Mass, centre of mass and moments of material curves and surfaces

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Find the static moment and moment of inertia of a semicircular arc of radius aa with respect to the diameter through its two endpoints.

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Find the coordinates of the centroid of the circular arc: x=acos⁡φx=a\cos \varphi, y=asin⁡φy=a\sin \varphi (∣φ∣⩽α⩽π\left|\varphi \right|\leqslant \alpha \leqslant \pi).

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Find the coordinates of the centroid C(φ0,r0)C(\varphi_{0},r_{0}) of the arc OPOP from the point O(−∞,0)O(-\infty ,0) to the point P(φ,r)P(\varphi ,r) on the logarithmic spiral

r=aemφ(m>0) r=ae^{m\varphi }\quad (m>0)

As the point PP moves, what curve does the point CC trace out?

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Find the static moments MxM_{x} and MyM_{y} of the curve1.

Footnotes

  1. In the problems of this section take ρ=1\rho = 1 unless stated otherwise. ↩

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(1)

x/a+y/b=1,x⩾0,y⩾0x / a+y / b = 1, x \geqslant 0, y \geqslant 0;

(2)

x2+y2=a2,y⩾0x^{2}+y^{2} = a^{2}, y \geqslant 0;

(3)

y2=2x,0⩽x⩽2;y^{2} = 2 x, 0 \leqslant x \leqslant 2 ;

(4)

y2−p2=2px,x⩽0y^{2}-p^{2} = 2 p x, x \leqslant 0

(5)

x2/a2+y2/b2=1,y⩾0,a>bx^{2} / a^{2}+y^{2} / b^{2} = 1, y \geqslant 0, a > b

(6)

y=ach⁡(x/a),0⩽x⩽ay = a \operatorname {ch}(x / a), 0 \leqslant x \leqslant a.

Find the static moments MxM_{x} and MyM_{y} of the curve1.

Footnotes

  1. In the problems of this section take ρ=1\rho = 1 unless stated otherwise. ↩

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(1)

x=asin⁡t,y=bcos⁡t,0⩽t⩽π/2,a>bx = a \sin t, y = b \cos t, 0 \leqslant t \leqslant \pi / 2, a > b;

(2)

x=asin⁡3t,y=bcos⁡3t,0⩽t⩽π/2x = a \sin^{3} t, y = b \cos^{3} t, 0 \leqslant t \leqslant \pi / 2;

(3)

x=a(t−sin⁡t),y=a(1−cos⁡t),0⩽x⩽2πx = a(t-\sin t), y = a(1-\cos t), 0 \leqslant x \leqslant 2 \pi.

Find the static moments MxM_{x} and MyM_{y} of the curve1.

Footnotes

  1. In the problems of this section take ρ=1\rho = 1 unless stated otherwise. ↩

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(1)

r=2acos⁡φ,0⩽φ⩽π/2r = 2 a \cos \varphi , 0 \leqslant \varphi \leqslant \pi / 2;

(2)

r=a(1+cos⁡φ),−π⩽φ⩽πr = a(1+\cos \varphi ),-\pi \leqslant \varphi \leqslant \pi;

(3)

r=aeφ,0⩽φ⩽2πr = a e^{\varphi }, 0 \leqslant \varphi \leqslant 2 \pi.

Prove that the static moment of the arc ABA B of a parabola relative to the axis of the parabola equals R0(RA−RB)/3R_{0}\left(R_{A}-R_{B}\right) / 3, where R0,RAR_{0}, R_{A} and RBR_{B} are the radii of curvature of the parabola at its vertex and at the points AA and BB respectively.

