using the laws of consines and sines

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staceyrho

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Jan 12, 2007
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I am trying to work a two part question.
consider two vestors A and B and their result and A+B are, respectively, 15.1 and 5.4 and they act at 110 degrees to each other.

use the law of cosines to clculate the magnitude of the resultant vector A+B?

Now use the law of sines to clculate the angle between the direction of the resultant vector A+B and the direction of the vector A.
answer in units of degrees.

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staceyrho said:
I am trying to work a two part question.
consider two vestors A and B and their result and A+B are, respectively, 15.1 and 5.4 and they act at 110 degrees to each other.

use the law of cosines to clculate the magnitude of the resultant vector A+B?

Now use the law of sines to clculate the angle between the direction of the resultant vector A+B and the direction of the vector A.
answer in units of degrees.

R^2 = 15.1^2 + 5.4^2 - 2(15.1)5.4cos(70º)

The angle between the resultant vector and vector A derivrs from
sin/_/5.4 = sin(70º)/R^2
Click to expand...
 
I don't understand what you are doing. What is R in this formula stand for? I'm really confused by your reply.
Also in this formula below you have sin/_/5.4 what is the _ for?
sin/_/5.4 = sin(70º)/R^2
 
staceyrho said:
I don't understand what you are doing. What is R in this formula stand for? I'm really confused by your reply.
Also in this formula below you have sin/_/5.4 what is the _ for?
sin/_/5.4 = sin(70º)/R^2
Click to expand...
Hold on there, stacyrho. It will help you more if you take the time to think about it, rather than just throwing it back.

What do you THINK "R" is? What is the point of the problem? I agree it should have been defined explicitly, but it is not too hard to see that "R" is the resultant vector. You could have figured that out from the use of the Law of Cosines.

What do you THINK the symbol /_/ means? What is the Law of Sines? It's just a couple of division problems. Therefore, /_/ can't be much else besides a typo. Simple division was the intent and there should be an argument for that sine function. Something is wrong. Go ahead and fix it. Also, what's that "R^2" doing on the other side? Should that be there or is something else consistent with the Law of Sines?

Do you believe everything your teacher says without ever questioning or wondering how it is consistent with previous statements? You must think and put stuff together. The world doesn't start all over each time you approach a problem statement. Hang in there. Relax a bit. Don't be so hard on yourself. You'll get it.
 
Okay so i'm using the formula you are showing me but my answer is still incorrect. Here is what i'm doing you tell me if I'm doing this wrong. I've never worked with Sines or Cosines and I'm not really understanding it. I've searched the internet to define it so I can better understand it but no luck. I'm taking this physics class and i don't have a clue. I'm so lost but I can't just drop the class at this point. so here I am stuck and trying to teach myself stuff I've never worked with in order to understand this asignment. So if you could please tell me what it is that i'm not calculating correctly. here's what i've done.


You show above R^2 = 15.1^2 + 5.4^2 - 2(15.1)5.4cos(70º) and
sin/5.4=sin(70Decrees)/R^2

My calculations are r^2 = -201.1 so then I am using that to imput in the other formula correct?

there my answer was -189.2

I think i'm really doing something wrong here. Any suggestions on my errors? I do thank you for all the help you've given me. Belive me I've sat her for quite a while trying to figure this out. I enter my answers onto a specified website for class and it will tell me if my answer is correct or not. so far no luck on this one.
 
okay I redid the problem and came up with the 201.393355 but that's incorrect according to my homework assignment. That is supposed to be the resultant vector of A=+B correct?
 
staceyrho said:
I've never worked with Sines or Cosines...
Click to expand...
From another one of your threads, we know that you are taking a physics course. Is this is another exercise from that class?

If so, please note that you should never have been placed in a trig-based physics course without ever having seen, let alone taken a course in, trigonometry.

You might want to have a serious talk with your academic advisor regarding appropriate course placement.

Eliz.
 
Well when I signed up for this course it was suppsed to be a physical science class woith no pre req. Then after I signed up paid for it and everything they changed it to physics that is algebra based. But this is just out of what I know. i've never taken trig and it's been 15 years since I've taken algebra. I don't know what to do anymore.
 
staceyrho said:
okay I redid the problem and came up with the 201.393355 but that's incorrect according to my homework assignment. That is supposed to be the resultant vector of A=+B correct?
Click to expand...

Stacey, WHAT is the darn answer "according to your assignment"?
If r^2 = 201.393355, then r = sqrt(201.393355) = ~14.2
 
staceyrho said:
I don't understand what you are doing. What is R in this formula stand for? I'm really confused by your reply.
Also in this formula below you have sin/_/5.4 what is the _ for?
sin/_/5.4 = sin(70º)/R^2

My apologies for theh brevity since I assumed you had some familiarity with the focus of theh problem.

Let R = the resultant vector you seek.

The angle opposite R is 70º

From the general Law of Cosines,

A^2 = B^2 + C^2 - 2BCcosA

R^2 = 15.1^2 + 5.4^2 - 2(15.1)5.4cos(70º)

Let /_ represent an angle and / = divided by.

The angle between the resultant vector R and vector A derives from
sin/_/5.4 = sin(70º)/R^2
Click to expand...
 
TchrWill said:
The angle between the resultant vector R and vector A derives from sin/_/5.4 = sin(70º)/R^2
Click to expand...
Will, I've entirely forgotten "physics, vectors et al..."; so dividing by R^2 is ok?
 
staceyrho said:
...this course...was suppsed to be a physical science class woith no pre req. Then...they changed it to physics that is algebra based.... i've never taken trig and it's been 15 years since I've taken algebra. I don't know what to do anymore.
Click to expand...
If the course requirements changed after you signed up, that would almost certainly be something for which you could demand a refund, since you'd paid for one thing and were assigned something else.

Also, since their physics course requires a current knowledge base of algebra and trigonometry, then it would be advisable to drop the physics course and sign up for the required prerequisite courses.

It should only take a year or two of classroom studies to get what you need. Once you have the necessary background material, you can then try the physics again.

My best wishes to you.

Eliz.
 
thanks to all of you who have helped. I contacted my advisor to see what my options are. Being I currently get financial aid I didn't actually pay for this class out of pocket so i'm not sure what choices I have for taking another course. I may just have to take the f and live with it. But I certainaly would not have signed up for anything requiring knowlegde of even algebra being it's been so long since i've looked at it.
Oh well such is life! I just have to roll with the punches. this website has been wonderful and i will continue to keep it in my favorites so i can utilize it when I do take another math class.
 
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