We are asked to find the mass $m$ and the center of mass $(\bar{x}, \bar{y})$ of a lamina bounded by the curves $x=0$, $x=4$, $y=0$, and $y=3$. The density function is given by $\delta(x, y) = y + 1$.

Applied MathematicsCalculusDouble IntegralsCenter of MassDensity Function
2025/5/25
Okay, I will solve problem #1 from the image.

1. Problem Description

We are asked to find the mass mm and the center of mass (xˉ,yˉ)(\bar{x}, \bar{y}) of a lamina bounded by the curves x=0x=0, x=4x=4, y=0y=0, and y=3y=3. The density function is given by δ(x,y)=y+1\delta(x, y) = y + 1.

2. Solution Steps

First, we find the mass mm using the double integral:
m=δ(x,y)dAm = \int \int \delta(x,y) \, dA
In this case,
m=0403(y+1)dydxm = \int_{0}^{4} \int_{0}^{3} (y+1) \, dy \, dx
We first evaluate the inner integral:
03(y+1)dy=[12y2+y]03=12(32)+30=92+3=92+62=152\int_{0}^{3} (y+1) \, dy = \left[ \frac{1}{2}y^2 + y \right]_{0}^{3} = \frac{1}{2}(3^2) + 3 - 0 = \frac{9}{2} + 3 = \frac{9}{2} + \frac{6}{2} = \frac{15}{2}
Now we evaluate the outer integral:
m=04152dx=15204dx=152[x]04=152(40)=1524=152=30m = \int_{0}^{4} \frac{15}{2} \, dx = \frac{15}{2} \int_{0}^{4} dx = \frac{15}{2} [x]_{0}^{4} = \frac{15}{2} (4 - 0) = \frac{15}{2} \cdot 4 = 15 \cdot 2 = 30
Next, we find MyM_y using the formula:
My=xδ(x,y)dAM_y = \int \int x \delta(x,y) \, dA
My=0403x(y+1)dydxM_y = \int_{0}^{4} \int_{0}^{3} x(y+1) \, dy \, dx
First, we evaluate the inner integral:
03x(y+1)dy=x03(y+1)dy=x[12y2+y]03=x152\int_{0}^{3} x(y+1) \, dy = x \int_{0}^{3} (y+1) \, dy = x \left[ \frac{1}{2}y^2 + y \right]_{0}^{3} = x \cdot \frac{15}{2}
Now we evaluate the outer integral:
My=04x152dx=15204xdx=152[12x2]04=15212(4202)=15416=154=60M_y = \int_{0}^{4} x \cdot \frac{15}{2} \, dx = \frac{15}{2} \int_{0}^{4} x \, dx = \frac{15}{2} \left[ \frac{1}{2}x^2 \right]_{0}^{4} = \frac{15}{2} \cdot \frac{1}{2} (4^2 - 0^2) = \frac{15}{4} \cdot 16 = 15 \cdot 4 = 60
xˉ=Mym=6030=2\bar{x} = \frac{M_y}{m} = \frac{60}{30} = 2
Next, we find MxM_x using the formula:
Mx=yδ(x,y)dAM_x = \int \int y \delta(x,y) \, dA
Mx=0403y(y+1)dydxM_x = \int_{0}^{4} \int_{0}^{3} y(y+1) \, dy \, dx
First, we evaluate the inner integral:
03(y2+y)dy=[13y3+12y2]03=13(33)+12(32)=273+92=9+92=182+92=272\int_{0}^{3} (y^2+y) \, dy = \left[ \frac{1}{3}y^3 + \frac{1}{2}y^2 \right]_{0}^{3} = \frac{1}{3}(3^3) + \frac{1}{2}(3^2) = \frac{27}{3} + \frac{9}{2} = 9 + \frac{9}{2} = \frac{18}{2} + \frac{9}{2} = \frac{27}{2}
Now we evaluate the outer integral:
Mx=04272dx=27204dx=272[x]04=272(40)=2724=272=54M_x = \int_{0}^{4} \frac{27}{2} \, dx = \frac{27}{2} \int_{0}^{4} dx = \frac{27}{2} [x]_{0}^{4} = \frac{27}{2} (4-0) = \frac{27}{2} \cdot 4 = 27 \cdot 2 = 54
yˉ=Mxm=5430=2715=95\bar{y} = \frac{M_x}{m} = \frac{54}{30} = \frac{27}{15} = \frac{9}{5}

3. Final Answer

The mass is m=30m=30. The center of mass is (xˉ,yˉ)=(2,95)(\bar{x}, \bar{y}) = (2, \frac{9}{5}).

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