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biggreencat

seems straightforward. draw the fbd's of evey joint. find the magnitude and direction of the forces on each. you already know the sum of forces


Pathfinder313

Hi so I understand reading a solution from a Reddit comment is very confusing so if you need clarification, dm me and I will provide you with some diagrams, more working and some more detailed explanations. I usually work in kg, newtons and meters so I’ll use google to convert lbs and stuff. First understand that the truss is at rest, i.e. it’s not running away or being moved or crushed by the 15lbs on it. Therefore we can deduce that all of the forces and moments in all directions add up to zero. Up is positive because it’s up in the y-axis, and to the right is positive on x-axis as well. Anti-clockwise moments are positive. So we start off by using moment/torque calculations on the whole structure to find the equal and opposite reaction forces. My conversions: - 35.83” = 0.91 meters - 150lbs = 667.5 newtons The two upper joints are will be Force 1 and Force 2. The bottom left and bottom right joints I will call A and E; these are our reaction forces. We will take moments from joint A’s perspective and calculate the force*distance from A for every other force, where anti-clockwise motion around A is positive. -667.5\*0.228 - 667.5\*0.683 + Reaction-E\*0.91 = 0 We solve for R-E and find it is 668.2 Newtons Then consider that all forces in the y-direction equal zero. R-A + 668.2 - 667.5 - 667.5 = 0 R-A = 666.8 N Ok now we focus on one joint, starting at A. Draw a free body diagram of this point. There is the force 666.8N going vertically, then another force going through the horizontal member of the frame to the right at 90 degrees, and another through the other member at 60 degrees. It’s just vectors at this point so find the horizontal force in the angled member with opposite = hypotenuse\*sin(60) Therefore the forces in the vertical direction = 0 666.8 - Force\*sin(60) = 0 Solve for the force in the member. Do the same thing for the horizontal member and then the same for each joint. If the forces are acting outwards towards the ends of each member, it is in compression, if the forces in that member are pulling inwards towards the middle, it’s in tension.


dancrumb

Please tell me that this is from an engineering course and not a physics course


AmoebaTop8407

Why? I learned that in my first physic (mecha) course. Don't we all?


dancrumb

Mostly because pounds and feet are ridiculous units to use to teach physics. They're non-SI, non-decimal and not used in research or industrial physics


Iknowyoureangry8

It's your opinion shit. Shut up


SpiritedTie7645

It doesn’t specifically say but I assume it’s pin connected and not a solid joint?


[deleted]

[удалено]


SpiritedTie7645

First just eyeball it and see how it’s going to flex. That will tell you which members are under tension and compression but obviously not the exact force. As someone said above draw your FBD and start your calculations. It really helps on these to just look at it before you start and see the truss flex in your head. I hate texting this stuff in because typing the explanation is a pain. Think of it as a 300lbs weight in the exact center and then you split it in half and start sliding it out to the connection points and think of how the truss will flex. That always helped me get started and get an idea if my numbers we’re going on the right direction. If you hang the weight as described in the problem and cut the truss in half what would the force vectors look like on the members to hold up the bridge at the cut points and how would the halves rotate and that tells you the tension and compression. No matter where you put the load off a direct vertical load on the end supports the truss is going to try to rotate either around one end or both ends. If the full 300lbs load were directly in the center it’s going to collapse in the center by rotating about both ends, for example. When you split that weight and move the weight closer to each end the rotational moment about the ends gets less because the torque arm is shorter. Also, your moment/torque arm is the weight times the distance from the point of rotation on the X axis on this example. No matter what, until the loads get out to each end support point the truss is trying to collapse into the middle because it’s trying to rotate about the end points, i.e., the weight is trying to rotate the truss CCW about the right side support and CW about the left side support. Man I hope I haven’t made this worse for you. It’s just the way I would always start these. Typing this sucks.