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lucy3964
10.03.2020 •
Mathematics
A bag of sand originally weighing 144 lb is lifted at a constant rate. As it rises, sand also leaks out at a constant rate. At the instant when the bag has been lifted to a height of 18 feet, exactly half of the original amount of sand remains. (Neglect the weight of the bag and the lifting equipment.) How much work was done lifting the sand this far?
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Ответ:
Step-by-step explanation:
The bag of sand wave is
W=144lb at x=0
The bag of sand is lifted at a constant rate, i.e the it is not accelerating, so a=0m/s²
Let W be the mass of the sand,
Sand leaks out at a constant rate
dW/dx= C
At a height=18ft, half of the original mass.
i.e x=18ft, M=72lb
We are asked to find the work at 18ft
Work is given as
Work=∫F•dx,
Where F is the force and also the weight of the sand, F=W
The Weight of the bag is a linear function,
dW/dx= C
Then, using variable separation
dW=Cdx
Integrating both sides
∫dW=∫Cdt
W=Cx + B
So, at x=0, W=144
144=B
B=144
W=Cx+144
Also at x=18, W=72lb
72=C×18+144
72-144=18C
-72=18C
C=-4
Then, the weight function becomes
W=-4x+144
W=144-4x
Then applying work formula
Work=∫W•dx
Work=∫(144-4x)dx. x=0 to x=18
Work = 144x-4x²/2. x=0 to x=18
Work=144x-2x² x=0 to x=18
Work=144(18)-2(18²) -0 -0
Work =2592-648
Work =1944 ft lbs
The work done in lifting the sand to 18ft is 1944 ft lbs
Ответ: