Big objects have their mass spread out all over the place which makes it hard to do calculations. Things are much easier when all the mass is concentrated in one small point. Turns out that for a big object, it is possible to fake such a tiny point. It is called the CENTRE OF MASS and it makes the calculations work. The centre of mass is the pint at which the whole mass of an object SEEMS to be.
You are going to find the centre of mass of a weird shaped object.
1.
Find a bit of a cardboard box. (If you really can't find any cardboard anywhere in the house, try it with a piece of paper - preferably glue more than one piece of paper together to make it thicker).
Draw a shape onto the cardboard. Make it more complicated than a square, circle or triangle please! I choose to draw Great Britain but you don't have to be that complicated.
2
. Cut the shape out. Force a round pen through it and hang it off the edge of a table with a heavy book on the pen. Make the hole slightly bigger than the pen so that when you pull the shape to one side, it swings backwards and forwards a bit.
3. Find a piece of string or a shoelace or something stringy. (
If you REALLY can't find that, see point 4 below on what to do instead) Tie the string round a rubber or a heavy-ish object and hang it from the pen.
Hold the cardboard and the string still with one hand and then as best you can draw a line down the string.
4. If you really can't find anything stringy, let the shape swing freely and then hold a ruler as near to vertical as you can against the pen. Hold the ruler and the cardboard still with one hand and draw down the ruler as best you can with the other hand. This is not as good as the string method because you have to decide whether it is vertical. Gravity will pull the rubber on the string so the string really will be vertical.
5. Do this for at least 4 different hanging points on your object. The lines should roughly go through the same point.
That point is called the centre of mass.
Notice that if you hang an object, the centre of mass will always be on a line directly below the hanging point.
