Robin Ford
I am a retired Associate Professor from the School of Mechanical and Manufacturing, UNSW Australia and hold several teaching awards. I like to look for new ways of presenting Engineering Mechanics.
Location I live in Sydney Australia.
Activity
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Robin Ford made a comment
For fun - and to educate a dentist who is interested in electric cars - I have been analysing a model radio controlled electric car: a Tamiya Grasshopper.
https://www.tamiya.com/english/products/58346grasshopper/index.htm
It's notionally 1/10 scale, and so far we've got it just over 21 km/h.
The Grasshopper isn't the car that the dentist wants, but...
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Robin Ford replied to Thomas Toseland
They all go together - engineering mechanics, mechanics of materials, materials science, fluid mechanics and more. Design is what holds it together as I see it. The divisions into these separate 'disciplines' is artificial, but nevertheless in its way, useful. The whole field of engineering would be overwhelming if you tried to grasp it in a lump.
When...
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Robin Ford replied to Patrick Scoley
There is a "part 2", it's called "Through engineers' eyes: expanding the vision" It starts on 12 Nov.
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I too thought so for a long time, so I thougt I'd share the revelation with you!
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Robin Ford replied to Keith McGraw
It does. 1440 rpm betrays my history: 30 years in England followed by the rest in Australia (give or take a year and a half in the USA). I take it that the nominal speed in the USA will be 1440*(60/50) - where 60 and 50 Hz are the frequencies of AC grids in USA and UK/Europe respectively.
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It's good to get responses - specially when they are positive!
In the case that "too much engineering mechanics is barely enough", you might like to continue with the companion course:
"Through Engineers' Eyes: Expanding The Vision".
It starts 12 Nov 2018.
There are some very quick and easy, yet instructive, experiments on centre of gravity and...
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Robin Ford replied to Jonathan Cooke
There is a follow-on course "Through Engineers' Eyes: expanding the vision. It explores three applications of statics (centres of gravity, friction, wind loads and rolling resistance; then makes a start on work/energy, collisions and projectile motion. It starts two weeks after this one finishes - 12 November 2018.
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Yes. This week completes the basic tool kit for engineering statics. It is simple once you get it, but it can be mysterious until you do. It took Newton and a whole lot of other people to work it out.
For a while I tried to find who devised the FBD, but its origins are lost in the mists of time.
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Robin Ford replied to Keith McGraw
I would normally use something like the notation that you are familiar with. For some reason I chose this different notation when working on this problem. It does demonstrate that you can choose from a wide range of notations - provided that you make it clear. Maybe I could have made it clearer.
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Robin Ford replied to Adam Melańczuk
There are two features for each item: direction of rotation and direction of torque. The direction of the torque on the input shaft is the same as the direction of rotation of the input shaft. On the other hand, the direction of the torque on the output shaft is opposite to the direction of rotation of the output shaft.
Does that help?
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Robin Ford replied to Keith McGraw
There's a little movie on the padlet wall.
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Robin Ford replied to Keith McGraw
I just cut away some of the structure of a broken balance. I'll post photos and explanations on the Padlet wall.
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Robin Ford replied to Keith McGraw
I came across one over the weekend. I replaced two 'spring balances' that are used in lifting windows to balance out the weight of the window. You have to pretension them by twisting them. They specified 6 turns.
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The job of a propeller is to convert torque from the motor to a thrust that drives the boat along.
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Robin Ford replied to Keith McGraw
Not really. I wanted a speed and this one came to mind, no doubt because of its practical links. Thank you for your question. Next time I'll explain it so that the learners are let into the the story.
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Robin Ford replied to Paul Crooke
Maybe that is cryptic. Sometimes when I'm drawing an FBD I overlook a connection with the body's surroundings, which means that although I had intended to isolate the body completely there was still a connection with the outside world, which could introduce errors.
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Robin Ford replied to Keith McGraw
1440 rpm is the commonly stated speed for a 4-pole induction motor running off a 50Hz supply
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Robin Ford replied to Keith McGraw
@KeithMcGraw Yes, it is a universal test m/c, and maintaining calibration is vital. There's a wide range of kit in the labs. I'll see if I can get a listing - just out of interest.
