# Magnitude at pivot point and A

1. Mar 27, 2014

### 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

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2. Mar 27, 2014

### SteamKing

Staff Emeritus
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.

3. Mar 27, 2014

### 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

4. Mar 28, 2014

### SteamKing

Staff Emeritus
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.

5. Mar 28, 2014

### Johann

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

6. Mar 28, 2014

### SteamKing

Staff Emeritus
Yes.

7. Mar 28, 2014

### 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?

8. Mar 29, 2014

### SteamKing

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