# Search results

1. ### Very simple question about calculating weight

Okay thanks. It just strikes me as odd that the formula has mass, but yet you end up with weight. It's also crazy since this means the weight of the truss is only 10lbs and yet it holds up a 10,000lb load...But then again this is just a 2D model of what would really be a 3D structure.
2. ### Very simple question about calculating weight

Homework Statement Calculate the weight of each member of the truss structure. Homework Equations density = mass/volume The Attempt at a Solution This is part of a lab that we are doing in my mechanics of materials course. I have designed a structure, and I must calculate the weight of each...
3. ### Thermodynamics -- Internal Energy of Water

Oh yes, oops. So then ΔU = 2093.5 kJ And then: V = m/ρ = 100kg / 1000kg/m3 ==> V = 0.1m3 ΔH = ΔU + VΔP = 2093.5 kN*m + (0.1m3)*(900 kN/m2) =2183.5 kN*m ==> ΔH = 2183.5 kJ
4. ### Thermodynamics -- Internal Energy of Water

Oh okay I see now, so C = 4.187 kJ/kgK. So I found that ΔU = (100kg) * (4/187 kg/kgK) * (278.15K) = 116,461.41 kJ. Now I am ready to use that in the equation for ΔH = ΔU + VΔP. So in this case, volume does not change? And if so, would I have to find volume by using V = m/ρ ?
5. ### Thermodynamics -- Internal Energy of Water

Right, I was originally thinking that. So is C for water the same in all of its physical states?
6. ### Thermodynamics -- Internal Energy of Water

OK thanks, that's what I thought. So my tables that I am provided start at P = 25 bar. I have been doing a lot of searching on the internet, since some of my other problems involve compressed liquid. I found something called Compressed Liquid Approximation. It states that: u(T,P) ≅ uf(T) v(T,P)...
7. ### Thermodynamics -- Internal Energy of Water

Homework Statement Water is initially at P = 1 bar and T = 20°C. 100kg of water is pumped to a higher pressure at which P = 10 bar and T = 25°C. Find ΔU and ΔH Homework Equations H = m*h du = c*dT dh = c*dT + v*dP The Attempt at a Solution So far I have looked in my table and found that at P...
8. ### Internal Energy Thermodynamics

I am confused because I do not understand which value to choose for Cp if it changes with volume. Even if I were to integrate I would have: ∫dU = m∫Cp dT But how can I do that? The Cp is really throwing me off, and I have no idea how to use it in this problem. The same goes for Cv, since that...
9. ### Internal Energy Thermodynamics

No I wouldn't think that mass would change. I went ahead and calculated the volume of the second state, although I'm not sure if it was needed. Would I then have to use dU = mcvdT ? And since it cv is not constant, would I need to integrate both sides of this equation?
10. ### Internal Energy Thermodynamics

Okay so I am familiar with the PV=nRT formula. In our class, we usually use PV=mRT, and use the specific R for the gas in question. Would my first step be to calculate the mass of CO2 in state 1 using this formula?
11. ### Internal Energy Thermodynamics

Homework Statement CO2 is at P=3atm, T = 295K and V=1.2m3. It is isobarically heated to T = 500K. Find ΔU and ΔH Homework Equations dU = cpdT The Attempt at a Solution I am having a hard time in general in this class. I understand that in this problem, ΔP = 0. Does this mean that there must...
12. ### Thermodynamics - Need help with inclined plane problem

Oh okay that makes sense, thank you both for your help. I appreciate it!
13. ### Thermodynamics - Need help with inclined plane problem

Right, so the equation is the same, and we get the same answer. So why exactly are you guys pointing out my equation and saying it is dimensionally inconsistent?. What does that mean?
14. ### Thermodynamics - Need help with inclined plane problem

Oops, should be: Qout = 352.77MJ - 105.83MJ = 246.94MJ And that equation came from my notes. Why is it plus and not minus?
15. ### Thermodynamics - Need help with inclined plane problem

So Qin = Wout/0.30 = 3.53GJ dE/dt = Qin - Qout - Wout 0 = 3.53GJ - Qout - 105.83MJ Qout = 3.53GJ - 105.83MJ = 3.42GJ
16. ### Thermodynamics - Need help with inclined plane problem

