Lab 1: Measurement, Precision, and Accuracy

Physics, like other sciences, relies on observation and experiment. Physicists routinely take many different kinds of measurements—length, volume, mass, and temperature, among others. Doing this well depends on the ability to measure both accurately and precisely.

Suppose you use a ruler to measure a piece of string that is actually 30.48 cm (0.3048 m) long. Accuracy describes how close a measurement comes to the true, correct value. If you measure the string three separate times and each result lands very close to 30.48 cm, your measurements are accurate.

Precision describes something different: how closely a group of repeated measurements match each other—in other words, how spread out your results are. Precision takes into account both the range between the measurements and how consistently you get similar results. For example, if your repeated measurements of the string stay close together, without wide variation, they are precise—even if they aren't 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, a spring scale, and an electric balance;
• how to measure distance using rulers, meter sticks, and string.

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

Volume is the amount of space 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 masses (whose true volumes are already known) using a graduated cylinder. You will also learn the displacement method: placing an object into a container holding a known volume of water. Because the object takes up space, the water level rises when the object is fully submerged, and the amount it rises tells you the object's volume. This method is especially useful for irregularly shaped objects, where measuring length, width, and height directly would be difficult or impossible.

Significant figures are the digits in a measurement that carry real meaning: all the figures you can read directly off the measuring instrument, plus one final "doubtful" digit that you estimate yourself. To figure out how many significant figures an instrument can support, look at its resolution (the smallest unit it can reliably measure) to determine the known digits.

A few rules for counting significant figures:

• Nonzero digits (1–9) are always significant.
• Zeros between nonzero digits (acting as placeholders) are significant.
• Zeros at the end of a whole number, such as the zeros in 1000, are not significant—unless they appear to the right of a decimal point, as in 2.00, in which case they are significant.
• Zeros to the left of the first nonzero digit, as in a decimal like 0.003, are not significant.