weighing scale error

[bookend]

I was watching Mythbusters on TV earlier and part of the program involved weighing what was essentially an archery arrow between 1-2 feet long.

The presenters weighed the arrow by placing it on a pocket mini scale similar to the one shown here:

http://www.everyonedoesit.co.uk/online_headsho p/Tanita_Digital_Mini_Scale__1479V.cfm?iProduc tID=57

Now I seem to remember being shown at school many years ago that you can't accurately weigh a long object in this way. I'll try to explain what I saw at school.

In the first part of a school experiment, a metal tube about a yard long was mounted on a small rectangular plate with a short rod in the centre - the whole thing looked like an upside down "T" - and the whole assembly was placed on the scale. The balance then provided us with weight X.

The second part of the experiment involved placing the same tube horizontally on the balance along with the rectangular plate assembly (in order to cancel out the weight of the plate from the first part of the experiment). This gave us a totally different weight, Y. Weight Y was less than weight X.

Now as I understand it, the difference in weight had something to do with the amount of overhang of metal when the tube was placed horizontally on the scale. In other words, part of the tube was not really being weighed in the horizontal position.

Is there a name for this phenomenon or a way of calculating or taking it into account beforehand?

[Teddio]

Are you thinking of the principle of moments? The moment of a force = force X perpendicular distance from the line of action of the force. If the clockwise and anticlockwise moments about an object (or an arrow) are equal, then that object will be in equilibrium.
If the centre of mass of a long object is above the balance pan then it will not fall off and an accurate measure of mass (or weight) can be made.