The hinged plate is supported by the cord AB. If the force in the cord is F = 340 lb, express this force, directed from A toward B, as a Cartesian vector. What is the length of the cord?
Image from: Hibbeler, R. C., S. C. Fan, Kai Beng. Yap, and Peter Schiavone. Statics: Mechanics for Engineers. Singapore: Pearson, 2013.
Solution:
This question involves expressing forces in Cartesian vector form. If you are unsure on how to do this, read the detailed guide on expressing forces in Cartesian notation.
Let us first determine the locations of points A and B.
Using the diagram, the locations of the points are:
A:(8i+9j+0k) ft
B:(0i+0j+12k) ft
We can now write a position vector from points A to B.
r_{AB}\,=\,\left\{(0-8)i+(0-9)j+(12-0)k\right\}=\left\{-8i-9j+12k\right\} ft
A position vector, denoted \mathbf{r} is a vector beginning from one point and extending to another point. It is calculated by subtracting the corresponding vector coordinates of one point from the other. If the coordinates of point A was (x_A,y_A,z_A) and the coordinates of point B was(x_B,y_B,z_B), then r_{AB}\,=\,(x_B-x_A)i+(y_B-y_A)j+(z_B-z_A)k
The magnitude of the position vector is also the length of the cord.
magnitude of r_{AB}\,=\,\sqrt{(-8)^2+(-9)^2+(12)^2}=17 ft
The magnitude is equal to the square root of the sum of the squares of the vector. If the position vector was r\,=\,ai+bj+ck, then the magnitude would be, r_{magnitude}\,=\,\sqrt{(a^2)+(b^2)+(c^2)}. In the simplest sense, you take each term of a vector, square it, add it together, and then take the square root of that value.
Let us write the unit vector for this position vector.
The unit vector is each corresponding unit of the position vector divided by the magnitude of the position vector. If the position vector was r\,=\,ai+bj+ck, then unit vector, u\,=\,\dfrac{a}{\sqrt{(a^2)+(b^2)+(c^2)}}+\dfrac{b}{\sqrt{(a^2)+(b^2)+(c^2)}}+\dfrac{c}{\sqrt{(a^2)+(b^2)+(c^2)}}
Finally, we can express the force in the cord in Cartesian vector form. To do so, we multiply the magnitude of the force by the unit vector.