Whenever the topic of strange units arose, my first boss (who was from a farming family) would always tell us the irrigator's unit of acre-feet per day.
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Not my field, but I have a colleague who might be able to shine some light on this. He's been away; I'll see when I can contact him.
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Robin Ford replied to John Phipps
I see your thinking. It is complicated by the unknown length of string used to suspend the rubber band transducer from the beam. The dimensions AB and CB are supplied so you can calculate the angles. The forces in the transducers are intended to be taken as given.
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Robin Ford replied to John Phipps
Can you please let me know where you found mention of the lengths 272 mm and 228 mm.
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Robin Ford replied to Keith McGraw
The bridge is probably the Tacoma Narrows bridge. It's a classic. There are many versions of it on youtube. I liked this one best.
https://www.youtube.com/watch?v=nFzu6CNtqec
When we split the 7 week course to two 4 week courses, I decided to expand the time spent on twist. I didn't think of the Tacoma Narrows bridge. Instead I added power transmission...
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Robin Ford replied to Justin Bulten
Thank you, Justin. I'll get on to it.
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Robin Ford replied to Justin Bulten
It's an excellent question. Questions like this prompt deep learning. Free Body Diagrams help sort this out. The FBD isolates one body from another. Newton's third law requires that the 'action' on one body is equal and opposite to the 'reaction' on the the other body. That's when the body is isolated. When you 'reassemble' the system, that's when the action...
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Robin Ford replied to Jonathan Hemp
Although this answer might seem improbable, it is correct. I'll try to convince you. Consider the springs one at a time. Assume there is no pre-tension in the springs when they are unloaded (this doesn't affect the conclusions). Then when you load the point where they join, one is a tension spring and one is a compression spring. The tension spring pulls back...
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Robin Ford made a comment
Good to see all that activity. The padlet wall shows how i made a spring from an aluminium can. It was fun.
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Robin Ford replied to Liz Hundy
I've posted the image.
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Robin Ford replied to Liz Hundy
I found a page from Brunel's sketchbook in "Brunel and his world", John Pudney, Thames and Hudson, 1974. I'll try posting it on Padlet. There are several mentions of his sketchbooks in "Isembard, Kingdom Brunel", LTC Rolt, Longmans, 1964. For example, "...Brunel's sketch and notebooks do reveal very clearly the mind of the engineer at work...Amidst drawings...
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Robin Ford replied to eddy mayne
Back in the day, we used different units for different kinds of power; mechanical power was horsepower, electrical power was Watts, heating power was British Thermal Units per hour; food power was Calories per day. Some non-standard terms still survive in general usage, for example although my gas bill charges in Joules (a Joule is a Watt-second), my...
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I'm glad you enjoyed it.
I've been wondering about workload. A video-game designer tells me that they might aim for 20 hours to complete, over 4 weeks. The net supplied interesting material. This quote for example "According to The Entertainment Software Association, the average game buyer is 34 years old and has been gaming for 12 years. At that age, they... -
Robin Ford replied to Khalaf Alshammari
If you told Ahel-A'azm 50 years ago that by now they would have a hand held device that would be a video phone, a camera, a calculator and connect them to much of the knowledge in the world you would have received a blank look. On the other hand they would still recognise the general layout of cars and planes.
Good luck with your project to go home. I hope...
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I'll see what I can do for next time. I assume the same will apply to some other sections. Maybe I can add extra guidance/explanation in a supplementary document.
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I agree that positive objectives are best. I try to change from negative to positive each time we revise the course. Somehow those ideologically less sound negative ones seem easier to dream up. I'll keep trying.
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Robin Ford replied to Carlos E
This got me thinking. It took a while but I finally got it.
I looked at it rather differently. If it is on the point of tipping, A=0 and N=mg. If N=mg then -a/(Us*h-2a) must equal 1, dividing top and bottom by a I got -(Us*h/a-2) =1, and with h/A=.9/.3 this gives -Us*3+2=1 or Us=1/3. If Us is less than I it will slide if it is more it will tip; now 0.35 is... -
That's great! I like the idea of a larger aircraft with thicker wings. Well done.
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Thank you for the feedback. We'll look at it when we review this run of the course.
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Thank you for your comprehensive note.
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It's a long job devising talking points. We update some of them each time. Thank you for encouraging us to keep at it.
