Why use unit vector for this, and not for that?

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The discussion clarifies the application of unit vectors in physics problems, specifically contrasting a car problem that utilizes a unit vector with a more complex problem that does not. The car problem requires a unit vector to calculate moments about an axis, represented as j or [0i, 1j, 0k]. In contrast, the second problem involves calculating moments about a point, necessitating a different approach without the use of unit vectors. This distinction highlights the importance of understanding vector applications in various contexts.

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viet_jon
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the car problem in my text uses the unit vector, and the second one doesn't...

how come?


zxcv.jpg


zxc-1.jpg
 
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The second problem is more complex and requires vectors to take care of the 3 directions involved. The first (car) problem has only one direction so no need for vectors.
 
but it did the opposite of just that.

the car used 'unit' vector...

while the second does not use the 'unit' vector
 
The first problem is asking for a moment about an axis. I'm pretty sure this is why you have to employ the unit vector in your calculations. If i recall correctly (been awhile since I was in statics) the unit vector here will be j or [0i 1j 0k]. Is that familiar?

The second problem is asking for a moment about a point.
 

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