# Completely not understanding moments! help!

#### physicsneek

there are 2 questions which are driving me insane because I jus don't understand what to do!
I hope that if someone could go through the questions in detail then I'll finally understand how to go about answering.

5. A rod AB of mass 6kg and length 3m has its centre of mass 1.2m from A. It is suspended horizontally from the ceiling by two vertical strings one attached at A and the other at point C where AC is 2.5m. Find the magnitude of the tensions in the strings.

7. A uniform plank AB is 6m long and is resting in a horizontal posotion on two supports at A and B. The mass of the plank is 20kg. A body of mass 30kg is placed on the plank 1m from A and a second body of mass 30kg is placed on the plank 4.5m from A. Calculate the reaction at A and B.

the answers for 5 are 30.6N, 28.2N
the answers for 7 are 417N , 368N

thank you!

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#### Mapes

Homework Helper
Gold Member
The first important step is knowing how to draw a free-body diagram correctly. How are you doing with that?

After the FBD, you just need to apply the principles of equilibrium to generate algebraic equations containing your unknowns, and solve those equations. The equilibrium statements are fantastically powerful tools: you often have three of them available (sum of the forces in the x-direction is zero, sum of the forces in the y-direction is zero, and sum of the moments is zero), and you can apply them in any location of your structure. Part of statics is learning to spot where to apply them most effectively.

Moments take some getting used to, to be sure. The moment is the dot product of the force and the distance vectors, or the product of the force magnitude and the perpendicular distance to the line of force. A torque is just added to the sum of the moments. A sign convention (e.g., clockwise is positive) keeps the sign straight.

Are you up to speed on all of the above?

#### physicsneek

I think my FBF diagrams are correct for the questions, I can usually do those adequately.
I think the main problem is that I have no idea what the questiona are asking me!

#### Mapes

Homework Helper
Gold Member
The tension and reaction values are unknown forces applied to the system.

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