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Find the coordinates xCx_{C} and yCy_{C} of the centroid of the curve:

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(1)

x=Rcos⁡φ,y=Rsin⁡φ,∣φ∣⩽α⩽πx = R \cos \varphi , y = R \sin \varphi ,\left|\varphi \right| \leqslant \alpha \leqslant \pi;

(2)

x2/3+y2/3=a2/3,x⩾0,y⩾0x^{2 / 3}+y^{2 / 3} = a^{2 / 3}, x \geqslant 0, y \geqslant 0;

(3)

y=ach⁡(x/a),∣x∣⩽by = a \operatorname {ch}(x / a),\left|x\right| \leqslant b;

(4)

x=y2/4−(ln⁡y)/2,1⩽y⩽2x = y^{2} / 4-(\ln y) / 2,1 \leqslant y \leqslant 2;

(5)

x=a(t−sin⁡t),y=a(1−cos⁡t),0⩽t⩽2πx = a(t-\sin t), y = a(1-\cos t), 0 \leqslant t \leqslant 2 \pi;

(6)

r=a(1+cos⁡φ),0⩽φ⩽πr = a(1+\cos \varphi ), 0 \leqslant \varphi \leqslant \pi;

(7)

r=aeφ,π/2⩽φ⩽πr = a e^{\varphi }, \pi / 2 \leqslant \varphi \leqslant \pi.

Prove that if a homogeneous curve has an axis of symmetry, then the centroid of the curve lies on that axis.

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Prove that the static moment of a curve relative to the axis passing through its centroid equals zero.

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Find the moment of inertia of a segment of length ll relative to an axis lying in the same plane with it and not crossing this segment, if the distance to the axis from one end of the segment equals aa, and from the other −b-b.

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Find the moment of inertia IxI_{x} of the curve:

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(1)

y=ex,0⩽x⩽1/2y = e^{x}, 0 \leqslant x \leqslant 1 / 2

(2)

x=Rcos⁡φ,y=Rsin⁡φ,0⩽φ⩽α⩽2πx = R \cos \varphi , y = R \sin \varphi , 0 \leqslant \varphi \leqslant \alpha \leqslant 2 \pi;

(3)

x2+(y−a)2=R2,a>Rx^{2}+(y-a)^{2} = R^{2}, a > R.

Find the moments of inertia IxI_{x} and IyI_{y} of one arch of the cycloid

x=a(t−sin⁡t),y=a(1−cos⁡t),0⩽t⩽2π. x = a(t-\sin t), \quad y = a(1-\cos t), \quad 0 \leqslant t \leqslant 2 \pi .
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Prove that the point of the plane, defined for a curve or figure by the formulas

xC=My/m,yC=Mx/m x_{C} = M_{y} / m, \quad y_{C} = M_{x} / m

(i.e. the centroid), does not depend on the choice of coordinate system.

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A right circular cone has height hh and base radius RR. Find the distance to the base of the cone from the centroid:

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(1)

its lateral surface;

(2)

its total surface (assuming they are homogeneous).

The base radii of a right frustum of a cone equal rr and 2r2 r, the angle between the generator and the base equals π/3\pi / 3. Find the centroid of its:

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(1)

the lateral surface;

(2)

the total surface.

Find the centroid of the shell that is:

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(1)

a spherical zone of height hh;

(2)

a segment with base radius RR and height R3/2R \sqrt{3} / 2, cut off from a paraboloid of revolution by a plane perpendicular to its axis.

Find the centroid of the part of the surface of a cylinder lying between the planes z=0z = 0 and z=hy/R,y⩾0z = h y / R, y \geqslant 0, if the cylinder is given by the equation:

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(1)

x2+y2=R2x^{2}+y^{2} = R^{2};

(2)

x2/3+y2/3=R2/3x^{2 / 3}+y^{2 / 3} = R^{2 / 3}.

Find the moment of inertia:

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(1)

the lateral surface of a cylinder with radius RR and height hh, relative to its axis;

(2)

the lateral surface of a cone with height hh and base radius RR, relative to its axis;

(3)

a sphere of radius RR, relative to its diameter.

