• Support PF! Buy your school textbooks, materials and every day products Here!

General intuition and tips for solving vector problems in mechanics?

  • #1
128
0

Homework Statement


I'm just looking for ideas, like how to proceed with various problems and how to counter them.
Almost all problems I have encountered so far haven't been very much like the examples, but instead kind of vague and dependant on trigonometry and vectors.

Right now I'm stuck trying to solve problems about Moment of Force.

The thing is that I don't have a PhD in trigonometry so the questions are kind of tricky for me. They even have the answer (just the answer) below the question, just so they can say that "Well, the answer is right in front of you!".

So when you have problems in which you don't have any numbers to go on, maybe an angle and a side or two. For example if you have a force vector between A and B, and you have the length from O to A and from O to B as well as the angle between those two lengths.
Together the two lengths and the vector AB form a triangle (not a right triangle) and you're supposed to find the moment about point O "as a function" of the angle.

Homework Equations


Mo = r x F = F*r*sin(a)


The Attempt at a Solution


I know that if the arm isn't perpendicular to the force then you have to find the shortest distance between the point about which I'm trying to find the "torque" and that is given by the length of the arm times sine of the angle between the arm and the force's line of action.
 

Answers and Replies

  • #2
tiny-tim
Science Advisor
Homework Helper
25,832
250
Hi Inertigratus! :smile:
I know that if the arm isn't perpendicular to the force then you have to find the shortest distance between the point about which I'm trying to find the "torque" and that is given by the length of the arm times sine of the angle between the arm and the force's line of action.
That's right! :smile:

So you know the lengths a and b (and maybe c?), and you also know the angle between them, and you have to find the sine of one of the other angles.

Using trig (there's also a Cartesian coordinate method, using the cross product ), you can find it by using the sine formula for triangles (sinA/a = sinC/c); if you don't know the third side of the triangle (c), you'll need to use the cosine formula also, to find that. :wink:
 

Related Threads on General intuition and tips for solving vector problems in mechanics?

  • Last Post
Replies
6
Views
1K
Replies
14
Views
406
Replies
6
Views
6K
Replies
3
Views
1K
  • Last Post
Replies
12
Views
119
Replies
1
Views
4K
Replies
10
Views
9K
  • Last Post
Replies
5
Views
704
Replies
3
Views
3K
Replies
4
Views
1K
Top