# Couple moments

1. Jun 24, 2015

### J-dizzal

1. The problem statement, all variables and given/known data
For F = 70 lb, compute the combined moment of the two forces about (a) point O, (b) point C, (c) point D. The moments are positive if counterclockwise, negative if clockwise.

2. Relevant equations
M = F d , M = r x F

3. The attempt at a solution

2. Jun 24, 2015

### SteamKing

Staff Emeritus
The force F is a vector, and your calculations treat it as if it were a scalar.

Do you understand how to apply M = r x F ?

3. Jun 24, 2015

### J-dizzal

would it be like M = r x F where r= displacement vector from A to 0?

4. Jun 24, 2015

### J-dizzal

i tried but i must be making a mistake

5. Jun 24, 2015

### SteamKing

Staff Emeritus
The moment arm is the vector rOB , which is measured from point O to point B. Your calculation shows the opposite.

The moment arm should be measured from the reference point of the moment to the head of the force vector.

6. Jun 24, 2015

### J-dizzal

So when using M = rF, the vector r is always measured from the point of moment to the to the head of the vector. so the would look like to vectors with the heads pointing toward eachother? And what about M = Fd? the distance line should always travel to the head of the vector? unless you negate the vector then the tail is ok?

7. Jun 24, 2015

### Drakkith

Staff Emeritus