Find the mass, distributed with linear density ρ(x;y)\rho (x ; y) along the arc ABA B of the plane curve Γ\Gamma, if:

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(1)

Γ\Gamma — the segment AB,A(1;1),B(2;3);ρ(x;y)=2x+yA B, A(1 ; 1), B(2 ; 3) ; \rho (x ; y) = 2 x+y;

(2)

Γ\Gamma — the segment AB,A(1;0),B(4;6);ρ(x;y)=y+2/xA B, A(1 ; 0), B(4 ; 6) ; \rho (x ; y) = \sqrt{y+2} / x;

(3)

Γ:y=x2/2,A(1;1,5),B(2;2);ρ(x;y)=y/x\Gamma : y = x^{2} / 2, A(1 ; 1,5), B(2 ; 2) ; \rho (x ; y) = y / x;

(4)

Γ:y2=x,A(1;1),B(4;2);ρ(x;y)=y\Gamma : y^{2} = x, A(1 ; 1), B(4 ; 2) ; \rho (x ; y) = y;

(5)

Γ:y=2x3/2/3,A(0;0),B(4;16/3);ρ=ks\Gamma : y = 2 x^{3 / 2} / 3, A(0 ; 0), B(4 ; 16 / 3) ; \rho = k s, where ss is the arc length from the point (0;0)(0 ; 0).

Find the mass of the entire curve y=ach⁡(x/a),x∈Ry = a \operatorname {ch}(x / a), x \in R, with linear density ρ=1/y2\rho = 1 / y^{2}.

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Find the mass, distributed with linear density ρ\rho, along the plane curve Γ\Gamma:

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(1)

Γ:r=acos⁡2φ;ρ=kr\Gamma : r = a \sqrt{\cos 2 \varphi } ; \rho = k r;

(2)

Γ:r=a(1+cos⁡φ);ρ=kr\Gamma : r = a(1+\cos \varphi ) ; \rho = k \sqrt{r};

(3)

Γ:x=a(t−sin⁡t),y=a(1−cos⁡t),0⩽t⩽2π;ρ=y3/2\Gamma : x = a(t-\sin t), y = a(1-\cos t), 0 \leqslant t \leqslant 2 \pi ; \rho = y^{3 / 2};

(4)

Γ:x=acos⁡3t,y=asin⁡3t,0⩽t⩽π/2;ρ=y3\Gamma : x = a \cos^{3} t, y = a \sin^{3} t, 0 \leqslant t \leqslant \pi / 2 ; \rho = \sqrt[3]{y};

(5)

Γ:x=ln⁡(1+t2),y=2arctg⁡t−t,0⩽t⩽1;ρ=ye−x\Gamma : x = \ln \left(1+t^{2}\right), y = 2 \operatorname {arctg} t-t, 0 \leqslant t \leqslant 1 ; \rho = y e^{-x};

(6)

Γ:x2/a2+y2/b2=1,x⩾0,y⩾0;a>b;ρ=y\Gamma : x^{2} / a^{2}+y^{2} / b^{2} = 1, x \geqslant 0, y \geqslant 0 ; a > b ; \rho = y;

(7)

Γ:x2/3+y2/3=a2/3;ρ=∣xy∣\Gamma : x^{2 / 3}+y^{2 / 3} = a^{2 / 3} ; \rho = \left|x y\right|;

(8)

Γ:x2+y2=ax;ρ=x2+y2\Gamma : x^{2}+y^{2} = a x ; \rho = \sqrt{x^{2}+y^{2}}.

Find the mass, distributed with linear density ρ\rho, along the space curve Γ\Gamma:

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(1)

Γ:x=cos⁡t,y=sin⁡t,z=t,0⩽t⩽2π;ρ=(x2+y2+z2)−1\Gamma : x = \cos t, y = \sin t, z = t, 0 \leqslant t \leqslant 2 \pi ; \rho = \left(x^{2}+y^{2}+z^{2}\right)^{-1};

(2)

Γ:x=at,y=at2/2,z=at3/3,0⩽t⩽1;ρ=2y/a\Gamma : x = a t, y = a t^{2} / 2, z = a t^{3} / 3,0 \leqslant t \leqslant 1 ; \rho = \sqrt{2 y / a};

(3)

