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千千万万遍
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此套试卷由23道选择题以及3道大题组成
每道大题含有不同数量的小题
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Part A选择题部分:
Part B简答题部分
Problem 1
(a) To compare the efficiencies, we need to convert them to the same unit:
3.785 L/gal
USA car efficiency = 30 mpg ⇒ =7.84 L/100km
(30 mpg)×(1.609 km/mile)
European car efficiency = 7.80 L/100km
The European car is the most efficient.
(b)
0.2 × 7.8 L/100km =1.56 L/100km ⇒ European car new efficiency = 6.24 L/100km
0.2 × 30 mpg =6 mpg ⇒ USA car new efficiency = 36 mpg
3.785 L/gal
USA car new efficiency = 36 mpg ⇒ =6.53 L/100km
(36 mpg)×(1.609 km/mile)
Therefore the European car is still the most efficient, by 6.53−6.24 ×100% = 4.65%. Note that a 20% improvement
6.24
in efficiency measured in L/100km is more important than a 20% improvement in efficiency measured in mpg.
(c) The distance traveled in each part of the trip is given by the velocity during this part, multiplied by the travel time, so the total distance traveled is:
ΔX =0.5h × 60km/h +0.5h × 120km/h +0.5h × 80km/h +0.5h × 100km/h = 180km
(d) The work done by the the drag force is the sum of the work in each part of the motion:
4
W =i=1 FiΔXi = FiViΔti = 240N ×60km/h ×0.5h+540N ×120km/h ×0.5h+320N ×80km/h ×0.5h +420N ×100km/h ×0.5h = 73.4 MJ
(e) The total gasoline used is given by the sum of the amounts used in each part of the motion:
4
FiViΔti
l ==
i=1 35 MJ/L×Efficiencyi 240N×60km/h 540N×120km/h 320N×80km/h 420N×100km/h
0.5h × (+ ++)/(35 MJ/L) = 16.7 L
0.075 0.15 0.11 0.135
(f) The amount of gasoline used per kilometer travelled (denoted by lkm) for each part of the motion is then given by:
FV ΔtF 1 J/m F 1 L
== × = ×× 10−3
lkm 35MJ/L×Efficiency×V Δt Efficiency 35 MJ/L Efficiency 35 km
For the different speeds of the car the lkm can be calculated:
V = 60km/h ⇒ lkm =9.14 L/100km V = 120km/h ⇒ lkm = 10.3 L/100km V = 80km/h ⇒ lkm =8.31 L/100km V = 100km/h ⇒ lkm =8.89 L/100km
Therefore gasoline is used most efficiently when the car is traveling at 80km/h .
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