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# FURTHER APPLICATIONS OF INTEGRATION

8. FURTHER APPLICATIONS OF INTEGRATION. FURTHER APPLICATIONS OF INTEGRATION. 8.3 Applications to Physics and Engineering. In this section, we will learn about: The applications of integral calculus to force due to water pressure and centers of mass.

## FURTHER APPLICATIONS OF INTEGRATION

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1. 8 FURTHER APPLICATIONS OF INTEGRATION

2. FURTHER APPLICATIONS OF INTEGRATION 8.3Applications to Physics and Engineering In this section, we will learn about: The applications of integral calculus to force due to water pressure and centers of mass.

3. APPLICATIONS TO PHYSICS AND ENGINEERING As with our previous applications to geometry (areas, volumes, and lengths) and to work, our strategy is: • Break up the physical quantity into small parts. • Approximate each small part. • Add the results. • Take the limit. • Then, evaluate the resulting integral.

4. HYDROSTATIC FORCE AND PRESSURE Deep-sea divers realize that water pressure increases as they dive deeper. • This is because the weight of the water above them increases.

5. HYDROSTATIC FORCE AND PRESSURE Suppose that a thin plate with area A m2 is submerged in a fluid of density ρkg/m3 at a depth d meters below the surface of the fluid.

6. HYDROSTATIC FORCE AND PRESSURE The fluid directly above the plate has volume V = Ad So, its mass is: m =ρV =ρAd

7. HYDROSTATIC FORCE Thus, the force exerted by the fluid on the plate isF = mg =ρgAd where g is the acceleration due to gravity.

8. HYDROSTATIC PRESSURE The pressure Pon the plate is defined to be the force per unit area:

9. HYDROSTATIC PRESSURE The SI unit for measuring pressure is newtons per square meter—which is called a pascal (abbreviation: 1 N/m2 = 1 Pa). • As this is a small unit, the kilopascal (kPa) is often used.

10. HYDROSTATIC PRESSURE For instance, since the density of water is ρ = 1000 kg/m3, the pressure at the bottom of a swimming pool 2 m deep is:

11. HYDROSTATIC PRESSURE An important principle of fluid pressure is the experimentally verified fact that, at any point in a liquid, the pressure is the same in all directions. • This is why a diver feels the same pressure on nose and both ears.

12. HYDROSTATIC PRESSURE Equation 1 Thus, the pressure in anydirection at a depth d in a fluid with mass density ρis given by:

13. HYDROSTATIC FORCE AND PRESSURE This helps us determine the hydrostatic force against a verticalplate or wall or dam in a fluid. • This is not a straightforward problem. • The pressure is not constant, but increases as the depth increases.

14. HYDROSTATIC F AND P Example 1 A dam has the shape of the trapezoid shown below. • The height is 20 m. • The width is 50 m at the top and 30 m at the bottom.

15. HYDROSTATIC F AND P Example 1 Find the force on the dam due to hydrostatic pressure if the water level is 4 m from the top of the dam.

16. HYDROSTATIC F AND P Example 1 We choose a vertical x-axis with origin at the surface of the water.

17. HYDROSTATIC F AND P Example 1 The depth of the water is 16 m. • So, we divide the interval [0, 16] into subintervals of equal length with endpoints xi. • We choose xi* [xi–1, xi].

18. HYDROSTATIC F AND P Example 1 The i th horizontal strip of the dam is approximated by a rectangle with height Δx and width wi

19. HYDROSTATIC F AND P Example 1 From similar triangles,

20. HYDROSTATIC F AND P Example 1 Hence,

21. HYDROSTATIC F AND P Example 1 If Ai is the area of the strip, then If Δx is small, then the pressure Pi on the i th strip is almost constant, and we can use Equation 1 to write:

22. HYDROSTATIC F AND P Example 1 The hydrostatic force Fi acting on the i th strip is the product of the pressure and the area:

23. HYDROSTATIC F AND P Example 1 Adding these forces and taking the limit as n → ∞, the total hydrostatic force on the dam is:

24. HYDROSTATIC F AND P Example 2 Find the hydrostatic force on one end of a cylindrical drum with radius 3 ft, if the drum is submerged in water 10 ft deep.