Oh okay so it should be 105.83MJ. Efficiency of a heat engine is Wout/Qin So would it be: .30 = Wout/Qin ?
17. ### Thermodynamics - Need help with inclined plane problem

Oh oops, okay so there are two horizontal components acting to the left. Friction and mg*sin(20). So Fpush = 8kN + 40.26kN = 48.2kN. For work, the distance would be: d = 750/sin(20°) = 2192.85m And so the work would be: w = Fd = (48.2)*(2192.85) = 105.83kJ Now where does the efficiency come in?
18. ### Thermodynamics - Need help with inclined plane problem

Oh I believe I know what I'm doing wrong. I didn't realize that the friction force is equal to the horizontal component of gravity. Here is a new FBD: If this is correct, then Fpush (force required for the train to go up) is equal to 8kN? Also, where would the 30% efficiency come in? Would be...
19. ### Thermodynamics - Need help with inclined plane problem

For some reason I though I needed to. And gravity acts directly downwards, but I tilted the incline onto the x-axis to make it easier to work with. Is my diagram wrong? I am treating this as an incline plane problem, is that wrong?
20. ### Thermodynamics - Need help with inclined plane problem

If its 12000kg that means its 12,000 kg * 1000g/1kg = 12,000,000g = 12000x103 g, right? Also, I assumed that the force would be the F||. Now I am little confused, since I am used to having the friction force on the other side, and so F|| and F⊥ are just the components of the gravitation force...
21. ### Thermodynamics - Need help with inclined plane problem

Homework Statement A train engine climbs a hill. The engine is 30% efficient. The train has a mass of 12000kg. The hill is 750m in height, with an incline of 20° form the horizontal. Friction exerts a force opposing motion of 8000N throughout the climb. Find w, Qin and Qout for the engine...
22. ### How to determine direction of a moment

Okay so I will first say this is not my work, I am just studying the problem. They do explicitly define that anticlockwise is positive (it is to the left of the moment equation). I as well defined anticlockwise as positive. Then by judgement, I got the equation that CWatters got...
23. ### How to determine direction of a moment

Homework Statement I am following the first problem on this online pdf: http://www.ce.udel.edu/courses/CIEG212/Homework_1_2007.pdf Homework Equations Equilibrium equations for forces and moments. The Attempt at a Solution I know how to solve the problem, that part is straight forward. However...
24. ### Normal Stress: Compression vs Tension

Okay, so simply put, in a problem of this sort, if the member is in compression, take the cross-section where the pin is not?
25. ### Normal Stress: Compression vs Tension

Oh okay, that makes sense. I did include the entire problem, question, diagram and all, so that is all I had. Therefore they must have expected me to assume these things, although I honestly could not find it anywhere in the section. Essential, what I get out of your first explanation is that...
26. ### Normal Stress: Compression vs Tension

When I look at the solutions to these problems, I keep seeing that when a two force member is in tension, they use A = t(w-d) , as depicted by this image: When the member is in compression, they just use t*w, which to me (correct me if I'm wrong) means anywhere else along the member where...
27. ### Normal Stress: Compression vs Tension

Homework Statement Homework Equations Equation of Equilibrium (Horizontal and Vertical Forces, Moments) Normal Stress = F/A The Attempt at a Solution I have already solved the solution for this problem. For part (a), I simply found the force in the link, and used the cross area where the...
28. ### Shear Stress, Area of Projection

For some reason I was thinking of bearing stress, when it was actually asking for shearing stress. It just so happens that the area must include d and t. Either way, I am starting to understand this a little more. It is asking for the stress between the plate and the shaft, which is the area of...
29. ### Shear Stress, Area of Projection

I am learning about shearing stress, and I am a little confused about the area of projection mentioned in my book. When it introduces it, it shows a plate with a rivet through it. The plate is of thickness t, and the diameter of the rivet, d. It shows the plate and the rivet cut in half by a...
30. ### Beginner Question About Series Parallel

Right, that makes sense, here is my drawing now, which I think is correct: So in this drawing, the reason R3 is in series with (R1 || R4) and then that whole quantity (enclosed in the box) is parallel with R4 since they both share the Green and Blue nodes. R5 is then in series with all of this...