# Vector and cosine law?

1. Jan 11, 2012

### saac

Vector and cosine law??

1. The problem statement, all variables and given/known data
Three deer, A, B, and C, are grazing in a field. deer B is located 62m from deer A at an angle of 51 north of west. deer C is located 77 degree north of east relative to deer A. The distance between deer B and C is 95m. What is the distance between deer A and C?

2. Relevant equations
fairly new to physics and im stuck on these types of problems. Not sure how to draw the graph or complete the calculations. Any tips would be appreciated

3. The attempt at a solution

2. Jan 11, 2012

### Simon Bridge

Re: Vector and cosine law??

Just draw the triangle.
Get some graph paper and select a scale (hint 1cm = 1m).
Pick a direction for north (hint: up the page)
[if north is the y axis, then south is -y, east is +x and west is -x.]

Mark point A someplace handy.
Use protractor to get 51 degrees upwards from the -x axis (N of W)
Draw a line - to scale - 62m long.
Mark the end of the line with a point, label it B.

Follow the instructions like that for the rest.

You can either measure the length needed with a ruler or use trigonometry.

The trig method either resolves each position vector wrt to the x and y axis to get the points B and C, then compute |C-B|
OR use the cosine rule ... for which you need the angle between the two position vectors.
To do that you need to realize that the E-W line forms 180 degrees.
Your diagram will show you how this relates to the angles you are given.

3. Jan 11, 2012

### saac

Re: Vector and cosine law??

So i have drawn what i think is correct. Do i now separate the large triangle into two separate ones and then solve Hypotenuse for triangle A-B?

4. Jan 12, 2012

### Staff: Mentor

Re: Vector and cosine law??

No. You have a triangle ABC where you know the lengths of two sides, and you know one angle. Using your knowledge of trigonometry, that is sufficient information to determine all angles and all sides.

Do you know the sine rule? If not, try a google search.

5. Jan 12, 2012

### Simon Bridge

Re: Vector and cosine law??

If you know two sides and the angle between them, that's the cosine rule.
The sine rule is where you know one side and two angles - one opposite the known side.
But there is more than enough information here that normal trigonometry is sufficient.

You have drawn the triangle ABC?
You have two angles about vertex A (at the origin)?

There are two methods - both equivalent.

1.0 normal trig
1.1 put point A at the origin and +y pointing N
1.2 find the coordinates of B and C (trigonometry)
1.3 compute a = |B-C| (pythagoras)

2.0 shortcut
1.1 put b=|AB|, c=|AC|, find θ = angle BAC
1.2 put these numbers into the cosine rule formula

6. Jan 13, 2012

### Staff: Mentor

Re: Vector and cosine law??

We don't know |AC|. So attempting to use the cosine rule yields a quadratic to be solved.

Not such a shortcut.

7. Jan 14, 2012

### Simon Bridge

Re: Vector and cosine law??

Oh you are correct, I misread.
In that case you do need the sine rule to find angle BCA - it is a Side-Side-Angle problem.
Well spotted.

8. Jan 17, 2012

### saac

Re: Vector and cosine law??

Thanks for the replies but i've had to move on as the class is quickly progressing