Posts Tagged ‘newtons laws’

Bowl of SoupInertia to an engineer it can be fundamental to what they are doing and can be troublesome if there is too much or indeed too little, to an organisation it can be the difference between never getting going or becoming an unstoppable machine, but what is this thing we call inertia.

I was once in a restauraunt when one of the people I was with had a bowl of soup, the soup bowl was also on a plate.  They rotated the plate intending to rotate the soup bowl but the only thing that rotated with any significance was the plate, the bowl barely moved.  They exclaimed ‘Why?’, my response ‘inertia’.  Now this is a simple and trivial example but it does offer quite a nice introduction to the subject.

If we are first take a look at what we have and what would be required for the bowl to rotate. We start with the plate a rest and we know from Newton’s first law of motion that basically states that a body, or a plate, will remain at a constant velocity, and we need to remember that stationary is a constant velocity of zero, unless an external force is applied. We have a plate that is stationary, has zero velocity, and has a rotational force imparted by the person’s hand causing it to change its velocity. We have one small problem with this in that we haven’t defined a mechanism of transferring the force available from the person’s hand to the plate but clearly there is one and it is one that were are all familiar with and that mechanism is known as friction.  Whilst we might not all realise it we are all familiar with friction, it’s the force that stops us falling over when we walk along a pavement and often in the winter the presence of ice often reduces the friction, when that happens we can often end up in the horizontal rather than the vertical.  Now that we can associate the word friction with what we know happens in our everyday lives we can be confident that we can generate enough friction between our fingers and a plate to impart a force large enough to rotate it so we can leave that part and from now on just be happy that the hand rotated the plate.  Having said that it is worth mentioning that there is also a frictional force between the plate and the table but that it is sufficiently small that it doesn’t prevent our rotation of the plate so we won’t dwell on it.

So we have a rotating plate and on that plate is a bowl and there was clearly an expectation that the bowl would rotate, so that must be part of our everyday experience also, but I think it is worth just a little thought of how that may work.  In the same way that we needed friction between our hand and the plate to impart a force we also need a frictional force between the plate and the bowl otherwise there isn’t a force to cause the bowl to move and we know that we need a force to change its velocity. Now you might think that we have the answer as to why the bowl didn’t rotate, there wasn’t enough friction, but we haven’t mentioned the word inertia once so why was my answer inertia?

Well inertia really is the answer as to why the level of friction available wasn’t enough.  Now I didn’t tell you this earlier but if the plate had been rotated slowly and the speed gradually increased the bowl would also have started to rotate.  Did the level of friction change?  Well I can answer that with a resounding no, the force available from friction is determined by the force pushing the two surfaces together, the weight of the soup and bowl, and the nature of the surfaces, which don’t change regardless of the speed with which the plate is initially rotated, within limits I might add but within the bounds of what would be reasonable for a person there is no change. So if the friction didn’t change why would the bowl rotate if the rotation started slowly and the answer is inertia.

So inertia will allow something to be rotated, or moved, if it is done gently but not if it is done more vigorously? Well sort of, inertia is basically a body’s resistance to a change in its velocity, so something with a high inertia doesn’t want to change velocity, or speed if you like, so you have to push it harder to make it do it, something with a low inertia is quite happy to change and doesn’t require much force.  So why will our bowl, that seemingly has a high inertia because it stays still when the plate it rotated, change its speed if rotated slowly?  Well it was a bit of a deception on my part and in the human mind, the bowl will actually start to rotate at the same rate regardless of whether we start to spin the plate fast or slow, but our expectations are different.  When we spin the plate fast we expect the bowl to stay in sync with the plate which it doesn’t, but when we rotate the plate slowly we have the same expectation and this time it is met, the bowl stays in sync with the plate.

Now you might now be thinking OK but as the title states why should we care, because let’s face it, it doesn’t really matter whether your bowl of soup stays in sync with your plate if you so happen to spin your plate.  Well for me there are two reasons, one is all to do with motors and one is to do with expectations with regard to people.

I’ll tackle the motors question first. At Cornfield we are often required to design a system that can move something arround and move it in a very specific way.  To cause something to move on an item of industrial machinery, which after all is what we are working with, requires the impartation of a force, remember Newton’s first law, to cause a… you get the gist.  Now for industrial machinery there are two main ways of imparting a force, one is a pneumatic actuator where we use compressed air acting on an area to provide a force, such as a pneumatic cylinder, or an electric motor where we use electricity to create a set of magnetic fields that are setup to repel each other causing a shaft to rotate.  Now to create rapid, controller precise movements motors are good and a particular class of motor, along with its associated controller, is very good and these are called servo motors.

As you have probably guessed these motors come in various sizes from little ones that are about as big as a mug up to large ones that are as big as your dustbin. It is important to select the correct one, too small and you won’t be able to do what you want, you either don’t have enough force to move the piece of mechanics you want to move or you don’t have enough force to control the piece of mechanics, either situation is not good.  However, select one too big and you will pay extra for the motor and the controller, and these things aren’t cheap, you will have to fit more physical bulk into your machine, which is often more difficult than you might think, and you will use more electricity than necessary giving higher energy bills and contributing to climate change over the life of the machine. Cornfield are a systems partner for a company called Lenze who manufacture this type of kit and they have a principle that they call ‘Right Sizing’ which in essence means selecting the components that are just big enough for the task, not too big, not too small but the right size.

Now the second reason why we should care is to do with people in organisations, for me that is Hope Community Church, organisations have inertia and it is a product of the people within the organisation.  An organisation that has a large inertia requires a larger effort to cause a change in direction than one with a smaller inertia, so when contemplating change for an organisation it is important to understand its inertia characteristic, and use the appropriate level of force over the appropriate level of time.  Just like with mechanical systems if you apply too greater force, because of a desire to change direction quickly, you can break the very thing you are trying to move.  Also if you apply a force that is small enough not to break the organisation but don’t apply it for long enough you will not effect a sufficient change.  The principle of right sizing is useful to organisations as much as mechanical systems, apply the right force for the right amount to change for the best outcome.

The trick is to come up with the right levels of force and time, when dealing with a mechanical system there are nice formulas that can be used to nail down some of the variables.  With an organisation this is more difficult, but at Hope we have God that is big enough to work this our, our job is to trust His judgement and act upon His instruction, which we gain through reading His word, the Bible, and praying, both of which align us to His will for our lives and His church.


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