Basic Engineering Mechanics (trigonometry)

In summary: But if you're asking specifically for the angle from the x axis, then it's from the -x axis.But now:In summary, the trigonometry calculation yielded the resultant angle with the horizontal to be 93.3 degrees.
  • #1

Homework Statement


Two forces P and Q are applied to the lid of a storage bin as shown (see picture). Knowing that P = 48N and Q = 60N, determine by trigonometry the magnitude and direction of the resultant of the two forces.
IMG_20130903_180133_524_zpse7aada23.jpg



Homework Equations


"The triangle method" where you put each vector tip-to-tail, and then use components of one vector to make a right triangle (see picture).
IMG_20130903_180803_648_zps4cc30f74.jpg

Also, law of sines, law of cosines (??), and sin, cos, tan.
Pythagorean Thm

The Attempt at a Solution



As you can see, I turned the picture somewhat sideways so that Q lays flat.
I used simple trig to find Px = 48cos30o = 41.56 and Py = 48sin30o = 24. From there, I have a right angle and I know that one leg of the triagle is Q+Px and the other is Py. Using Pythagorean theorem, I got 104N, which is the right magnitude for the resultant.

My problem is the angle of the resultant. When I solve for the angle θ (between Q and R) in my picture, I get 13.3.
I used tanθ = 24/101.56
as well as the law of sines (104/sin150) = (48/sinθ)
both yield the 13.3o.
Even adding 13.3 to the 25 degree angle lid and past the 55 degree Q vector, I still don't get the right answer (which is 86.7 degrees, with an angle symbol pointing into the second quadrant, not sure what that means either).

Thanks for any help.
 
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  • #2
What is the resultant angle with the horizontal? If measured from the second quadrant?
 
  • #3
voko said:
What is the resultant angle with the horizontal? If measured from the second quadrant?

I guess that's what I'm trying to figure out.
I also find the wording in this problem poor because since the answer is 86.7 pointing to the second quadrant (??), does that mean it's 86.7 from the already 25 degree slant? or from the 1st quadrant x axis? or from where?
 
  • #4
ellieahlquist said:
I guess that's what I'm trying to figure out.

Are you saying you cannot answer the questions? You have calculated, correctly, the resultant angle with Q. But Q itself is not horizontal. Do you know its angle with the horizontal? What is the resultant angle with the horizontal then?
 
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  • #5
voko said:
Are you saying you cannot answer the questions? You have calculated, correctly, the resultant angle with Q. But Q itself is not horizontal. Do you know its angle with the horizontal? What is the resultant angle with the horizontal then?

This was my first thought:
The lid is 25 degrees from horizontal plus an additional 55 degrees to get to Q. 25+55=80
So I am at 80 degrees, and if the resultant vector is 13.3 degrees from Q, that's 93.3 degrees.
I'm still missing something because the answer is 86.7 degrees.

But now:
I think I understand what you meant by saying "if measured from the second quadrant." For some reason I didn't take that as from the -x axis. So I have 180 degree horizontal, minus the 25, minus the 55, then I have 100. and 100-13.3 is 86.7 degrees. But I definitely wouldn't have figured that out. Thanks.
 
  • #6
This is just something to keep in mind. This is done frequently, even if it is confusing. We just prefer angles less than 90 degrees.
 

1. What is the importance of trigonometry in basic engineering mechanics?

Trigonometry is essential in basic engineering mechanics because it helps in solving problems involving angles and sides of triangles. It is used to calculate forces, velocities, and distances, which are crucial in designing and analyzing structures and machines.

2. How is trigonometry used in resolving forces in basic engineering mechanics?

Trigonometry is used to break down forces into their horizontal and vertical components, which are then used to determine the net force acting on an object. This is important in analyzing the stability and equilibrium of structures and machines.

3. Can trigonometry be used to calculate distances and heights in basic engineering mechanics?

Yes, trigonometry can be used to calculate distances and heights in basic engineering mechanics. By using the trigonometric functions sine, cosine, and tangent, the lengths of sides and angles of triangles can be determined, allowing for the calculation of distances and heights.

4. How is trigonometry applied in basic engineering mechanics to analyze motion?

Trigonometry is used to analyze motion by calculating the displacement, velocity, and acceleration of an object. By using trigonometric functions and equations of motion, the path and speed of an object can be determined, which is crucial in designing and optimizing machines and structures.

5. Are there any other applications of trigonometry in basic engineering mechanics?

Yes, there are many other applications of trigonometry in basic engineering mechanics. It is used in determining the angles of inclination for ramps and roads, calculating the tension in cables and ropes, and analyzing the forces acting on bridges and buildings. Additionally, it is also used in geotechnical engineering to calculate the slope stability of structures and slopes.

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