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Chapter 5

Chapter 5. Applications of Integration. 5.1. Areas Between Curves. 5.2 and 5.3. Finding Volumes of Objects. 5.2. Volume by Slicing. Disk Method (solid object, no hole). Practice example:. Washer Method (solid object, with hole). Practice example:. Both Disk and Washer

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Chapter 5

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  1. Chapter 5 Applications of Integration

  2. 5.1 Areas Between Curves

  3. 5.2 and 5.3 Finding Volumes of Objects

  4. 5.2 Volume by Slicing

  5. Disk Method (solid object, no hole)

  6. Practice example:

  7. Washer Method (solid object, with hole)

  8. Practice example:

  9. Both Disk and Washer methods can also be used around the y-axis!

  10. (Washer) (Disk)

  11. 5.3 Volume by Shells

  12. For objects with or without holes: Gallery

  13. Add a series of “cylindrical shells”:

  14. More general: object not limited at the bottom by the x-axis

  15. Practice example:

  16. Example with shell about the x-axis: • Two cases: • R revolving about the x-axis • R revolving about the line y=-2.

  17. SUMMARY Disk & Washer about x-axis Disk & Washer about y-axis Shell about x-axis Shell about y-axis When to use what? Answer: It will depend on the shape and symmetry of the object.

  18. Often, more than one method can be used, but usually one would be easier than the other(s). Example: Volume of object using WASHER then SHELL (about x-axis). Ans: V = p/5

  19. 5.4 Physical Application: Work

  20. “Work” is the total amount of effort required to perform a task Work has units of Force  Distance For a Constant Force over a distance d: W = Fd For a Variable Force f(x) from a to b: The UNITS of WORK: • SI Units: If F is measured in newtons and d in meters, then the unit for W is a newton-meter, which is called a joule (J). • US Units: If F is measured in pounds and d in feet, then the unit for W is a foot-pound (ft-lb), which is about 1.36 J. Conversion formula: 1 ft-lb = 1.36 J

  21. How much work is done in lifting a 1.2-kg book off the floor to put it on a desk that is 0.7 m high? Use the fact that the acceleration due to gravity is g = 9.8 m/s2. How much work is done in lifting a 20-lb weight 6 ft off the ground? Example 1: Work of a constant force: 1(a) – Solution The force exerted is equal and opposite to that exerted by gravity, so F = mg = (1.2)(9.8) = 11.76 N And the work done is W = Fd= (11.76)(0.7)  8.2 J 1(b) – Solution Here the force is given as F = 20 lb, so the work done is W = Fd = 20  6 = 120 ft-lb • Notice that in part (b), unlike part (a), we did not have to multiply by g because we were given the weight (which is a force) and not the mass of the object.

  22. When a particle is located a distance x feet from the origin, a force: f(x) = x2 + 2x pounds acts on it. How much work is done in moving it from x = 1 to x = 3? Solution: ft-lb. The work done is 50/3 ft-lb. Example 2: Work of a non constant force

  23. Hooke’s Law states that the force required to maintain a spring stretched x units beyond its natural length is proportional to x: f (x) = kx where k is a positive constant called the spring constant. Work of a force in the case of a spring: Hooke’s Law holds provided that x is not too large (a) Natural position of spring (b) Stretched position of spring

  24. Example 3: Work on a spring A force of 40 N is required to hold a spring that has been stretched from its natural length of 10 cm to a length of 15 cm. How much work is done in stretching the spring from 15 cm to 18 cm? Solution:According to Hooke’s Law, the force required to hold the spring stretched x meters beyond its natural length is f(x) = kx.

  25. Example 3 – Solution cont’d When the spring is stretched from 10 cm to 15 cm, the amount stretched is 5 cm = 0.05 m. This means that f(0.05) = 40, so 0.05k = 40 k = = 800 Thus f(x) = 800x and the work done in stretching the spring from 15 cm to 18 cm is = 400[(0.08)2 – 0.05)2] = 1.56 J

  26. 5.5 Average Value of a Function

  27. To compute the average value of a function y = f (x), over an interval [a,b]: We start by dividing the interval [a, b] into n equal subintervals, each with length x = (b – a)/n. Then we choose points x1*, . . . , xn* in successive subintervals and calculate the average of the numbers f (x1*), . . . , f (xn*): (For example, if f represents a temperature function and n = 24, this means that we take temperature readings every hour and then average them.) Since x = (b – a)/n, we can write n = (b – a)/x and the average value becomes: Average Value of a Function

  28. If we let n increase, we would be computing the average value of a large number of closely spaced values. The limiting value is: Therefore the average value of f on the interval [a, b] is: Average Value of a Function

  29. Find the average value of the function f (x) = 1 + x2 on the interval [–1, 2]. Solution: With a = –1 and b = 2 we have Example

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