How Do You Calculate the Magnitude of Vector C in Vector Subtraction?

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To calculate the magnitude of vector C, which is the difference between vectors A and B, it's essential to accurately determine the components of each vector based on their angles and magnitudes. Vector A has a magnitude of 5 m at 55° and vector B has a magnitude of 2 m at 33° with the negative x-axis, placing B in the fourth quadrant when inverted. The angle between A and B is not 90°; it is 92°, calculated by subtracting the sum of their angles from 180°. Using the correct component values and the Pythagorean theorem, the magnitude of vector C can be found by summing the squares of the x and y components. Properly sketching the vectors and applying the correct geometry is crucial for an accurate calculation.
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Homework Statement


https://smart.physics.illinois.edu/images/phys101/Homework/02/vector.gif
Two vectors A and B are shown in the x-y plane. Vector A has a magnitude of 5 m and makes an angle of 55° with the positive x-axis vector B has a magnitude of 2 m and makes an angle of 33° with the negative x-axis. (See the figure above.) Vector C (not shown in the diagram) is the difference of A and B (C = A - B).
What is the magnitude of C?

Homework Equations


[v] = sqrt(vx2-vy2)

The Attempt at a Solution


A=5cos55deg+5sin55deg
B=-2cos33deg+2sin33deg
So I plugged it into the equation above and got 6.391
But apparently my answer is not correct. I am wondering where I went wrong
 
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Hmm, I get an answer a little lower than yours for the magnitude of C. Maybe you made a calculator error? Show us what you've done.
 
T54 said:

The Attempt at a Solution


A=5cos55deg+5sin55deg
B=-2cos33deg+2sin33deg

In what quadrant is -B?
 
TaxOnFear said:
Welcome to PF!

Hmm, I get an answer a little lower than yours for the magnitude of C. Maybe you made a calculator error? Show us what you've done.

sqrt(2.868+4.096)2-(-1.677+1.089)2)
sqrt(48.497-.346)
= 6.989
This time I got a little higher than my previous answer.
 
Rapier said:
In what quadrant is -B?
Qaud II
 
I think I was making it to complicated.
The angle between vector A and vector B is 90°, so if I just the pythagorean theorem
I would get magnitude of vector C = sqrt(5² + 2²) = sqrt29
which is 5.385
 
T54 said:
Qaud II

There is part of your problem. B is in Quad II. When you take the inverse of a vector, like -B, you extend the line right out of his butt in the opposite direction and of the same magnitude. -B is in Quad IV.

EDIT: I want to add something to make sure this is clear. A - B = A + (-B), just as if these were regular variables.

If you draw that line of B backwards into Quad IV, you have -B and the angle is -33° (or 327°, if you prefer)...and keeping that 33° reference angle.

When you are dealing with vectors, there is two methods to deal with signs. Either you reduce everything to reference angles and then tack on the signs for the appropriate quadrant or you ignore the signs entirely and let the angles take care of it.

In Quad I: cos 33° is positive and sin 33° is positive
In Quad IV: cos 33° is negative and sin 33° is negative, so you either need to tack on the negatives yourself or use the -33° or 327° angle measures.

Once you understand A-B the rest is pretty easy peasy.

The magnitude of the combined vector will be the square root of the squares of the sums of the X and Y components. Which is a fancy way of saying that:

(A-B)=√( (Ʃx)^2 + (Ʃy)^2)

Try recalculating with the new information and give a shout if something is still unclear.
 
Last edited:
T54 said:
I think I was making it to complicated.
The angle between vector A and vector B is 90°, so if I just the pythagorean theorem
I would get magnitude of vector C = sqrt(5² + 2²) = sqrt29
which is 5.385

The angle between A and B is NOT 90°. Add up the angles you were given (33° + 55° = 88°). We know that the total angle measures from 0 to ∏ is 180°. 180°-88°=92° between the two angles.

I know it's very tempting, but you need to avoid slamming numbers into equations. You can bang your head against the wall for quite a while that way. Draw your sketch and label your vectors and then create your equations based upon the geometry, not by the utility of any particular equation.
 
T54 said:

Homework Statement


https://smart.physics.illinois.edu/images/phys101/Homework/02/vector.gif
Two vectors A and B are shown in the x-y plane. Vector A has a magnitude of 5 m and makes an angle of 55° with the positive x-axis vector B has a magnitude of 2 m and makes an angle of 33° with the negative x-axis. (See the figure above.) Vector C (not shown in the diagram) is the difference of A and B (C = A - B).
What is the magnitude of C?

Homework Equations


[v] = sqrt(vx2-vy2)


The Attempt at a Solution


A=5cos55deg+5sin55deg
B=-2cos33deg+2sin33deg
So I plugged it into the equation above and got 6.391
But apparently my answer is not correct. I am wondering where I went wrong

Note: if C = A - B, it follows that C + B = A

What vector must you add to B, in order to get A, for that is indeed C.
 

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