Forces - Centre of Mass

Balancing an object is all about the centre of mass. Objects with a wide base and low center of mass are more stable than those with a narrow base and high center of mass. When you are on a bus, train, trolley car etc and have to stand up, spreading your legs a little wider than normal helps to make you more stable.

Formula one race cars can go round corners much faster than a family sedan because they have a wide base and a very low center of gravity.

The term 'centre of mass' represents a unique point in an object or system through which the entire mass of the object acts. Assuming the mass of an object acts at a point allows us to calculate how objects will respond to different forces. It is sometimes referred to as the center of gravity. Finding the center of mass for a regularly shaped object with a uniform density is easy, it can be done by drawing.

If you have an irregular shaped or non-symmetrical object, you need to do that by experiment. You will need to drill a hole into the object and hang it up. Then, you hang a plumb line (that's a piece of string with a mass attached to the end) from the same support point and carefully draw a straight line down the object. Next, you repeat the experiment but by drilling a hole on a different place in the object. Where the two lines cross is the center of mass. You can repeat it even more times to improve the accuracy.

But why does that work? When an object is suspended from a point around which it can freely turn, the center of mass will be directly below the suspension point. This is illustrated by a pendulum. If a pendulum is pushed or pulled sideways, the center of mass is no longer directly beneath the suspension point. The pendulum is out of balance and therefore 'falls' back towards the point at which it is balanced. It will then swing backwards and forwards, until air resistance and friction bring it to a halt, with the center of mass directly below the suspension point. The point of balance is actually the point of the lowest gravitational potential energy and represents the greatest stability. Interestingly, the time taken by a pendulum to make one swing does not depend on the mass of the pendulum, but its length. In senior high school, you need to know the formula that relates the time taken for a pendulum to complete one full swing (the time period) is related to the number of swings it makes in one second (the frequency).