Picking the pivot point is an important step. It can simplify your problem or make it more complex.
Any force that is exerted at the pivot point will not exert torque on the system because it will have a lever arm of zero metres. It does not matter if the force is due to weight or "applied" or friction, etc.
Often, the pivot point is chosen to minimize the number of forces you would need to know. counter
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negative
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In (a), I did not mention the torque associated with the applied force of the supporting pivot point. It has no lever arm and thus exerts no torque on the system. It can be included in the net torque statement (and probably ought to be). I wanted to keep things simple.
I did mention the weight of the supporting plank because students can often forget to think about where the centre of mass of an object would be. If the centre of mass is not over the pivot point then the weight of the object will exert a torque on the system.
sys
sys
negative
C
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c
positive
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note:
In problem #4, at the instant the girder is about to rotate off of the concrete slab, the only upward applied force would be that exerted by the edge of the slab (pivot point). The left side of the girder at that point would not be supported by the concrete slab.
left end not supported
by slab
at the brink of rotating
In problem #6 you need to consider what forces are acting on each end of the ladder as well as the weight of the ladder.
You will need to think about the horizontal forces and the vertical forces.
You will also need to identify, when needed,
the lever arm for each force. I find it easier to draw little diagrams to isolate each torque.
ml = 5.00 kg
#6 is meant to be a very simple introduction to the firefighter example shown in the text book on page pp. 496 ­ 498. It is a good final question to see if you understood the conditions for static equilbrium.
We are not doing any problems that are as
hard as the firefighter example. You can chose to read it over for enrichment. And, you can then try #7. Note, I haven't had time to sit to solve it yet.
However, I disagree with the text's determination of the friction force. I believe they should have found the lever arm for the combined centre of mass for the ladder and man system (something we have not discussed in class but is mentioned in your text).