bucket that weighs 3 lb and a rope of negligible weight are used to draw water from a well that is 50 ft deep. The bucket is filled with 42 lb of water and is pulled up at a rate of 2.5 ft/s, but water leaks out of a hole in the bucket at a rate of 0.25 lb/s. Find the work done in pulling the bucket to the top of the well.Show how to approximate the required work by a Riemann sum. (Let x be the height in feet above the bottom of the well. Enter xi* as xi.)lim n→∞ n
Question
bucket that weighs 3 lb and a rope of negligible weight are used to draw water from a well that is 50 ft deep. The bucket is filled with 42 lb of water and is pulled up at a rate of 2.5 ft/s, but water leaks out of a hole in the bucket at a rate of 0.25 lb/s. Find the work done in pulling the bucket to the top of the well.Show how to approximate the required work by a Riemann sum. (Let x be the height in feet above the bottom of the well. Enter xi* as xi.)lim n→∞ n
Solution
The work done in pulling the bucket to the top of the well can be calculated using the formula for work, which is force times distance.
Step 1: Calculate the total weight of the bucket and water initially. This is 3 lb (bucket) + 42 lb (water) = 45 lb.
Step 2: Determine the rate at which the weight is decreasing. This is due to the water leaking out of the bucket at a rate of 0.25 lb/s.
Step 3: Calculate the force needed to pull up the bucket. This is the weight of the bucket and water (which is decreasing over time) times the acceleration due to gravity. Since we're using lb for weight and ft for distance, we'll use the conversion factor of 32.2 ft/s² for gravity. So the force is (45 - 0.25t) lb * 32.2 ft/s² = 1449 - 8.05t lb*ft/s².
Step 4: Calculate the work done in pulling the bucket up 1 ft. This is the force times the distance, or (1449 - 8.05t) lbft/s² * 1 ft = 1449 - 8.05t lbft²/s².
Step 5: Integrate this expression from 0 to the time it takes to pull the bucket up 50 ft. Since the bucket is being pulled up at a rate of 2.5 ft/s, this time is 50 ft / 2.5 ft/s = 20 s. So the total work done is ∫ from 0 to 20 of (1449 - 8.05t) dt lb*ft²/s².
Step 6: Evaluate this integral to get the total work done. This comes out to be 28,980 lbft²/s², or 28,980 ftlb of work.
To approximate this using a Riemann sum, we would divide the 20 s interval into n subintervals, calculate the work done over each subinterval using the formula from step 4, and then sum these up. As n approaches infinity, this sum approaches the exact value of the work done.
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