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Vector problem..help please!

by confusedaboutphysics
Tags: problemhelp, vector
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confusedaboutphysics
#1
Sep24-05, 01:28 PM
P: 36
Vector V1 is 8.94 units long and points along the -x axis. Vector V2 is 4.13 units long and points at +35.0 to the +x axis.
(a) What are the x and y components of each vector?
so i got V1x=-8.94 and V1y=0 which is right

but i dont know how to get the V2x and V2y
i got V2x=4.99 or 5.00 and V2y=-2.76 but i got it wrong.

I solved for V by using V= V1 + V2 so i got V=-8.94 + 4.13 = -4.81. i got V2x by using Vx=Vcos(angle) so Vx=-4.81cos(35) = -3.94. Then i did -3.94=-8.94 + V2x and solved for V2x which i got 4.999 or 5.00. I got V2y by using Vy=Vsin(angle) so Vy=-4.81sin(35) = -2.76. Then i used Vy=V1y + V2y which is -2.76 = 0 + V2y and solved for V2y and got -2.76.

Am i doing this right? if i'm not how do i solve for them???
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phreak
#2
Sep24-05, 02:24 PM
P: 149
Your explanation is a bit difficult to read, but I think I know you're problem. You said V = -8.94 + 4.13. This wouldn't work since vector addition is not the same as regular addition. You have to find the COMPONENTS, add those up, then using trigonometry, find the final vector.

Here's how you would do it. Because for V2 the angle is 35 degrees and pointing along the +x axis, to find the y component, just multiply the length of the vector by sin(35), and then to find the x component, multiply the length of the vector by cos(35) and you have both answers for V2.

To find the final vector, add the x-components (that is, V1(x) + V2(x)), then add the y components (V1(y) + V2(y)). Then use the Pythagorean Theorem to find the magnitude of the vector and trigonometric formulae to find the angle.

Caution: Be sure that your signs are correct, as V1(x) is pointing in the opposite direction of V2(x).
BobG
#3
Sep24-05, 02:36 PM
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Quote Quote by confusedaboutphysics
Vector V1 is 8.94 units long and points along the -x axis. Vector V2 is 4.13 units long and points at +35.0 to the +x axis.
(a) What are the x and y components of each vector?
so i got V1x=-8.94 and V1y=0 which is right

but i dont know how to get the V2x and V2y
i got V2x=4.99 or 5.00 and V2y=-2.76 but i got it wrong.

I solved for V by using V= V1 + V2 so i got V=-8.94 + 4.13 = -4.81. i got V2x by using Vx=Vcos(angle) so Vx=-4.81cos(35) = -3.94. Then i did -3.94=-8.94 + V2x and solved for V2x which i got 4.999 or 5.00. I got V2y by using Vy=Vsin(angle) so Vy=-4.81sin(35) = -2.76. Then i used Vy=V1y + V2y which is -2.76 = 0 + V2y and solved for V2y and got -2.76.

Am i doing this right? if i'm not how do i solve for them???
Are you solving the right problem? From what you said, you just need the x and y components of each vector. You don't have to add them at all. In other words, if you want the x component of V2, then multiply the magnitude of V2 (4.13) by the cos(35) and multiply v2 by the sin(35) to get the y component.

And phreak is right about you using the wrong method to add vectors together if what you really want are the x and y components of the sum of V1 and V2. You still have to find the x and y components for each vector before you can add them together. Then add the x components together to get the x of you new vector and add the y components together to get the y of your new vector.


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