Determine the moment of the force about point O

[sHatDowN]

Homework Statement

Determine the moment of the force about point O.

Relevant Equations

M = F.d

There are components of 500N:

500cos(45)= 353.55
500sin(45)= 353.55

Radius is 3 then

M = (353.55*5.12) - (353.55*2.12) = 1060.65is that correct?

 

[erobz]

Homework Statement: Determine the moment of the force about point O.
Relevant Equations: M = F.d

View attachment 324887

There are components of 500N:

500cos(45)= 353.55
500sin(45)= 353.55

Radius is 3 then

View attachment 324888

M = (353.55*5.12) - (353.55*2.12) = 1060.65is that correct?

It's always good practice to show (or state) your convention with the diagram, and show units in computation. Sig figs are probably too many as well. In this case you chose counter-clockwise as positive moment.

Computationally...the calculation is correct.

 

[BvU]

Yes. It's a bit cumbersome that way. More straightforward:

The blue vector is ${1\over 2}r\sqrt 2$. That times the 500 from $F$ is 1061 Nm

$\$

 

[sHatDowN]

It's always good practice to show (or state) your convention with the diagram, and show units in computation. Sig figs are probably too many as well. In this case you chose counter-clockwise as positive moment.

Computationally...the calculation is correct.

Thanks a lot.

 

[jbriggs444]

M = (353.55*5.12) - (353.55*2.12) = 1060.65

So you are splitting the applied force into its vertical and horizontal components. You computed the torque from the vertical component by multiplying by the horizontal component of the moment arm for the point of application (5.12, 2.12). By inspection, this is a counter-clockwise torque.

You computed the torque from the horizontal component by multiplying against the horizontal moment arm to the same point of application (5.12, 2.12). This time the torque is clockwise, so it will subtract.

That is a viable approach. Straight, by the book, crank and grind.

The approach that I took was different.

The torque from a given force is the same no matter where that force is applied, as long as the revised point of application is somewhere along the "line of action" of the original force.

The drawing makes it clear that the line of action passes through the point (3.00, 0). That simplifies the math. Now the vertical moment arm is zero and we need only consider the vertical force component of 353.55 and the horizontal moment arm of 3.00:$$3.00 * 353.55 = 1060.65$$By inspection, this is a clockwise counter-clockwise torque.

 

[sHatDowN]

The drawing makes it clear that the line of action passes through the point (3.00, 0). That simplifies the math. Now the vertical moment arm is zero and we need only consider the vertical force component of 353.55 and the horizontal moment arm of 3.00:By inspection, this is a clockwise torque.

I think you wrong it's counter-clouckwise because in this case when we applied a force horizontal it's counter-clouckwise.

 

[haruspex]

it's counter-clockwise

I agree.

 

[jbriggs444]

I think you wrong it's counter-clouckwise because in this case when we applied a force horizontal it's counter-clouckwise.

You are right, of course.