Introduction: Balance Bird
Balance and centre or gravity (COG), go hand in hand.
The centre of gravity is the point that an object will balance if supported at that point.
However, the centre of gravity is not specifically fixed at a point as it depends upon the distribution of weight about the object in question.
Taking a Seesaw as an example.
It balances at a point equidistant from each end unless the weight on each end is different.
But if we could move the balance point with the dissimilar weighted ends we could yet again balance the see saw.
Real world Seesaw's don't allow you to move the balance point which is why the lighter of two friend tends to be stranded in mid air requiring additional friends to try and add more weight at the lighter end or requiring the heavier individual to move closer to the centre in an attempt to regain balance.
This is the same principle of a balance scale were an unknown weight is placed on one side and a variety of known weights are placed on the other side until balance is achieved.
Using a STEAM approach and very few materials you can get an insight into COG in a creative format with the Balance Bird.
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Calculating Centre of Gravity.
COG = (w1d1+w2d2+w3d3)/(w1+w2+w3)
w1 = weight of see saw, d1 = balance point from reference end.
w2 - weight of individual2, d2 = distance from reference end.
w3 = weight of individual2, d3 = distance from reference end.
reference end = point were distance measurements are referenced.
A5 sheet of card
White/Blue Tack, Playdoh, Plasticine or similar.
Scissors or Modelling Knife
Pencil, Pen or Marker.
Step 1: Creation
Paper may not appear very technical but is the product of technical processes.
I created the basic artistic rendition of a bird form (coloured blue), and printed it out.
A PDF version of the bird form is included which you can print out to use as a template.
This was used as a template by placing it over a piece of card, taping one edge to stop it moving and then using a modelling knife* to cut around the template and through to the card beneath.
*Scissors may also be used as an alternative for cutting.
Card ls a much more suitable medium as we will need a little rigidity in the form for it to function as required.
The centre of gravity of the bird is ~37mm from the beak & the distance from the beak to the wing tips is ~27mm.
Width 13cm, length 11cm.
Therefore, to balance the bird on the beak requires moving the COG forward and similar to a Seesaw with individuals of dissimilar weight we have to add, remove or reposition the weight.
As by design we will be moving the COG to the beak we will need to add weight to the wing tips as the distance from the beak to the wing tip is fixed.
Carefully cut out the bird from the card with a suitable cutting implement.
Step 2: Art
Having cut out the form in the card.
Now is it opportunity to add some customisation, shading, colour or any other highlights.
Step 3: Balance
Using some tack, create two pea size balls, each weighing ~1g.
It only requires a small weight to balance the bird.
One ball is then attached to the outer edge of each wing approximately 1cm from the wing tip.
The aim is to be able to balance the bird by its beak while the bird balances almost horizontally,
Too much weight and the tail will point up as in a dive, either reduce the size of the balls or move then a little further back along the wing.
Too little weight and the tail will point down, either increase the size of the balls or move them closer to the wing tip.
The two pea sized balls do not have to be exactly the same weight, some variation can be tolerated which will be evident in a tilt. This will only really be an issue if the bird topples over at the slightest touch.
Similar adjustments will be required based on the rigidity of the card.
The physical COG was also verified to illustrate that you can balance the bird without the weights and identify this measurement for further analysis. However, without the weights its much more unstable.
Step 4: Some Maths
Finally just some calculations to verify the previous empirical results.
Where do you place the 2 * 1g weights on the wing tip to balance the bird.
Length of bird (lob) = 11cm.
cog = 6.4cm from wing tip
cog to Balance point cog2bp = 3.7cm
Balance point (bp) = cog2bp - cog = 2.7cm
cog = (w1d1+w2d2)/(w1+w2)
w1 = weight of bird referenced to the weight of an A4 sheet 0.279 * 0.21 = 0.0624m2 * 80g/m2 = 4.99g/4 = 1.25g
Divide by 4 is due to the bird being ~1/4 the size of an A4 sheet & therefore estimated at ~1g.
d1 = cog
w2 = sum of weights = (1g * 2) = 2g
d2 = (((bp * (w1 + w2)) - (w1*d1)))/w2 = (((2.7*(1+2))-(1*6.6))/2 = 0.85cm, estimated at 1cm.
Step 5: Further Activities
If you found this informative and useful and want something a little more robust than I have also created a 3D printed version of the bird form.
It's a heavier than the paper version weighing in at 15g and therefore requires heavier weights to balance it .
The BlocksCAD file is included within this Instructable whilst the OBJ file can be found at the following link.
Step 6: Finally
Hope you found it informative.
Now 1, 2, 3 make.
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