Solving 2D moments about point O

In summary: F and O in the wrong direction (and with the wrong sign) :wink:it needs to be perpendicular to F, remember … draw a diagram! :DIn summary, to find the magnitude of force, F, when it causes a clockwise moment of 15 N-m about point O at an angle θ=30˚, you must find the distance between the line of force and point O by drawing a perpendicular line and using trigonometric functions to set up the equation. Similarly, to find the angle θ when
  • #1
majunee
5
0
(a) The moment the force, F, makes about O is 15 N-m clockwise, and θ = 30˚, find the magnitude of F.
(b) If the force F = 100 N, determine the angle θ so the force develops a clockwise moment about point O
of 20 N-m. Limit your answers to a range of 0º ≤ θ ≤ 90º.

I'm a little lost can someone please help me with how to set up equations so i can figure out the answers. Thanx
 

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  • #2
welcome to pf!

hi majunee! welcome to pf! :smile:

start by completing the diagram …

continue the line of force backwards, and find its perpendicular distance from O :wink:​
 
  • #3
thanx so is this correct so far
-15 KN-m = F[-(sin30°)-(50mm)-(300sin60°)]
 
  • #4
hmm … you're going to need another hint:

call that corner "C"

draw the line G through C parallel to F

draw the two perpendicular lines from O to G and from C to F …

how long are they? :wink:​
 
  • #5
Sorry I am really having trouble understanding the concept...
wouldnt 0-G be 300sin60 and c-f be 50 + xsin30
 
  • #6
ahhh, you seem to be measuring vertical distances

no, to find the moment of a force, you need to measure distance(s) perpendicular to the line of the force :wink:

start again :smile:​
 
  • #7
how? if force has continuing line of action shouldn't we measure along the yaxis which would be like i did?
 
  • #8
why the y axis? :confused:

(are we using the same diagram? … the force is at 30°)
 
  • #9
yes we are
so would it be -300sin60 + 300 cos 60 - 50 + xsin30 - x cos 30
 
  • #10
why the y axis? :confused:

and what is the "x" in your equation?
 

FAQ: Solving 2D moments about point O

1. What is the concept of solving 2D moments about point O?

The concept of solving 2D moments about point O involves finding the moment of a force or a system of forces about a specific point in a 2D plane. This is done by taking into account the magnitude, direction, and distance of each force from the chosen point.

2. Why is it important to solve 2D moments about point O?

Solving 2D moments about point O is important because it helps in analyzing and understanding the stability and equilibrium of a structure or object. It also allows for the calculation of torque, which is essential in designing structures and machines.

3. What are the steps involved in solving 2D moments about point O?

The steps involved in solving 2D moments about point O include:
1. Identify the point O and draw a free body diagram of the object or structure.
2. Determine the magnitude, direction, and distance of all the forces acting on the object.
3. Calculate the moment of each force about point O using the formula M = F x d, where F is the force and d is the distance from the point O.
4. Sum up all the moments in either clockwise or counterclockwise direction.
5. Set the sum of moments equal to zero to find the unknown force or moment.

4. What are the units of measurement for moments about point O?

The units of measurement for moments about point O are Newton-meters (Nm) in the SI system and pound-feet (lb-ft) in the imperial system.

5. Can moments about point O be negative?

Yes, moments about point O can be negative. This indicates that the force is creating a clockwise rotation, while a positive moment indicates a counterclockwise rotation. The direction of rotation is important when determining the equilibrium of an object or structure.

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