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Robin Ford replied to Erika T.
Well done for getting this far. And thank you for your contributions; I'll miss them.
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The main thing is to keep the cardboard horizontal, otherwise the horizontal distances from suspension to centre of gravity get corrupted.
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Robin Ford replied to Ian Kershaw
That's an interesting and important point. As long as you keep track of things it doesn't matter where you take moments (provided you always use the same axis). As you got the right answer, I assume that you DID keep track of it. When finding the balance mass for the glider, there's (at least) one effective axis to use - see my solution (perhaps after you've...
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Robin Ford replied to Allehn ~
It might make more sense when you can see how it's used in dynamics.
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Robin Ford replied to Brian Rogers
I'll see what is possible.
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It depends on what you are looking for. In this case it helped me to assess the situation. But if it doesn't help you then I;d leave it out.
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This might be called 'ill conditioned'; to get the horizontal comp you are subtracting one fairly large number from another, so the result can change quite a lot with small differences in the two subtracted numbers. This situation can occur quite frequently in eng calcs.
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Yes. The weight of the strut is included when finding the pin force in part 1, but has been left off in part 2 by error. We'll fix it. Thank you for pointing it out.
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This has come about because I rounded things off differently in the two locations. I'll check the FBD in the video.
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That's a common use of torque. In SI it's Newton Metres of course. It's the twist at the output shaft.
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I explained this earlier I think but it has disappeared. % error is usually found by dividing the error by the true value and multiplying by a hundred. This works fine for the vertical force, but for the horizontal force the true value is zero so you can't divide by it usefully. So I divided by the vertical force to give some idea of the scale of the error in...
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Robin Ford replied to Allehn ~
This opens up a long-standing debate on the usefulness of the concept of inertia forces, of which 'centrifugal force' is one. Some say that they are pernicious and should be shunned; others find them a useful concept, particularly with dynamics of rigid bodies. I was once a true believer in the 'centrifugal force is a pernicious' camp (following Meriam and...
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Robin Ford replied to Harold Bowyer
For each question the arrows represent the additional load mentioned in the description. What caused the loads is not specified. Although the cause doesn't affect the FBDs I can see that it would add an extra note of realism if the cause were indicated. Something for when we update the new course.
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Robin Ford replied to Allehn ~
It's some time ago that I did the experiments. I may have used a different 'force transducer'.
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Robin Ford replied to ANNE-MARIE STEWARD
L and W are two forces - the lift and the weight of the balloon. I have put the tail of each arrow roughly in the centre of the balloon; perhaps it would have helped if I had put a mark so that it's clear there are two arrows.
In the next diagram I have put the head of the WT arrow roughly in the centre of the box; it represents the weight of the basket. T is... -
If the wheels are not braked the wheels are 'rollers' and the forces are at right angles to the surface. When a wheel is braked there is a component along the surface, but only on the braked wheel.
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Point taken about the 'new way'. What was new to me was the freedom that the MOOC concept gave me for following a storyline rather than a syllabus. I could go where my instinct led me, and this lightened my spirit and (I hope) kept the presentation fresh.
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Robin Ford replied to Allehn ~
Thank you for finding the typo. I'll get it corrected.
The definition of transducer was from my head. Now that you have raised the issue I just checked out various sources.
On the web I found:
+ a device that converts variations in a physical quantity, such as pressure or brightness, into an electrical signal, or vice versa.
+ a device that converts one... -
Robin Ford replied to Allehn ~
I see what you mean. A four-function one would do, but why not get some practice in on the scientific calculator so you are ready for week 2?
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That's a good idea. I think I had something like it in the course development in an early version, but somehow it seems to have ended up on the cutting room floor,
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Robin Ford made a comment
I've noticed recently that the word 'curator' has become fashionable - as in someone 'curating' an art exhibition for example. A curator selects items to display that demonstrate the development of a particular artist's work or develop a theme. I think that's what we have done here. As curators we have chosen our own guided tour through engineering mechanics -...
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Robin Ford replied to Taaha Malik
The meanings of sine, cosine and tangent are all you need for this course. And I find that learning these sorts of things are easier if you have an application in mind.
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Robin Ford replied to Robin Ford
It's an instructive - and scary - story.
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My view is that either the head or tail is placed where the force acts.
