How Do I Solve Complex Biomechanics Homework Questions?

[Hakan Can]

Hey all!

I just found out and joined the forum. I am a Biomedical Engineering student and in desperate need of help. We were given couple of questions to hand in next week. I managed to solve most of them, but I'm kinda lost at these 3 questions.

For question 2.1, I managed to calculate the values for a/b/c. But I couldn't find the mechanical energy to be invested in order to sustain the movement in these conditions.

Question 2.1: http://s24.postimg.org/ug6ccd8h1/Scan3.jpg
Solution I tried: http://s23.postimg.org/gec2oihgb/q2_1_sol.jpg
Related material: http://s18.postimg.org/42rtr72wp/q2_1_related.jpg

Question 4.2, the model given meets the standard formula for joint moment/reaction force calculations except one point. The object in hand. I could get this to the point of calculating by putting the angular acceleration, mass of forehand and mass of object in classical formula, but I'm not sure if that is the right way of solving the question.

Question 4.2: http://s23.postimg.org/8wtohtd5n/Scan.jpg
Solution I tried: http://s2.postimg.org/okxn35lix/q4_2_sol.jpg
Related material: http://s10.postimg.org/sroyitm21/q4_2_mat1.jpg
http://s28.postimg.org/70b0g1wbh/q4_2_mat2.jpg
http://s27.postimg.org/azf2dppw3/q4_2_mat3.jpg

Question 4.1 is where I am all lost. I know that moment of inertia is to be calculated considering the centre of mass of leg but the movement and lengths of y1 and y2, and calculating them related to centre of mass of leg is a mystery to me. I couldn't even have a try on this one.

Question 4.1: http://s11.postimg.org/ab1weck37/Scan2.jpg
No Solution
Related material: Nothing :(

I know I may be asking too much. But I'm honestly caught in the middle, I checked every resource I came up on internet but couldn't make much out of them. I would be extremely glad if you could help me on those.

Thanks in advance!
Hakan

 

[haruspex]

Hi Hakan, welcome to PH.
First, some advice on posting here:
- read the forum guidelines
- only put one problem per thread; if you have a number of similar problems then maybe it would be best to deal with one first, and that might show you how to handle the others;
- only use images for textbook pages and printed or hand drawn diagrams; any algebra or other typable working of your own should be typed in, preferably using Latex;
- always keep algebra purely symbolic (making up variables to represent given constants as necessary) until the final step; this has many advantages, not least making it much easier for others to follow what you are doing and spot errors;
- usually, using the template provided, quote any standard equations you believe may be relevant; but i see in this case you have some of them as a link, which is fine.
- always define your variables, including those in standard equations.

For now I'll just comment on 2.1.
You quote what appears to be a standard formula for min speed. Do you understand how it is obtained? The derivation involves calculating the energy, so using that equation is like starting at a point beyond where you need to get to.
Take it from first principles:
- how much energy has to be expended to get from A to B?
- how much to get from B to C?
- what distance has been covered?

 

[Hakan Can]

Hi haruspex,

Thanks for your suggestions and reply. I will read them thoroughly next time before posting.

As for Question2.1, I actually derived the formula from the conversation of energy, eliminating the kinetic energy at B, and calculating the velocity needed to go from A to B. But what is asked in the question is to put these Velocity values inside a formula to calculate invested energy.

I understand that at leg switch, the energy spent and put should be the same, as the velocity itself, but I'm having problem finding the Angle to put inside the formula. The actual angle between leg phases is 20degrees. But is it the same for the calculation of invested energy, Va and Vc seem a bit misaligned to leg angles to me.

I would be glad if you could help me on this, as I'm on the final stage of the calculation, then I will just divide this by the horizontal distance covered.

Thanks
Hakan

 

[haruspex]

what is asked in the question is to put these Velocity values inside a formula to calculate invested energy.

I don't read it that way. In order to calculate the minimum speed you had to think what that meant in terms of gait. The question is asking for the energy per unit distance traveled with the gait which achieves minimum speed. It does not require you to derive the answer from the calculated speed. You are entitled to derive it directly from consideration of the gait.
So, please answer my three questions in post #2.

 

[HalfLostAndSearching]

Hello Hakan,

I am glad to hear that you have joined the forum and are seeking help with your biomechanics homework. As a fellow scientist, I understand the importance of fully understanding and being able to solve these types of problems.

For question 2.1, it seems like you have made good progress in calculating the values for a, b, and c. However, in order to determine the mechanical energy required to sustain the movement, you will need to use the formula for kinetic energy (KE = 1/2mv^2) and the formula for potential energy (PE = mgh). You will need to calculate the kinetic and potential energy for each condition (A, B, and C) and then add them together to get the total mechanical energy required for the movement.

For question 4.2, you are correct in using the standard formula for joint moment/reaction force calculations. However, you will also need to take into account the weight of the object in the hand by adding it to the formula. This will give you the total joint moment/reaction force required to hold the object.

For question 4.1, you are correct in considering the moment of inertia of the leg and the center of mass. To calculate the lengths of y1 and y2, you will need to use the Pythagorean theorem (a^2 + b^2 = c^2). You can use the length of the leg as the hypotenuse (c) and solve for the lengths of y1 and y2. Once you have these values, you can use the formula for moment of inertia (I = mr^2) to calculate the moment of inertia for the leg.

I hope this helps guide you in the right direction for solving these questions. It is always important to fully understand the concepts and formulas being used in order to solve problems accurately. Keep up the good work and good luck with your homework!