Before you start addressing the internal forces, you should first have determined the unknown external forces and moments (if possible). Then, you draw a free body diagram that includes your cross section of interest. You try to choose a free body that includes as few of the external forces and moments as possible; this reduces the amount of work you need to do, but doesn't affect the answer.princejan7 said:Homework Statement
http://postimg.org/image/4lvunjeoz/
Solution: http://postimg.org/image/v1d7gilef/
When you have to calculate the internal forces at a point, how do you decide which forces are included in the free body diagram?
Yes. But its effect is captured by the other external forces that are actually acting on the free body you have chosen.For D, why isn't the rectangular bit of the distributed load included? Doesn't it affect D?
Chestermiller said:Yes. But its effect is captured by the other external forces that are actually acting on the free body you have chosen.
Only the insignificant part between B and D.princejan7 said:But the question says that D is located to the left of point B. Wouldn't that mean the rectangular distributed load is also actually acting on the body at D?
Chestermiller said:Only the insignificant part between B and D.
That's OK. Your free body diagram does not include the part to the left. And, as I said earlier, the effect of the rest of the triangular distributed load is accounted for by the reactions at B and A. To prove this to yourself, use the free body to the left of E instead of the one to the right of E, and see if you get a different result for the internal forces and moments at E.princejan7 said:oh ok, but then what about the diagram for point E?
The triangular distributed load is included in the diagram even though the important part is quite a bit to the left?
Free body diagrams are graphical representations of an object or system that show all the external forces acting on it. These diagrams are used to analyze the forces and motion of an object.
Selecting internal forces for free body diagrams is important because it allows for a more accurate analysis of the external forces acting on an object. Internal forces are the forces between different parts of an object, and they can affect the overall motion and behavior of the object.
To select internal forces for free body diagrams, you must first identify all the external forces acting on the object. Then, you can use Newton's third law of motion, which states that every action has an equal and opposite reaction, to determine the internal forces.
One common mistake is forgetting to consider all external forces, which can lead to inaccurate diagrams and analysis. Another mistake is incorrectly applying Newton's third law and incorrectly identifying internal forces as external forces.
Yes, one technique is to use symmetry. If an object or system is symmetric, then the internal forces will also be symmetric, making it easier to identify them. Another technique is to break the object into smaller, simpler parts and analyze the forces on each part separately.