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Can y explain the part B and B because I don't understand why change the direction. Please.

Hi joseotilio25, this is a personal preference. I choose to make every vector subtraction problem an addition problem. When subtraction comes along, as in part b) here, I choose to make it an addition problem by adding the second vector in the opposite direction. This technique has the advantage that every vector problem, including subtraction, involves putting the tail of the second vector on the head of the first. To illustrate why this works, consider $3 - 2 = 1$. This can also be written as an addition problem by adding the opposite of the second number. The second number is $2$, so adding it's opposite means adding $-2$. It looks like $3 + (-2) = 1$. It works out to the same answer, as it should, and with arithmetic there's no advantage to making subtraction into an "adding the opposite" question, whereas with vectors, changing subtraction to "adding the opposite" has the advantage that you can keep on using the "head to tail" method. In the end, it's an issue of personal preference. You could instead learn a "head to head" method, and create the resultant by connecting the remaining tails if you want a different technique.

what about the last part of the problem that asked to find the magnitude and direction ?

Hi samkvertus, thanks for the question. Both the magnitude and directly are actually given here. Direction in this case is "to the left" when the resultant is negative, whereas it's "to the right" when positive. The magnitude is the number when ignoring the negative sign. This is explained more in the video, so consider giving it a second view.

All the best,
Mr. Dychko