Thomas Calculus 13th [Solutions]

474 Chapter 6 Applications of Definite Integrals
distance through which F( y ) must act to lift the slab to the level of 2 ft above the top of the tank is about
(12 y ) ft, so the work done is
y 0 ft to y 10 ft is approximately
2
W 56 100 y (12 y) y lb ft. The work done lifting all the slabs from
10
0
10 2 10 2
0 0
2
W 56 100 y (12 y) y lb ft. Taking the limit of these
Riemann sums, we get W 56 100 y (12 y) dy 56 100 y (12 y)
dy
10 2 3
100 y 12 y y
y y y dy y
0
2 3 4
56 1200 100 12 56 1200
2 3 4 10
10,000 10,000
56 12,000 4 1000 (56 ) 12 5 4
5
(1000) 967,611ft-lb. It would cost
2 4 2
(0.5)(967,611) 483,805¢= $4838.05. Yes, you can afford to hire the firm.
0
23.
dv dv
dt dx
F m mv by the chain rule
1 2 2 1 2 1 2
2 2 1 2 2 2 1
m v ( x ) v ( x ) mv mv , as claimed.
x2 dv
x2
dv 1 2
x dx x dx 2 x
W mv dx m v dx m v ( x ) x
1 1 1
2
24. weight 2 oz
2
lb; mass
16
1
weight 8 1
32 32 256 slugs; W 1 1
2
2 256
slugs (160 ft / sec) 50 ft-lb
25. 90 mph
90 mi 1 hr 1 min 5280 ft
1 hr 60 min 60 sec 1 mi
0.3125 lb 0.3125
32 ft/ sec 32
132ft/sec; m
2
slugs;
W
1 0.3125 lb 2
2 2
(132ft/sec)
32 ft/sec
85.1 ft-lb
0.1lb 1
32 ft/ sec 320
26. weight 1.6 oz 0.1 lb m
slugs;
2
W
1 1
2 320
slugs (280ft/sec) 122.5ft-lb
2
27.
ft ft
1 sec 2
sec
0.125 lb 1
32 ft/ sec 256
v 0 mph 0 , v 153 mph 224.4 ; 2 oz 0.125 lb m slugs;
2
x
1
2
1 2 1 2 1 1 2 1 1 2
2 2 2 1 2 256 2 256
W F( x) dx mv mv (224.4) (0) 98.35 ft-lb
x
28. weight 6.5 oz
6.5
lb m
6.5
slugs;
16 (16)(32)
W
1 6.5
2 (16)(32)
slugs (132ft/sec) 110.6ft-lb
2
29. We imagine the milkshake divided into thin slabs by planes perpendicular to the y -axis at the points of a
partition of the interval [0, 7]. The typical slab between the planes at y and y y has a volume of about
2
2
y 17.5 3
V (radius) (thickness) y in . The force F( y ) required to lift this slab is equal to its
14
weight:
4 4 y 17.5
9 9 14
2
F( y) V y oz. The distance through which F( y ) must act to lift this slab
to the level of 1 inch above the top is about (8 y ) in. The work done lifting the slab is about
4 ( y 17.5)
9 2
14
2
W (8 y) y in oz. The work done lifting all the slabs from y 0 to y 7 is approximately
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