How to Determine Forces on a Right-Angle Slender Bar?

[Johann]

So this is the question that is given

The right-angle uniform slender bar AOB has mass m. If friction at the pivot O is neglected,
determine the magnitude of the normal force at A and the magnitude of the pin reaction at O.

Now I don't really know where to start whit this or what to do at all. Any help with this would me greatly appreciated

Thanks
Joey

 

[SteamKing]

Start by drawing a free body diagram. It would appear that both arms of the L would have the same mass per unit length, so you can use this assumption to find the weight of each arm in order to calculate the reactions at A and O. Once you have your FBD constructed, then you can write equations of static equilibrium.

 

[Johann]

Thanks for the quick response. I have thought of that but how do I go about finding the masses without any information besides that AO = 2l/3, OB=l/2 and that 30o degree angle at A.

With the free body diagram, Ill have downwards force(gravity), a force pushing back on A, anti-clockwise force at A and the clockwise force at B right?

This parts of mechanics is just beating me at this stage

Cheers
Joey

 

[SteamKing]

Thanks for the quick response. I have thought of that but how do I go about finding the masses without any information besides that AO = 2l/3, OB=l/2 and that 30o degree angle at A.

Like I suggested in my reply, assume a constant mass per unit length, ρ, and develop the mass of the two arms. Your final reactions will be expressions containing ρ, unless it cancels out somewhere along the line.

 

[Johann]

Does this mean that AO will have a mass of (2l/3)(p) and OB will have a mass of (l/2)(p)?

 

[SteamKing]

Yes.

 

[Johann]

So ill add both of these together to get the complete mass of the object, Should i then treat it as AO and then OB or do you wrok with the whole object i.e can it be two separate ladders leaning against a point or not?

 

[SteamKing]

AOB is a single bar in the shape of a lazy L, not a couple of ladders.