Finding the Moment of a Force about a Point

[sdoug041]

Homework Statement

I need to find the moment of A about D. We are given the F_g on the object to be 49N and the distance from A to D is 27.5cm. I've attached a drawing to help demonstrate.

Homework Equations

M_D= r_A/D X F

The Attempt at a Solution

I know the radius between A and D is 27.5cm but I'm not sure how to find out how much of the force is actually acting on D! I know I need to use sin or cos of a deduced angle (15 degrees or one relating to that). I'm not sure which trig operation or angle to use.

Would anyone be able to shed some light on how to go about? Much thanks.

 

[berkeman]

Homework Statement

I need to find the moment of A about D. We are given the F_g on the object to be 49N and the distance from A to D is 27.5cm. I've attached a drawing to help demonstrate.

Homework Equations

M_D= r_A/D X F

The Attempt at a Solution

I know the radius between A and D is 27.5cm but I'm not sure how to find out how much of the force is actually acting on D! I know I need to use sin or cos of a deduced angle (15 degrees or one relating to that). I'm not sure which trig operation or angle to use.

Would anyone be able to shed some light on how to go about? Much thanks.

Welcome to the PF. The equation that you wrote is a vector equation, so you only need to write the vectors r and F, and take their cross product. The definition of the cross product includes the trig function that you need to use. Does that help?

 

[sdoug041]

Muchly appreciated I understand right now. Simply break the radius and force vector into it's components and cross. I knew it was easier than I thought; my frustration was unjustified!

Thanks man.