Which of the following is a valid assumption for lab 2 (The Nerf gun lab)? KE =

Question

Which of the following is a valid assumption for lab 2 (The Nerf gun lab)? KE = 1/2 mv^2 Energy can be converted from one for to another, but total energy remains constant n = n_experimental/n_theoretical For the theoretical calculations, the air resistance in negligible. All of the above are valid assumptions. An object with a m ss of 30 Ibm accelerates from 10 ft/s to 20 ft/s. What is the work of the acceleration? 1500ft.Ibf 46.6 ft. Ibf 4500ft.Ibf 140ft.Ibf None of the above A heat engine can have an efficiency of 100%. Always true Always false Sometimes true If a heat engine is operating between a source of 500 degree C and 20 degree C, what is the carnot efficiency? 100% 96.0% 62.1% 37.9% None of the above

Explanation / Answer

24. All the options are correct. Hence option e is correct.

25. Given mass, m = 30 lbm

Initial velocity, u = 10 ft/s

Final velocity, v = 20 ft/s

Work of acceleration

W = 0.5m(v2-u2) = 0.5 X 30 X (202 - 102) = 4500 lb,-ft2/s2

W = 4500 /g ft-lbf = 4500 / 32.174 = 140 ft-lbf (g = gravitational acceleration)

Hence option d is the correct one

26. Carnot efficiency of heat engine is given by

eff = (1-T2 / T1)

where T1 is the higher temperature (source) temperature

T2 is the lower temperature (sink) temperature

For eff to be 100 % = 1, T2 should be equal to zero kelvin, which is impossible to be achieved.

Hence heat engine can never have 100 % efficiency

Option b is correct option

27. Given

T1 = 500 oC = 500 + 273.15 = 773.15 K

T2 = 20 oC = 20 + 273.15 = 293.15 K

Carnot efficiency

eff = (1 - T2 / T1) X 100 = (1 - 293.15 / 773.15) X 100 = 62.1 %

Option c is correct

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