × 104 kg is flying at a height of 1.35 × 103 m at a speed of
250.0 m/s. Which is larger: its translational kinetic energy or its gravitational potential energy
with respect to the Earth’s surface? (Support your answer with numerical evidence.) (A: TKE =
4.69 × 108 J; GPE = 1.99 × 108 J. Therefore, the TKE is greater than GPE.)
Need: TKE is _____ (greater/the same/ less than) GPE
Know: m = 1.50 104 kg, h = 1.35 103 m, v = 250. m/s
55. miles per hour.
Need: Gasoline equivalent of car of mass 1.00 × 103 kg traveling at 55. mph
Know: Gasoline energy = 1.30 105 J/gallon; 55. mph = 25. kg/s
4.11. Suppose the 1.00 kg book in Section 4.5 falls from a height of 2.5 meters. What would be
the final energy of the classroom?
Need: Classroom energy after thermal due to book fall is lost.
Know: GPE of book = mgh = 1.00 9.81 2.5 [kg][m/s2][m] = 24.5 J
4.12. A vehicle of mass 1.50 × 104 kg is traveling on the ground with a TKE of 4.69 × 108 J. By
means of a device that interacts with the surrounding air, it is able to convert 50% of the TKE into
GPE. This energy conversion enables it to ascend vertically. To what height above the ground
does it rise?
Need: Height = _____ m
Know: m = 1.50 10 4 kg, TKE of 4.69 108 J
4.13. Aeronautical engineers invented a device that achieves the conversion of kinetic to potential
energy as described in Exercise 4.12. The device achieves this conversion with high efficiency. In
other words, a high percentage of the translational kinetic energy of motion is converted into
vertical “lift” with little lost to horizontal “drag.” What is the device called? (Hint: This is not
rocket science.)
4.14. A hypervelocity launcher is an electromagnetic gun capable of shooting a projectile at very
high speed. A Sandia National Laboratory hypervelocity launcher shoots a 1.50 gram projectile
that attains a speed of 14.0 km/s. How much electromagnetic energy must the gun convert into
TKE to achieve this speed? Solve in SI. (A: 1.5 × 102 kJ.)
Need: Energy converted into TKE = _____ J
Know: m = 1.50 10-3 kg, v = 14. 103 m/s
Kosky, Balmer, Keat and Wise: Exploring Engineering, Fourth Edition, Solution Manual, Chapter 4
4.15. Solve Exercise 4.14 in Engineering English units. (Also check your answer by converting
the final answer to Exercise 14 into Engineering English units.)
Need: Energy converted into TKE = ____ ft-lbf
Know: m = 1.50 10-3 kg = 3.30 10-3 lbm, v = 14. 103 m/s = 4.60 104 ft/s
and gc = 32.2 lbm ft/lbf s2
4.16. Micrometeoroids could strike the International Space Station with impact velocities speeds
of 19 km/s. What is the translational kinetic energy of a 1.0 gram micrometeoroid traveling at that
speed? (A: 1.8 × 105 J.)
Need: TKE = _____ J
Know: m = 1.0 g = 1.0 10-3 kg ; v = 19 km/s = 1.9 104 m/s
4.17. Suppose a spaceship is designed to withstand a micrometeoroid impact delivering a TKE of
a million joules. Suppose that the most massive micrometeoroid it is likely to encounter in space
has mass of 3 g. What is the maximum speed relative to the spaceship that the most massive
micrometeorite can be traveling at for the spaceship to be able to withstand its impact?
Need: Maximum micrometeoroid speed = ____ m/s
Know: m = 3 gram = 3 10-3 kg, TKE = 1 106 J.