Γ:x=tcos⁡t,y=tsin⁡t,z=t,0⩽t⩽2π;ρ=x2+y2+z2\Gamma : x = t \cos t, y = t \sin t, z = t, 0 \leqslant t \leqslant 2 \pi ; \rho = \sqrt{x^{2}+y^{2}+z^{2}};

(4)

Γ:x=aetcos⁡t,y=aetsin⁡t,z=aet,−∞<t⩽0;ρ=kz\Gamma : x = a e^{t} \cos t, y = a e^{t} \sin t, z = a e^{t},-\infty < t \leqslant 0 ; \rho = k z;

(5)

Γ\Gamma — the arc of the curve y=x2/2,z=x3/3y = x^{2} / \sqrt{2}, z = x^{3} / 3 with initial point A(0;0;0)A(0 ; 0 ; 0) and terminal point B(4;82;64/3);ρ=kx2+y2B(4 ; 8 \sqrt{2} ; 64 / 3) ; \rho = k \sqrt{x^{2}+y^{2}};

(6)

Γ\Gamma — the arc of the curve y2−4x2=3z2,y2=x,z⩾0y^{2}-4 x^{2} = 3 z^{2}, y^{2} = x, z \geqslant 0, with initial point A(0;0;0)A(0 ; 0 ; 0) and terminal point B(1/4;1/2;0);ρ=zB(1 / 4 ; 1 / 2 ; 0) ; \rho = z;

(7)

Γ={x2+y2+z2=a2,x+y+z=a},ρ=x2\Gamma = \left\{ x^{2}+y^{2}+z^{2} = a^{2}, x+y+z = a\right\} , \rho = x^{2}.

Find the coordinates of the center of mass, distributed along the plane curve Γ\Gamma with linear density ρ=1\rho = 1:

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(1)

Γ:y=ach⁡(x/a),∣x∣⩽a\Gamma : y = a \operatorname {ch}(x / a),\left|x\right| \leqslant a;

(2)

Γ:x=a(t−sin⁡t),y=a(1−cos⁡t),0⩽t⩽2π\Gamma : x = a(t-\sin t), y = a(1-\cos t), 0 \leqslant t \leqslant 2 \pi;

(3)

Γ\Gamma — the arc of the circle r=R,∣φ∣⩽φ0⩽πr = R,\left|\varphi \right| \leqslant \varphi_{0} \leqslant \pi;

(4)

Γ\Gamma — the cardioid r=a(1+cos⁡φ)r = a(1+\cos \varphi );

(5)

Γ:x2/3+y2/3=a2/3,y⩾0\Gamma : x^{2 / 3}+y^{2 / 3} = a^{2 / 3}, y \geqslant 0;

(6)

Γ:x+y=a\Gamma : \sqrt{x}+\sqrt{y} = \sqrt{a};

(7)

Γ:y2=x2/3+x3,x⩾0\Gamma : y^{2} = x^{2} / 3+x^{3}, x \geqslant 0.

Find the coordinates of the center of mass, distributed with linear density ρ\rho, along the arc of the helix x=Rcos⁡φ,y=Rsin⁡φ,z=hφ/2π,0⩽φ⩽φ0x = R \cos \varphi , y = R \sin \varphi , z = h \varphi / 2 \pi , 0 \leqslant \varphi \leqslant \varphi_{0}, if:

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(1)

ρ=ρ0=\rho = \rho_{0} = const;

(2)

ρ=ρ0e−z/h\rho = \rho_{0} e^{-z / h}, take φ0=2πn,n∈N\varphi_{0} = 2 \pi n, n \in N.

Find the coordinates of the center of mass of the homogeneous curve

x=e−tcos⁡t,y=e−tsin⁡t,z=e−t,0⩽t<∞. x = e^{-t} \cos t, \quad y = e^{-t} \sin t, \quad z = e^{-t}, \quad 0 \leqslant t < \infty .
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Find the coordinates of the center of mass of the homogeneous edge of the surface x+y+z=a\sqrt{x}+\sqrt{y}+\sqrt{z} = \sqrt{a}.