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Robin Ford replied to Allehn ~
I'll be interested in your impressions later on. Have you looked at later weeks? Engineering mechanics is based on physics of course, but adopted and adapted by engineers. The concepts that we cover are met in university level courses, as a glance at the classic textbooks used in universities will show, but we hope to do It in a way that works for people with...
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Sometimes the correct statement involves a wrong answer.
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Robin Ford replied to Ashish poudel
You are right; answer C has the forces in the wrong direction, but the question asks 'Which of the diagrams shows the wrong sense for some forces?' Now 'sense' answers the question 'which way' so the wrong sense means that the line of action is ok but the arrow head points in the opposite direction to the right one. 'C' has the forces in the wrong sense so it...
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Robin Ford replied to Brian Rogers
Although it is definitely 'retro', you might like to try the graphical solution that Rafal mentions. It's surprising how accurate it can be if you are careful, and I think it gives you a feel for what is going on. I'll see how well I can do it and post my effort on the padlet wall.
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This is a point well made. Perhaps the person choosing the cables and shackles would do well to check that out too.
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Robin Ford replied to ANNE-MARIE STEWARD
That was my experience too. And we used logarithms for multiplying. Then I got a slide rule, and in 1976 an electronic calculator. And more recently a spreadsheet and a smart phone. But the same basic concepts are behind sine, cosine and tangent, however you find them.
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Robin Ford replied to Khaled Zakaria
We've spoken to the FutureLearn team about the problem with the 'completed' button and they suggest clearing your browser's cache and cookies. Instructions on how to do so for various browsers can be found here:
https://kb.iu.edu/d/ahicIf this doesn't work, please contact them and let them know what browser you're using via the support button at the...
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Robin Ford replied to Brian Rogers
You won't need the sine rule (or the cosine rule). I've worked directly from the definitions of sine, cosine and tangent.
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Robin Ford replied to Khaled Zakaria
I'll pass this on and report back.
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Sorry I couldn't see your experiment. I have managed to upload small videos in past presentations of this course.
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Robin Ford made a comment
In the last few weeks there have been a couple of balcony collapses on the East coast of Australia that have led to serious injuries. I wonder who will take the blame; the designer, the builder, the supplier of materials, the owner, or even the user?
http://www.abc.net.au/news/2017-04-30/tree-breaks-womans-fall-from-balcony-during-housewarming/8483924... -
Robin Ford replied to Robert James
Thank you for your careful review. I'll add it to the 'update list'.
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Robin Ford replied to Robert James
See 'engineers' eyes' for more explanation. I think I might have resolved your issue. I have specified 'tangent stiffness' for the stiffness of the non-linear spring, which is usual (for example in vibrations analysis). The stiffness calculated from 1500/0.1 is what is called the 'secant stiffness'. It is sometimes used in special applications, but was not the...
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Robin Ford replied to Robert James
Here's a fresh look.
Definition: stiffness = (change in force)/(change in length)
To find this at any load point, you can find the length at a load just below the load point, then at a load the same amount above the load point, then find the stiffness by:
stiffness = (change in force)/(change in length)
If the two loads are close enough, you.will...
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Robin Ford replied to Robert James
The question asked if it was a softening spring. By definition a softening spring becomes softer (less stiff) as the load <increases>, which can be determined without knowing the units or whether the stiffness was determined by a test where load was increased or decreased. Maybe it should show the origin.
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Robin Ford replied to Robert James
In engineering mechanics we have special meanings for weight and mass. Mass is the quantity of matter while weight is the force of gravity acting on the mass. In SI units, mass is in kg; force (including weight) is in Newtons. The weight of 1 kg (in Sydney) is 9.8 N. In imperial measure (US customary units) mass is in 'pounds mass'; force is in 'pounds force'...
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Robin Ford replied to Robert James
I'm not sure I've got the right question here. Which step number is it?
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Robin Ford replied to Robert James
Thank you for the feedback.
RT1.5 I think this is ok; weight is a force
RT1.6 This is a bit clunky. I took 'load' to be a force - weight in this case (the effect of gravity on the mass). Maybe I should have used 'force'. More on forces in week 2.
RT1.8 The schematic represents two springs connected together with forces at either end. The springs could be in... -
What an interesting insight.
