TEACHER NOTE: In this procedure, students should develop their own approach to the weighing technique that was observed in this SAF segment. Your role as instructor is to facilitate this process as the students engage higher thinking skills to attain this goal. Guide them, but do not tell them how this is accomplished. The desired outcome should be a volumetric weighing technique that is based upon differences in the observed weight of both the "weigher" and the item being weighed.

PROCEDURE

4. Make a prediction. Suppose you were to stand on the scale holding the bag of rice. How much weight should the scale read?
(Answers will vary be this should be the sum of the student's weight and the weight of the rice bag.)

6. Weigh yourself with the 5-pound bag. Was your prediction correct? Why or why not? Explain any differences.
(Scale may not retain its accuracy over a range in weight measurements).

9. Apply the strategy you developed in step 7, to measure out 0.5 liters from this 2-liter container. By what percentage must the container's original weight be decreased in order to remove 0.5 liters?
(About 25%).

Questions

1. Why did you need to repeat the weighing measurements?
   (In order to assure repeatability, the weighing was repeated. This repeatability was a measurement of the precision of the weighing device. The final average was a more reliable number than any of the individual values.)
2. Would the weight of the container in which the rice or water is stored affect the accuracy or precision of the measurement? Explain.
   (The weight of the container will offset the recorded value by a small, fixed value. This affects the accuracy of the measures. If we assume that the container's weight does not interfere with the scale's mechanism, we should observe the same repeatability of measure (precision) in all readings.)
3. Make a prediction. How would using molasses instead of water affect the weight of the measured and removed liquid?
   (Since molasses is a thick, sugar-rich solution, the observed weight measurements will be greater.)

EXTENSIONS

High Heel Pressure
In most schools, spiked high-heeled shoes are not allowed on a wooden gym floor. The reason for this ban has to do with science - not sociology! When you stand on a mostly flat sole, your body weight is distributed over the entire area of the contact surface. So if you weighed 150 pounds and you stood on one foot that contacted 30 square inches of surface, you'd produce a pressure of about 5 pounds/square inch. Suppose you only made contact with the spike of a high-heeled shoe. How would this effect pressure? Explain. (The heel's smaller contact area will assume the entire weight. Therefore, this single but small area contact point would produce a large increase in pressure - often strong enough to produce "dings" in the wooden floor surface.)