# Mechanics question

1. Oct 12, 2007

### engineer_dave

1. The problem statement, all variables and given/known data

Calculate the moment of the 90N force about point O for the condition theta=15 degrees.

2. Relevant equations

Moment= Force* perpendicular distance

3. The attempt at a solution

I am doing the first step

Fx= 90sin theta
Fy= 90costheta

What i got was Moment= (Fx*0.8) + (Fy*0.6)= 72sintheta + 54costhetha

When I looked at the worked solution, they got a -Fx*0.8, i dont understand how the negative sign came about. Can you help out? Thanks

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2. Oct 13, 2007

### Staff: Mentor

Clockwise moments are consider +, counterclockwise moments are considered -. Righthand rule.

3. Oct 18, 2007

### minger

You need to look at your units as well. As an early student, write your units out for everything. Degrees/radians are unitless. All you have right now is a force times a unitless quantity, your resultant is N. You need Nm, which means you need to still multiply by a distance.

You were correct to take the sin/cos to find the horizontal and vertical components of the force, but you still need to multiply it by the distance from the center.

So, for the vertical direction:

M = -90N*cos(theta)*600mm*(1 m / 1000 mm) = x Nm

As Astro said, negative because of sign convention. Do the same for the other component, making sure its in the direction that you think.