Lab 1: Measurement, Precision, and Accuracy

Physics, like other fields of science, relies on careful observation and experiment. Scientists in physics routinely measure many different things, including length, volume, mass, and temperature. Making these measurements well depends on the ability to be both accurate and precise.

Suppose you use a ruler to measure the length of a piece of string that is actually 30.48 cm (or 0.3048 m) long. Accuracy refers to how close a measurement comes to the true, correct value. If you measure the string three different times and all three measurements come out very close to 30.48 cm, your measurements are accurate.

Precision is a little different. It refers to how much a group of repeated measurements agree with each other — in other words, how tightly grouped, or "spread out," your results are. Precision looks at the range between different measurements and how consistently you get similar results. For example, if your repeated measurements of the string don't vary much from one another, they are precise — even if they aren't all perfectly accurate.

In this lab you will learn:

• how to measure volume using the displacement method;
• how to measure mass using a triple beam balance, spring scale, and electric balance;
• how to measure distance using rulers, meter sticks, and string.

Activity 1: Measuring Volume (TEKS 2H; 2J)

Volume is the amount of space that a substance or object takes up, or the amount of space enclosed inside a container. In this activity, you will measure the volumes of three objects with known masses, using a graduated cylinder (a container marked with measurement lines).

You will also learn the displacement method, a way to measure volume by placing an object into a container holding a known amount of water. When the object is fully placed underwater, the water level rises. That change in water level tells you the volume of the object. This method is especially useful for measuring the volume of oddly shaped objects, since it can be hard to measure their dimensions directly with a ruler.

When recording measurements, scientists use significant figures — the digits in a measurement that are known for certain, plus one final "doubtful" digit that is estimated by the person taking the measurement. To figure out how many significant figures a measuring tool can give you, look at how finely that tool is marked, since this tells you how many digits you can know for sure.

Here are some rules for counting significant figures:

• Nonzero digits (1–9) are always significant.
• Zeros can act as placeholders. They are significant if they appear between two nonzero digits.
• Zeros at the end of a whole number, such as in 1000, are usually not significant — unless they appear after a decimal point, as in 2.00, in which case they are significant.
• Zeros at the beginning of a number, such as in a decimal like 0.0025, are not significant.