Finding angle of inclination. Hanging object.

Click For Summary

Homework Help Overview

The discussion revolves around finding the angles of inclination for two segments, AB and BC, which are part of a system involving forces and moments. The original poster expresses uncertainty about how to approach the problem, particularly in relation to the geometry of the setup.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss drawing forces and taking moments about a pivot point. There are questions about how to handle the geometry when the system is not linear, and participants explore the concept of perpendicular distances in relation to forces.

Discussion Status

There is an ongoing exploration of the relationships between forces and angles, with some participants offering guidance on how to visualize the problem. Multiple interpretations of the setup and calculations are being considered, and participants are working through the implications of their reasoning.

Contextual Notes

Participants mention the need to understand the moments of forces and the geometry involved, including the center of mass and the angles formed by the segments. There is also a reference to the challenge of language comprehension affecting understanding.

EVriderDK
Messages
55
Reaction score
0

Homework Statement



I have to find the angles from horizontal of AB=30cm and BC=40cm.
http://peecee.dk/upload/view/356001
I don't know how to tackle this one.

I was told that i could see them as lever arms, and find out at which angle they cancel each other out. I'm not sure how to do that.

Thanks you in advance.
 
Physics news on Phys.org
Welcome to PF!

Hi EVriderDK! Welcome to PF! :wink:

Draw the forces (the two weights), and take moments about B …

what do you get? :smile:​
 
Thank you :)

I could do that, if it was a straight line, but how to do it when it is bend in 90 degrees?
 
Have you done moments of forces?
 
I have solved these ones fx:
http://peecee.dk/upload/view/355632
http://peecee.dk/upload/view/353915

Hope you don't mind the danish :)
 
But do you know how to calculate the moment of a general force about a general point? :confused:
 
Yes. It is just Force/length ?
 
EVriderDK said:
Yes. It is just Force/length ?

Noooo :redface:

You need to look it up in your book.

(and I'm off to bed :zzz:)
 
Woops meant F*r :D
 
  • #10
EVriderDK said:
Woops meant F*r :D

what is r ?
 
  • #11
Radius or length
 
  • #12
of what?

from what to what?
 
  • #13
From the "rotation point" B to the end of an arm/lever.
 
  • #14
EVriderDK said:
From the "rotation point" B to the end of an arm/lever.

ah, no :redface:

it's the perpendicular distance from B to the line of the force

(in this case, the force is the weight, so the line of the force is the vertical line through the centre of the rod)

another formula is F.d (or Fdsinθ)

where d is the vector from B to the centre of mass, and θ is the angle between F and d

see http://hyperphysics.phy-astr.gsu.edu/hbase/torq2.html :wink:
 
  • #15
Hmm.

B is a point. How can something be perpendicular to a point?

Centre of the mass? So i have to make a triangle out of it, and find the center of gravity?
 
  • #16
EVriderDK said:
B is a point. How can something be perpendicular to a point?
tiny-tim said:
it's the perpendicular distance from B to the line of the force
Centre of the mass? So i have to make a triangle out of it, and find the center of gravity?

i don't understand

either follow the diagram, http://hyperphysics.phy-astr.gsu.edu/hbase/torq2.html, or follow my instructions
 
  • #17
I'm looking at the site, but it doesn't make sense. Maybe I'm just not that good at english :) It's confusing.

I don't know what you mean by F.d. What does "." represent ?

If the Force is the force of gravity, and the force of gravity is a vertical force through the point B, how can i make a line perpendicular to the force(Fg) and B?
 
Last edited:
  • #18
EVriderDK said:
I don't know what you mean by F.d. What does "." represent ?

That's the dot product (or scalar product), see http://hyperphysics.phy-astr.gsu.edu/hbase/vsca.html

If you don't like the idea of dot product, just use the Fdsinθ formula :smile:
If the Force is the force of gravity, and the force of gravity is a vertical force through the point B, how can i make a line perpendicular to the force(Fg) and B?

No, you make it through B (or from B), and perpendicular to that vertical line.

In other words: if d is the distance from B to the centre of the rod,

then dsinθ is the horizontal distance from B to the vertical line through the centre of the rod.​
 
  • #19
So d is a line from the middle of AB, perpendicular to AB, which goes to F_g, which is the vertical force through B ?

Dot product and vector calculations is long gone. Cannot remember how it works, so another method is preferrable :)
 
  • #20
EVriderDK said:
So d is a line from the middle of AB, perpendicular to AB, which goes to F_g, which is the vertical force through B ?

No!

Read the instructions …

it's perpendicular to the force

and it's through B (the pivot point)​
 
  • #21
I found another solution, where the Fg goes vertically down in the middle of AB(a). From that point you draw a vertical line to the top, and a horizontal line to the pivot point B. The same with BC(c). To calculate the ange, i first have to find out, where the two moments are =0. So that will be F_g_a*Cb*sin(alpha)-F_g_b*Ca*cos(alpha)=0.

Right?
 
  • #22
EVriderDK said:
I found another solution …

That is the solution I've been telling you about!
… where the Fg goes vertically down in the middle of AB(a). From that point you draw a vertical line to the top, and a horizontal line to the pivot point B. The same with BC(c). To calculate the ange, i first have to find out, where the two moments are =0. So that will be F_g_a*Cb*sin(alpha)-F_g_b*Ca*cos(alpha)=0.

Right?

You mean F_g_a*Ba*sin(alpha)-F_g_c*Bc*cos(alpha)=0 ?

Right! :smile:

(if alpha is what I think it is)

Now what is the ratio of F_g_a and F_g_c? :wink:​
 
  • #23
60.64deg/29.538=2.053

1:2.053 ?
 
  • #24
tiny-tim said:
Now what is the ratio of F_g_a and F_g_c? :wink:​
EVriderDK said:
60.64deg/29.538=2.053

1:2.053 ?

uhh? :confused:

F_g_a/F_g_c = weight of BA / weight of BC = length BA / length BC
 
  • #25
3/4 ?
 
  • #26
Yup! :biggrin:

(why the question-mark? :wink:)

Now put it all together! :smile:​
 
  • #27
I was not sure what you wanted :)

atan(20^2/15^2)
atan(30^2/40^2)

That is much easier. I just had no clue, why this is possible. I lack the understading of it :/
 

Similar threads

Calculating x Position of a Moving Object
  • · Replies 10 ·
Replies
10
Views
3K
Rope between inclines with maximum length of hanging middle portion
  • · Replies 46 ·
2
Replies
46
Views
7K
Inclined Plane Forces - with a twist
  • · Replies 2 ·
Replies
2
Views
2K
Effects of changing the angle on an inclined plane
  • · Replies 1 ·
Replies
1
Views
3K
Find the range of an object coming off an inclined plane
  • · Replies 1 ·
Replies
1
Views
2K
A ball (sphere) rotating along a moving incline
  • · Replies 14 ·
Replies
14
Views
3K
For a Pendulum: Knowing Acceleration Find Maximum Angle
  • · Replies 26 ·
Replies
26
Views
4K
Angle of Inclination Homework: Find θ at Ladder's Verge of Slipping
  • · Replies 9 ·
Replies
9
Views
2K
A block lies on an inclined plane with an angle of elevation
  • · Replies 16 ·
Replies
16
Views
5K
Equilibrium of a uniform bar inclined at an angle
  • · Replies 41 ·
2
Replies
41
Views
4K