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

AI Thread Summary
The discussion contrasts the use of unit vectors in two problems, highlighting that the first problem, related to a car, employs a unit vector due to its focus on a single direction. In contrast, the second problem is more complex, involving multiple directions and moments about a point, which does not necessitate a unit vector. The first problem's calculations require the unit vector to represent the moment about an axis, specifically using the j component. The second problem's complexity arises from its need to account for three-dimensional vectors. Overall, the choice of whether to use unit vectors depends on the specific requirements of the problem being addressed.
viet_jon
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the car problem in my text uses the unit vector, and the second one doesn't...

how come?


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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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