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14.7 Assess Your Understanding

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Concepts and Vocabulary

  1. To convert a point P=(x,y,z) from rectangular coordinates to cylindrical coordinates (r,θ,z), use the equations: x =_____, y =_____, and z =_____.

x=rcosθ, y=rsinθ, and z=z

  1. Expressed in cylindrical coordinates, the circular cone x2+y2=4z2 has the form _____.

r=2z

  1. In cylindrical coordinates (r,θ,z), the differential dV of volume is _____

dV=rdrdθdz

  1. True or False In cylindrical coordinates, Erdrdθdz equals the volume of the solid E.

True

Skill Building

In Problems 5–12, find the cylindrical coordinates (r,θ,z) of each point with the given rectangular coordinates.

  1. (3,1,5)

(2,7π6,5)

  1. (1,3,4)

  1. (1,1,2)

(2,π4,2)

  1. (2,2,4)

  1. (2,0,4)

(2,0,4)

  1. (1,0,12)

  1. (0,3,4)

(3,π2,4)

  1. (0,1,3)

In Problems 13–18, find the rectangular coordinates (x,y,z) of each point with the given cylindrical coordinates.

  1. (2,π6,5)

(3,1,5)

  1. (4,π3,3)

  1. (1,0,8)

(1,0,8)

  1. (4,π6,2)

  1. (2,π2,0)

(0,2,0)

  1. (3,π2,1)

In Problems 19–22, give a geometric interpretation of each triple integral.

  1. 111x21x21x2z20dydzdx

The triple integral represents the volume of a hemisphere of radius 1 to the right of the xz-plane, that is y0.

955

  1. 101z201y2z20dxdydz

  1. 204x24x2x2+y20dzdydx

The triple integral represents the volume inside the vertical cylinder x2+y2=4, bounded below by the xy-plane and bounded above by the surface of the cone z=x2+y2.

  1. 309x29x2x2+z20dydzdx

In Problems 23–26, find each iterated integral.

  1. π/2π/630rsinθ0rcsc3θdzdrdθ

93

  1. π/2π/610sinθ0rcosθsinθdzdrdθ

  1. π/3010e10rdzdrdθ

π6e

  1. π/30sinθ0rsinθ0rdzdrdθ

In Problems 27–32, find each triple integral by converting to cylindrical coordinates.

  1. EdV, where E is the solid enclosed by the planes z=1 and z=4, and the cylinders x2+y2=1 and x2+y2=9.

24π

  1. EdV, where E is the solid enclosed by the xy-plane, z=3, and the cylinder x2+y2=4.

  1. EydV, where E is the solid enclosed by the planes z=1 and z=x+3, and the cylinders x2+y2=1 and x2+y2=4.

0

  1. ExdV, where E is the solid enclosed by the planes z=0 and z=x, and the cylinder x2+y2=9.

  1. ExydV, where E is the solid enclosed by the surfaces z=1xy and z=3xy, whose projection onto the xy-plane is the circle x2+y2=1.

0

  1. ExydV, where E is the solid enclosed by the surfaces z=0 and z=x2+y2, whose projection onto the xy-plane is the circle x2+y2=4.

Applications and Extensions

  1. Volume Find the volume of the solid enclosed by the intersection of the sphere x2+y2+z2=9 and the cylinder x2+y2=2.

V=4π(9773) cubic units

  1. Volume Find the volume of the solid enclosed by the intersection of the sphere x2+y2+z2=4 and the cylinder x2+y2=2x.

  1. Volume Find the volume V of the solid enclosed by z=x2+y2 and z=16x2y2.

V=64π cubic units

  1. Volume Find the volume V of the solid enclosed by z=x2+y2 and z=2x.

  1. Volume Find the volume V of the solid enclosed by z2=4x and x2+y2=2x.

V=128215 cubic units

  1. Mass Find the mass of a homogeneous solid of mass density ρ in the shape of a sphere of radius a.

  1. Mass Find the mass of a solid in the shape of a sphere of radius a, if the mass density ρ is proportional to the square of the distance from the center.

M=4kπa55, where k is the constant of proportionality

  1. Mass Find the mass M of an object in the shape of a right circular cylinder of height h and radius a, if its mass density is proportional to the square of the distance from the axis of the cylinder.

  1. Moments of Inertia Find the moments of inertia Ix and Iy for the solid region enclosed by the hemisphere z=9x2y2 and the xy-plane, if the mass density is proportional to the distance from the xy-plane.

Ix=Iy=2438kπ, where k is the constant of proportionality

  1. Center of Mass Find the center of mass of a homogeneous solid in the first octant enclosed by the surface z=xy and the cylinder x2+y2=4.

  1. Center of Mass Find the center of mass of a homogeneous solid enclosed by the surface x2+y2=4z and the plane z=2.

(ˉx,ˉy,ˉz)=(0,0,43)

  1. Center of Mass Find the center of mass of a homogeneous solid enclosed by the inside of the sphere x2+y2+z2=12 and above the paraboloid z=x2+y2.

  1. Center of Mass Find the center of mass of a homogeneous solid enclosed by the paraboloid z=x2+y2 and the plane z=4.

(ˉx,ˉy,ˉz)=(0,0,83)

  1. Mass Use cylindrical coordinates to find the mass of the homogeneous solid bounded on the sides by x2+y2=1, on the bottom by the xy-plane, and on the top by x2+y2+z2=2.

  1. Joint Between Two Rods Find the mass of the intersection of two rods with constant mass density ρ that is formed by the cylinders x2+y2=1 and x2+z2=1.

M=163ρ

  1. Volume of a Mountain The height of a mountain (in km) can be approximated by z=5.3e(x2+y2).

    1. (a) Sketch the mountain over the region x2+y24.
    2. (b) Find the volume of the mountain over the region x2+y24.

In Problems 49 and 50, each integral is given in cylindrical coordinates. Express each integral in rectangular coordinates. Do not integrate.

  1. 2π04016r2rz2r5cos4θdzdrdθ

4416x216x216x2y2x2+y2x4z2dzdydx

  1. π/20209r2z2r4sin4θdzdrdθ

956

Challenge Problems

  1. Volume A circular hole of radius r is drilled through a sphere of radius R>r, as shown in the figure.

    1. (a) Find r in terms of R so that the hole removes exactly half of the volume of the sphere. (Hint: set up a volume integral in cylindrical coordinates for the hole.)
    2. (b) Find r rounded to three decimal places if R=10cm.

  1. (a) r=R22/3122/3 cm
  2. (b) r6.083 cm
  1. Volume Find the volume enclosed on the top by the sphere x2+y2+z2=5 and on the bottom by the paraboloid x2+y2=4z.

  1. Volume of a Joint Two pipes intersect at right angles as shown in the figure. Find the inner radius r of the pipes to ensure the volume of the intersecting joint is 10m3.

r=3152 m