# Homework Help: Biomechanics Homework

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1. Dec 4, 2014

### 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.

Hakan

2. Dec 4, 2014

### haruspex

Hi Hakan, welcome to PH.
First, some advice on posting here:
- 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?

3. Dec 5, 2014

### Hakan Can

Hi haruspex,

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

4. Dec 5, 2014

### haruspex

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 travelled 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.