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Let the piecewise smooth curve Γ\Gamma be the union of smooth curves Γi,Γ=⋃i=1nΓi\Gamma_{i}, \Gamma = \bigcup_{i = 1}^{n} \Gamma_{i}, with masses mim_{i} and position vectors of the centers of mass ri,i=1,…,n\mathbf{r}_{i}, i = 1, \ldots , n. Let mm be the mass of Γ\Gamma, rCr_{C} the center of mass of Γ\Gamma. Prove that

rC=∑i=1nmimri \mathbf{r}_{C} = \sum _{i = 1}^{n} \frac{m_{i}}{m} \mathbf{r}_{i}
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Find the moment of inertia IxI_{x} of the circle x2+y2=R2;ρ=1x^{2}+y^{2} = R^{2} ; \rho = 1.

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Find the moment of inertia IyI_{y} of the circle x2+y2=2Rx;ρ=1x^{2}+y^{2} = 2 R x ; \rho = 1.

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Find the moments of inertia IxI_{x} and IyI_{y} of a single arch of the cycloid

x=a(t+sin⁡t),y=a(1−cos⁡t),∣t∣⩽π,ρ=1 x = a(t+\sin t), \quad y = a(1-\cos t), \quad \left|t\right| \leqslant \pi , \quad \rho = 1
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Find the moments of inertia Ix,Iy,IzI_{x}, I_{y}, I_{z} of a single turn of the helix

x=acos⁡t,y=asin⁡t,z=ht/2π,0⩽t⩽2π;ρ=1. x = a \cos t, \quad y = a \sin t, \quad z = h t / 2 \pi , \quad 0 \leqslant t \leqslant 2 \pi ; \quad \rho = 1.
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Find the moment of inertia IxI_{x} of the circle x2+y2+z2=R2,x+y+z=0;ρ=1x^{2}+y^{2}+z^{2} = R^{2}, x + y+z = 0 ; \rho = 1.

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Find the polar moment of inertia I0=∫Γ(x2+y2)dsI_{0} = \int_{\Gamma }\left(x^{2}+y^{2}\right) d s of the homogeneous plane curve Γ(ρ=1)\Gamma (\rho = 1) with respect to the origin, if:

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(1)

Γ:∣x∣+∣y∣=a\Gamma :\left|x\right|+\left|y\right| = a;

(2)

Γ:x2/3+y2/3=a2/3\Gamma : x^{2 / 3}+y^{2 / 3} = a^{2 / 3};

(3)

Γ:x=a(cos⁡t+tsin⁡t),y=a(sin⁡t−tcos⁡t),0⩽t⩽2π\Gamma : x = a(\cos t+t \sin t), y = a(\sin t-t \cos t), 0 \leqslant t \leqslant 2 \pi.

Determine the mass distributed:

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(1)

over the surface of the cube 0⩽x⩽a,0⩽y⩽a,0⩽z⩽a0 \leqslant x \leqslant a, 0 \leqslant y \leqslant a, 0 \leqslant z \leqslant a with surface density ρ=ρ0xyz\rho = \rho_{0} x y z;

(2)

over the sphere x2+y2+z2=R2x^{2}+y^{2}+z^{2} = R^{2} with density: а) ρ=ρ0x2+y2\rho = \rho_{0} \sqrt{x^{2}+y^{2}}, б) ρ=ρ0(x2+y2)\rho = \rho_{0}\left(x^{2}+y^{2}\right);

(3)

over part of the elliptic paraboloid x2+y2=2z,z⩽1x^{2}+y^{2} = 2 z, z \leqslant 1 with density ρ=ρ0z\rho = \rho_{0} z;

(4)

over part of the hyperbolic paraboloid x2−y2=2zx^{2}-y^{2} = 2 z, cut out by the cylinder x2+y2=1x^{2}+y^{2} = 1, with density ρ=ρ0∣z∣,ρ0=\rho = \rho_{0}\left|z\right|, \rho_{0} = const.

