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Inertial balance lab

What is an inertial balance?

Physics class doing labs

Physics class doing labs

“Inertial mass is measured with the use of an inertial balance, or spring-loaded pan. It is a dynamic measurement – that is, a measurement that can only be accurately recorded while the system is in a state of motion.

This method capitalizes on an object’s inertia, or its tendency to continue in its current state of motion, as a means of quantifying the amount of matter present.” (PhysicsLab Online)

We’re going to add masses to this pan, set it in motion, and then use a photogate to measure the resulting period.

The period is the time taken to repeat the pattern of motion once. This kind of motion is called an oscillation, or simple harmonic motion.

Here, the period (T) is the time it takes for the mass to go from the bottom, top, and then back to the bottom again.

Inertial Balance lab

This setup will be placed so that part of this oscillating pan will pass through a photogate, as shown below.

Inertial Balance lab 2.jpg

So what is a photogate?

An inertial balance is two metal pans connected by steel strips. We clamp one end to a table, and the other end is free to oscillate. As we add mass to the balance, it’s inertia increases, and so oscillates more slowly.

“By comparing the periods of the apparatus with known masses to the period of an unknown mass, the value of the unknown mass can be determined. This comparison can either be accomplished using a graphical program such as Excel or graphing the values by hand. … provides a definitive way to determine the inertial mass of an object directly, without relying on gravity.” (Arbor Science)

Pan, C-clamp, tabletop, and CPO timer.

Inertial balance

Inertial balance



* Learn how to find an object’s mass, without a need for gravity! (Scales only work if gravity pulls a mass downwards – so scales won’t work in zero-g situations.

Use data to produce a calibration curve and find the unknown mass of an object.

Learn how to make LoggerPro perform calculations for us

Learn how to precisely interpolate an answer from between data points


Inertial Balance, large C-clamp
Collection of known slotted masses
Masking Tape
CPO Timers, and CPO photo-gate, small C-clamps
Unknown Mass – get from teacher after completing data table


  • Record the mass of your inertial balance (150 g)

  • Clamp the inertial balance to the counter (not the table)

  • Add a mass to the pan (make sure that it is secured in place)

  • Tape cardstock to the outer edge of the pan. Set it up so that when it moves, the card will trigger the photo-gate as it passes through.

  • Set the CPO timer function to period.

  • Record the period of the pan oscillating.

    Don’t use the first reading – rather, take the 2 or 3rd . But take data before the oscillation slows down.

  • Now add more mass, set pan in motion. Record the new period and the mass in the table below.

  • Repeat adding masses until you have 8 data points.

    Use a wide range of mass ( 0 – 300 g) to get the best data.

  • Get the unknown mass and find it’s period.

  • Get the accepted value for the mass on the electronic balance

Be careful that the masses do not slide around in the pan as this causes damping.

Data Table:

Mass of balance = _______________ g
½ mass of balance = ______________ g

Mass, Period & Period2

Mass added to pan Total Mass (g)
in motion
Period   (seconds) Period2 (seconds2)
20 g 20+ ½ mass of inert bal  __  __
 __  __  __  __
 __  __  __  __
 __  __  __  __
 __  __  __  __
 __  __  __  __
 __  __  __  __

Unknown Mass : Data

Describe your unknown mass

Period (s)

Measured value of mass from electronic balance (g)


  1. Use Logger pro. In the X-axis plot [grams + 1/2 mass of balance]
    in the Y-axis plot Period.

  2. ‘Data’ -> ‘New calculated column’

  3. Name this new column “Period^2”, units “s^2”

  4. Under Equation, look for the “Variables (columns) >” box. Click, choose “Period^2”

    Choose “Period” then *,then “Period”

    Logger pro will then automatically create a new column for us.

  5. Click the Y-axis where it says ‘Period’; change it to ‘Period^2’.

  6. Autoscale button

  7. Use the “R=” button to produce a linear best fit.  Make sure that all data points are included in this regression analysis.

  8. Write down the equation for the lines

  9. Now we’ll use a process called interpolation to find the mass of the unknown object we measure: Under the Analyze tab, click Interpolate.

  10. Move the interpolation line until it’s period matches the period of our unknown: Now read off the corresponding mass!

  11. How did we do? Let’s compare our interpolated result with the actual mass measurement of the “unknown” mass, which we get from the digital scale.  calculate the percent error:

 Experimental error\

Questions : Use complete sentences

  1. Describe the motion of the pan. Will the pan ever be at rest? Explain.

  2. Would an inertial balance work in a zero gravity environment? Explain.

  1. Would a spring balance (like we used for the mass/weight lab) work in a zero gravity environment? Explain.

  1. Are mass and weight identical? Are mass and inertia equivalent? Discuss.

What is inertia? What is mass?
How is mass related to weight?

Inertia, mass and weight


Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework
HS-PS2-1. Analyze data to support the claim that Newton’s second law of motion is a
mathematical model describing change in motion (the acceleration) of objects when
acted on by a net force.

HS-PS2-10(MA). Use free-body force diagrams, algebraic expressions, and Newton’s laws of motion to predict changes to velocity and acceleration for an object moving in one dimension in various situations

A FRAMEWORK FOR K-12 SCIENCE EDUCATION: Practices, Crosscutting Concepts, and Core Ideas
How can one predict an object’s continued motion, changes in motion, or stability?

Interactions of an object with another object can be explained and predicted using the concept of forces, which can cause a change in motion of one or both of the interacting objects… At the macroscale, the motion of an object subject to forces is governed by Newton’s second law of motion… An understanding of the forces between objects is important for describing how their motions change, as well as for predicting stability or instability in systems at any scale.

Massachusetts Science and Technology/Engineering Curriculum Framework (2006)

1. Motion and Forces. Central Concept: Newton’s laws of motion and gravitation describe and predict the motion of most objects.

1.4 Interpret and apply Newton’s three laws of motion.
1.5 Use a free-body force diagram to show forces acting on a system consisting of a pair of
interacting objects. For a diagram with only co-linear forces, determine the net force acting on a system and between the objects.
1.6 Distinguish qualitatively between static and kinetic friction, and describe their effects on the motion of objects.
1.7 Describe Newton’s law of universal gravitation in terms of the attraction between two objects, their masses, and the distance between them.
1.8 Describe conceptually the forces involved in circular motion.

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