This lesson is based on California's Middle School Integrated Model of NGSS.
Science and Engineering Practice 4 - Analyzing and Interpreting Data
I find it beneficial to teach the kids how to manipulate a triple-beam balance scale as a separate activity before they use it in a lab or activity. They will use a triple-beam scale to analyze and interpret data they generate in future labs (S&E Practice 4 - Analyze and interpret data). This data by itself has very little meaning, what's important is that the students start inspecting their data for relevance. By generating appropriate data students can learn to inspect their data for trends, relationships, and evidence.
I always ask my students to review their measurements, in this case mass, to see if the numbers make sense. Does it seem reasonable for a pencil to weigh 500g? This activity is designed to assist students understanding of appropriate measurements (high school students have touble approximating metric measurements). My hope is that by continually having them review their measurements for appropriateness I can start building this skill for later activities.
TIP: A large part of this problem is that students in the U.S. do not grow up using the metric system and are not familiar with common metric approximations. The only way I can see improvement is to have students experience the metric system with hands-on activities.
I have my triple beam balance scales set-up on a counter ready for the students before I begin the activity.
Before I allow my students access to triple-beam balance scales, I instruct the following:
1) Proper carrying method – The first skill my students need to have is how to properly carry a triple beam balance scale. I require all of my students to carry the scales with two hands, one hand under the stage base and the second hand supporting the balance pointer column.
2) Naming the parts – A triple beam balance scale is made up of a several parts.
The stage, often called the pan, is the round stainless steel disk that would hold the object you wish to weigh.
The base is the metal structure that supports the triple beam balance scale.
The beams are the three measurement scales used for determining weight.
The calibration knob, or counter weight, is located underneath the stage.
The weights are the three weights that are manipulated for determining weight.
The balance pointer needle is the delicate measurement needle that shows when the object is properly weighed.
The zero mark is the mark opposite to the balance pointer needle.
3) Calibration – Setting the scale to zero without an object on the scale is critical for achieving an accurate measurement.
Step 1 – Remove any item from the stage and seat all three weights to their zero position.
Step 2 – Allow the balance pointer needle to come to a rest.
Step 3 – Turn the calibration knob one whole turn, in either direction, and allow the balance point needle to come to a rest.
Step 4 – Note if the balance point needle came close to the zero mark. If came close continue turning the balance point needle I the same direction, if not reverse the turning direction of the calibration knob.
Step 5 – Make smaller adjustments until the balance point needle exactly reaches the zero mark.
4) Demonstrate weighing an object – At this point I show the kids how to weigh an object.
Step 1 – Move all weight to zero. The balance pointer needle and the zero mark should meet.
Step 2 - Place an object on the stage. If the scale is properly calibrated the balance pointer needle will rise above the zero mark.
Step 3 – Move the largest weight (papa bear weight) to the right, making sure the weights sit ONLY on the notches. Continue moving this weight to the right until the balance point needle falls below the zero mark. Move this weight exactly one notch back to the left.
Step 4 - Move the medium weight (mama bear weight) to the right, making sure the weights sit ONLY on the notches. Continue moving this weight to the right until the balance point needle falls below the zero mark. Move this weight exactly one notch back to the left.
Step 5 – Using the sharp end of a pencil, careful move the smallest weight (baby bear weight) to the right, until the balance point needle is exactly pointing at the zero mark.
Step 6 – Read the weight. The largest weight (papa bear) represents the hundreds place. The medium weight (mama bear) represents the tens place and the smallest weight (baby bear) represents the one and the tenths place. The largest (papa bear) and the medium (mama bear) weights MUST be sitting in a notch on the beams.
Time permitting, I reinforce the concepts I just taught with one of two web denonstrations. These might work very well in a computer lab where students have a chance to manipulate a triple-beam balance scale before actually touching one. I gauge the students understanding and opt to skip these websites if I feel the students are understanding the concepts. Use at your discression.
Ohaus Web Tutorial - reviews the necessary steps on how to use a triple-beam balance scale
Wisc-Online Triple Beam Balance Introduction - uses photorealistic images to teach the process of using a triple-beam balance scale.
I now allow one member from each group to pick-up a triple beam balance scale from the counter and bring it to a student table for practice. I ask each group to informally choose several objects (wallet, pencil, cell phone, etc.) and practice weighing each object.
I don’t provide any worksheets to record their measurements; I only want them to practice what they see informally. As they are making measurements I ask them to turn to their shoulder partners and state out-loud what their object weighs and to use the correct label of grams after their stated number. As a closing skill I have one member from each group return the scales to the counter where they found them. I tend to run this activity as long as possible and I typically wait until the last few moments of a class period to have the students finish up what they are doing. Real experience will come with time through the manipulation of triple-beam balance scales in other labs.