25. HYDROSTATIC F AND P Example 2 In this example, it is convenient to choose the axes as shown—so that the origin is placed at the center of the drum. • Then, the circle has a simple equation: x2 + y2 = 9

26. HYDROSTATIC F AND P Example 2 As in Example 1, we divide the circular region into horizontal strips of equal width.

27. HYDROSTATIC F AND P Example 2 From the equation of the circle, we see that the length of the i th strip is: So, its area is:

28. HYDROSTATIC F AND P Example 2 The pressure on this strip is approximately So, the force on the strip is approximately

29. HYDROSTATIC F AND P Example 2 We get the total force by adding the forces on all the strips and taking the limit:

30. HYDROSTATIC F AND P Example 2 The second integral is 0 because the integrand is an odd function. • See Theorem 7 in Section 5.5

31. HYDROSTATIC F AND P Example 2 The first integral can be evaluated using the trigonometric substitution y = 3 sinθ. However, it’s simpler to observe that it is the area of a semicircular disk with radius 3.

32. HYDROSTATIC F AND P Example 2 Thus,

33. MOMENTS AND CENTERS OF MASS Our main objective here is to find the point P on which a thin plate of any given shape balances horizontally as shown.

34. CENTERS OF MASS This point is called the center of mass(or center of gravity) of the plate.

35. CENTERS OF MASS We first consider the simpler situation illustrated here.

36. CENTERS OF MASS Two masses m1 and m2 are attached to a rod of negligible mass on opposite sides of a fulcrum and at distances d1 and d2 from the fulcrum.

37. CENTERS OF MASS Equation 2 The rod will balance if:

38. LAW OF THE LEVER This is an experimental fact discovered by Archimedes and called the Law of the Lever. • Think of a lighter person balancing a heavier one on a seesaw by sitting farther away from the center.

39. MOMENTS AND CENTERS OF MASS Now, suppose that the rod lies along the x-axis, with m1 at x1 and m2 at x2and the center of mass at .

40. MOMENTS AND CENTERS OF MASS Comparing the figures, we see that: • d1 = – x1 • d2 = x1 –

41. CENTERS OF MASS Equation 3 So, Equation 2 gives:

42. MOMENTS OF MASS The numbers m1x1 and m2x2 are called the momentsof the masses m1 and m2 (with respect to the origin).

43. MOMENTS OF MASS Equation 3 says that the center of mass is obtained by: • Adding the moments of the masses • Dividing by the total mass m =m1 + m2

44. CENTERS OF MASS Equation 4 In general, suppose we have a system of n particles with masses m1, m2, . . . , mn located at the points x1, x2, . . . , xn on the x-axis. Then, we can show where the center of mass of the system is located—as follows.

45. CENTERS OF MASS Equation 4 The center of mass of the system is located at where m = Σmi is the total mass of the system.

46. MOMENT OF SYSTEM ABOUT ORIGIN The sum of the individual moments is called the moment of the system about the origin.

47. MOMENT OF SYSTEM ABOUT ORIGIN Then, Equation 4 could be rewritten as: m = M • This means that, if the total mass were considered as being concentrated at the center of mass , then its moment would be the same as the moment of the system.

48. MOMENTS AND CENTERS OF MASS Now, we consider a system of n particles with masses m1, m2, . . . , mn located at the points (x1, y1),(x2, y2) . . . , (xn, yn) in the xy-plane.

49. MOMENT ABOUT AXES Equations 5 and 6 By analogy with the one-dimensional case, we define the moment of the system about the y-axisas and the moment of the system about the x-axisas

50. MOMENT ABOUT AXES My measures the tendency of the system to rotate about the y-axis. Mx measures the tendency of the system to rotate about the x-axis.

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