Determine the static moment relative to the plane z=0z = 0 of a homogeneous (ρ=ρ0=\rho = \rho_{0} = const )) surface:

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(1)

x+y+z=a,x⩾0,y⩾0,z⩾0x+y+z = a, x \geqslant 0, y \geqslant 0, z \geqslant 0;

(2)

x2+y2+z2=R2,z⩾0x^{2}+y^{2}+z^{2} = R^{2}, z \geqslant 0.

Determine the applicate (z-coordinate) of the center of mass of the hemisphere x2+y2+z2=R2,z⩾0x^{2}+y^{2}+z^{2} = R^{2}, z \geqslant 0 with surface density:

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(1)

ρ=ρ0\rho = \rho_{0};

(2)

ρ=ρ0x2+y2\rho = \rho_{0} \sqrt{x^{2}+y^{2}};

(3)

ρ=ρ0(x2+y2),ρ0=\rho = \rho_{0}\left(x^{2}+y^{2}\right), \rho_{0} = const.

Determine the coordinates of the center of mass of homogeneous surfaces:

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(1)

x2+y2+z2=R2,x⩾0,y⩾0,z⩾0x^{2}+y^{2}+z^{2} = R^{2}, x \geqslant 0, y \geqslant 0, z \geqslant 0

(2)

z=R2−x2−y2,x⩾0,y⩾0,x+y⩽Rz = \sqrt{R^{2}-x^{2}-y^{2}}, x \geqslant 0, y \geqslant 0, x+y \leqslant R;

(3)

z=x2+y2,x2+y2⩽xz = \sqrt{x^{2}+y^{2}}, x^{2}+y^{2} \leqslant x;

(4)

z=2−(x2+y2)/2,z⩾0z = 2-\left(x^{2}+y^{2}\right) / 2, z \geqslant 0;

(5)

x=ucos⁡v,y=usin⁡v,z=v,u∈[0;1],v∈[0;π]x = u \cos v, y = u \sin v, z = v, u \in [0 ; 1], v \in [0 ; \pi ].

Calculate the moments of inertia relative to the coordinate planes of a homogeneous (ρ=ρ0=\rho = \rho_{0} = const )) surface:

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(1)

x+y+z=1,x⩾0,y⩾0,z⩾0x+y+z = 1, x \geqslant 0, y \geqslant 0, z \geqslant 0

(2)

z=hrx2+y2,x2+y2⩽r2z = \frac{h}{r} \sqrt{x^{2}+y^{2}}, x^{2}+y^{2} \leqslant r^{2}.

Calculate the moment of inertia of a homogeneous (ρ=ρ0=\rho = \rho_{0} = const) surface:

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(1)

x2+y2=2az,z⩽ax^{2}+y^{2} = 2 a z, z \leqslant a, relative to the axis OzO z;

(2)

x2/a2+y2/a2=z2/b2,0⩽z⩽bx^{2} / a^{2}+y^{2} / a^{2} = z^{2} / b^{2}, 0 \leqslant z \leqslant b, relative to the line y=0y = 0, z=bz = b.

Determine the electric charge distributed with density ρ=ρ0∣z∣\rho = \rho_{0}\left|z\right| over the surface:

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(1)

x2/a2+y2/a2−z2/c2=0,∣z∣⩽c;x^{2} / a^{2}+y^{2} / a^{2}-z^{2} / c^{2} = 0,\left|z\right| \leqslant c ;

(2)

z2−x2−y2=a2,∣z∣⩽a2z^{2}-x^{2}-y^{2} = a^{2},\left|z\right| \leqslant a \sqrt{2}.

References

[Dem97] Demidovich B.P. Сборник задач и упражнений по математическому анализу. 13-е изд., Издательство Московского университета / Издательство "ЧеРо", 1997.
[ККЧ+03a] Кудрявцев Л.Д., Кутасов А.Д., Чехлов В.И., Шабунин М.И. Интегралы. Ряды. 2-е изд., ФИЗМАТЛИТ, 2003.
[ККЧ+03b] Кудрявцев Л.Д., Кутасов А.Д., Чехлов В.И., Шабунин М.И. Функции многих переменных. 2-е изд., ФИЗМАТЛИТ, 2003.