Gold Coins

Logic Level 3

You have 10 identical bags containing 10 gold coins each. All of the coins weigh 10 grams, except for the coins in one of the bags which instead weigh 9 grams.

If you have a scale which tells you the total weight of whatever you place on it, then how many weighings are required to determine the bag whose coins are lighter?

Note: You can separate coins from their bag, and you can assume the bags have negligible weight.