Calculating Combined Moment for 70lb Forces

[J-dizzal]

Homework Statement

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.

Homework Equations

M = F d , M = r x F

The Attempt at a Solution

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[SteamKing]

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 ?

 

[J-dizzal]

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 ?

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

 

[J-dizzal]

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 ?

i tried but i must be making a mistake

 

[SteamKing]

The moment arm is the vector r_OB , 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.

 

[J-dizzal]

The moment arm is the vector r_OB , 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.

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 each other? 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?

 

[Drakkith]

The OP already has a thread on this here: https://www.physicsforums.com/threads/couple-moments.820602/
Any further replies should go into that thread.
Thread locked.