Newbury Park High School - Physics

The Inertial Balance

The inertial balance, a simple device for measuring the inertial mass of different objects independent of gravity, is shown below. The balance measures mass by using it’s inertia … that is, it’s resistance to acceleration. The more mass on the balance, the slower it will swing.

Procedure:

  1. Clamp the balance to a table as shown. Try to clamp it in an out of the way place and not on the side of the room where people are always walking by.
  2. Measure the period of the empty balance. Do this by measuring the time for 10 swings, and dividing by 10 to get the time per swing. Since the period of the balance is very short, it is difficult to count the vibrations visually. Hold a small piece of paper near one of the steel strips and count the audible snaps made by the paper when the blade just touches it. Small swings (about an inch or so) work best.
  3. Add one of the thin rectangular plates to the balance, and repeat the measurements of step #2. Mass the plate on an electronic balance, record the mass in grams.
  4. Continue in this manner, adding strips to the balance until you have a total of 6 strips in the balance. Be sure to measure the actual mass of each strip with the electronic balance.
  5. Sketch a graph of mass on the horizontal axis, and period on the vertical axis. You graph should be roughly linear. Redo any data points obviously in error.
  6. Take an object of unknown mass, like a small clamp, and measure its period. Refer to your graph, and using the graph determine the mass that corresponds to that period.
  7. Measure the actual mass on the unknown using the electronic balance. Compare the two masses and calculate a percent difference.

Your Lab Report

(aka What to turn in)

This lab report will be in memo format. A memo format lab report is a report like that you would turn into your boss if you were working in an industry setting. Your report must be TYPED and in your own words.

What your Report needs:

Title: (given)

Purpose: (given)

Data: Listed in neat tables. Be sure to label all data, include units, and the proper number of significant figures. Data tables can either be done using the table button (or function) directly in Word. Or you can cut and paste your table from your Excel document.

Graph: You graph must be done on the computer. I recommend using Excel. Graph the independent variable on the x-axis (in this case, mass) and the dependent variable on the y-axis (in this case period or time). Then using a LINEAR Trendline, determine the equation for the best fit line for your graph. Graphs should be at least half a page. They can be cut and pasted directly into your lab report document. All graphs need a title and axis labeled with units. The trendline equation can be moved off to the side of your graph so that it is easier to read.

Calculation: Using your trendline and period for the unknown object, calculate the mass of your unknown and the percent error using the known value from the scale. There needs to be sufficient detail to allow the reader (me) to follow your work and logic. Feel free in include comments about what is being done mathematically.

Discussion – Conclusions: The meat of the lab report. The most common mistake on lab reports is not enough depth to the discussion.

A good conclusion includes:

  • The purpose of the lab.
  • A brief description of what was done.
  • What your results were (including percent error)
  • A few sources of possible error and how this error would affect your data. This error should be unavoidable error. Errors such as not being able to work a stopwatch correctly do not count.
  • Possible ways to fix the error(s).

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