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Robin Ford replied to John Manning
Are you one of the folks who have skipped ahead to see what the later weeks include? There's something on each of these topics.
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Glad to have you on board. I think you've about got it. I have assumed that from symmetry the weight of the frame and washing is divided between the two sides. There is a strut each side, so you don't divide that by two; I assumed the worst case for a child hanging on would be at a corner, so you don't divide that by two either
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I can't help with the quantum physics, but you have encouraged me to look up my dictionary. "moment" comes from a Latin word meaning "to move" - presumably in this case a tendency to move in a rotary sense. The same Latin word gives us "momentum", which is used in dynamics for the product of mass x velocity. I also looked up "torque". That too comes from a...
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Robin Ford replied to Ali Kamali
For the next run of the course we are working up some additional materials to help people who get stuck. Perhaps we can also add some items for those who would like more details - or would like to see the beauty in some of the maths for example. The rope around a bollard is a candidate for this; I love the derivation.
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Robin Ford replied to Shibana Abdulsattar
When you load a spring there comes a point where it is permanently stretched (it won't go back to the starting length when you take the load off), and if you keep loading it breaks of course.
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That's an interesting one. A cyclist I know wanted to find out the effect of bumpy roads on rolling resistance. There are general rules for the best tyre pressures for various road roughness, but I didn't really get very far with analysis! I did notice when pushing a stroller with a child in I that it was harder to push on a rough surface. There are tables for...
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I hadn't really thought about its origins before. A web search was intriguing. There were definitions in US and UK dictionaries, for example the Cambridge dictionary had 'produced quickly with little preparation' and 'simple but good enough', and Wikipedia has "'old rough and ready', a nickname for US president Zachary Taylor'.
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Robin Ford made a comment
The poem was "The Sons of Martha" by Rudyard Kipling (1907). It's a rather dark view of the unrecognised responsibility that some people undertake - like Martha who, in the biblical story, worked in the background to make sure that visitors were comfortable when listening to Jesus, but when she suggested that her sister Mary might help out a bit, got told off....
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Robin Ford replied to taylor lemons
It's a strange one. You are correct. It seems that in a previous correction (in the first run of the course) the answer for the x coord of the combined cg was updated but not the value for A*x. I'm assuming that the reason you didn't get exact values for the other data in the table is that I didn't include sufficient significant figures in the coords for the...
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Robin Ford replied to taylor lemons
Were you happy about this in the end?
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Interesting thought for next time.
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Things seem to be changing. A search of the internet showed me that this range is now possible in several battery cars.
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Some of the small differences in the answers come about because of differences in the calculation methods. In the original solution I used the raw figures throughout whereas in the padlet solutions I rounded the figures off earlier. Sorry if it confused you - I'll sort it out for next time. And, yes, the sum of the two vertical forces in the wall anchors...
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Robin Ford replied to Sandra Evans
Some aircraft structures are machined out of solid to avoid 'discontinuities'. And composite structures can achieve a similar result by carefully building up layers of fibre and matrix.
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Robin Ford replied to Kevin Dullaghan
It's inconsistent, certainly, and confusing so I'll arrange an update. But it's not exactly 'wrong' because which one is T1 and which is T2 is just convention and looked at this way I have merely changed my convention for the moment. But it is an unnecessary diversion. Thank you for pointing this out.
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Robin Ford made a comment
Last night I came across a folding washing line held to an old paling fence with 'U' bolts ( https://padlet.com/ThroughEngineersEyes/Week3/wish/129308548 ). I was glad to hear that it is about to be replaced.
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It worked fine for me. Looks an interesting set-up.
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Robin Ford replied to Rd Le
If the concrete is poured after the cables are stretched wouldn't that be pre-tensioned? If the cables are in tubes they could have been tensioned after wards - post tensioned. When making concrete railway sleepers (ties) the steel strands are pre tensioned and the concrete poured directly around them. After waiting for the concrete to cure the tension is...
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Robin Ford made a comment
I've just done the calculation again and built the model too. I got 0.84 grams at the point of the straw by calculation. As you've noted there is room for slight differences in calculation details. I built the model and noted that the straw I had was 210 mm (rather than 213) which when recalculated gave 0.87 grams which I weighed out using the balance from...
