Measure Directly

Measuring a mass directly is just like stepping on a scale to determine your

weight. This approach is convenient to use when some uncertainty is acceptable

for the measurement and when it is easy to separate the material you are

massing from its container. The basic procedure for massing directly is as

follows:

1. Place something on the balance (weighing paper or a beaker).

2. Tare the balance so it reads zero when the weighing paper or beaker is on the

balance or record the mass of the weighing paper/beaker.

3. Place the material to be massed on the weighing paper or beaker and record

the mass.

Mass by Difference

Measuring a mass by difference is how I would determine the weight of babies in

my house. I would step on the scale while holding the baby to determine my

weight and that of the baby. Then I would step on the scale again without the

baby. The difference between the two weights is the weight of the baby.

Measuring by difference always infers a measurement based on two other

measurements. It is used when you want an accurate mass and when it is

difficult to separate an object at the end of an experiment.

As an example, say you wanted to measure how many grams of peanut butter

are left in a jar. You could try to scrape all the peanut butter out and measure the

mass directly, but it would be much easier to determine the mass by difference.

If you know the mass of the empty peanut butter jar, then all you need to

determine the weight of the peanut butter is the mass of the jar with the peanut

butter in it. Then you can take the difference between the masses to determine

the mass of peanut butter remaining.

When to Tare?

When taking initial masses of objects, it is often beneficial to think through to the

end of the experiment to decide if you will be taking a mass by difference at the

end or determining a mass directly. This dictates if you should tare the balance

with your container on it or include the mass of the container. For example, if you

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