Mechanics Vectors Watch

StylaBoy
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#1
Report Thread starter 7 years ago
#1
hi every1

this question isn't actually vectors, but it's in the same chapter. It's about understanding bearings and using trig.

I put it in a spoiler..

Spoiler:
Show


This is the question



and here are my workings.



ok, so let me guide you through my workings..

Step 1: I've drawn my diagram (hopefully right )

Step 2: The shortest way back is obviously the yellow line^^

Step 3: I know for a fact that the green angle is 120° :>

Step 4: And therefore we can work out the 3rd angle.. (360-120-110)

Step 5: Now, If I'm not mistaken, I can use the cosine rule, right?! Well.. as you can see I got a problem

Isn't that the right way to solve this:confused:

anyway, I'm confused right now but I'm sure you'll have a solution 4 thiz ^.^



thx

€: In the (2nd) picture, it's meant to be X² =
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ghostwalker
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(Original post by StylaBoy)
...
Don't know what that's meant to be at the end of the equation in the blue box.

You can use the cosine rule to find x, and then the sine rule to find the angle theta.

What's the problem you're having?
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StylaBoy
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Well, just ignore the last bit in the blue box

my problem is when I apply the cosine rule, I get -16.069.. and then I would have to square root, which I can't do with a negative number..

cheers
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tiny hobbit
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Your working is right, it must be something to do with the way you are putting it into the calculator
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Ben121
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Using the cosine rule:
x^2 = 20^2 + 15^2 - 2*20*15cos(130)
x^2 = 625- 600cos(130)
x = 31.8

Perhaps you typed it in wrong or are working in radians or something.
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v2p
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(Original post by Ben121)
Using the cosine rule:
x^2 = 20^2 + 15^2 - 2*20*15cos(130)
x^2 = 625- 600cos(130)
x = 31.8
Yeah I agree with Ben I got the same answer too


OP:
This is how I typed it on my calculator

20^2 + 15^2 - (2*20*15*cos(130))

= 1010.7
Then square root Answer
x= 31.8
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Sokka
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i got: 31.8 (3 s.f)
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ghostwalker
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(Original post by StylaBoy)

my problem is when I apply the cosine rule, I get -16.069.. and then I would have to square root, which I can't do with a negative number..

cheers
If you still have a problem, post your working. A couple of people have provided correct values for x.
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StylaBoy
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Ohhh yes thanks guys!

I know what my mistake was.. I've done 625-600

and then 25 Cos 130

thx again, you're the best
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anshul95
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(Original post by Sokka)
i got: 32.67084434 = 32.7 (3 s.f)

i mean it's fairly straight forward; here's my working:
the other length was 15 not 16
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Sokka
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(Original post by anshul95)
the other length was 15 not 16
ye man, I changed it asap x] I always get the lengths wrong for some reason :confused:
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