Teach lesson
Archimedes' principle: apparent mass
Students use the Buoyancy remote lab to connect immersed scale readings, displaced liquid, and Archimedes' principle without pretending the gram reading is exact force.
Learning Outcomes
Interpret the lab's dynamic gram reading as apparent mass/load, not direct force in newtons.
Estimate the mass of displaced liquid from liquid density and object volume.
Compare expected and observed apparent readings for sinking objects.
Explain how the lab supports Archimedes' principle with appropriate uncertainty.
Student activity preview
Activity Content
Preview only. In a class session, students can fill in responses and submit their work to the teacher.
What should change when an object is immersed?
12 min
Why does an object seem easier to hold under water than in air? The classic story says Archimedes noticed that a liquid pushes back on an immersed object. Whether or not the bath story happened exactly that way, the physics question is real: part of the object's weight is balanced by an upward push from the liquid.
When an object is lowered into a liquid, the liquid pushes upward on it. The lab shows this with a changing gram reading. Treat that reading as an apparent mass/load reading, not as an exact force in newtons.
Apparent mass model
The diagram shows why the sensor reading becomes smaller while the object is immersed. Use the formula below for sinking objects that stay supported by the scale.
Apparent reading estimate
\text{expected reading} \approx m_{\text{object}}-\rho_{\text{liquid}}V_{\text{object}}
In this lesson, how should you treat the changing Mass: ... g display?
Before opening the lab, explain which values you will need to estimate an apparent reading for one sinking object. What comparison will you make after you lower the object?
Measure apparent readings
28 min
The important comparison is between the fixed object data and the dynamic reading after the object is lowered. Read the changing display after the motion has mostly settled.
The lab may show more object cards than this activity uses. For the core comparison, use the ball full of marbles, the ball full of glycerin, and one floating case.
Open the Buoyancy lab
Open the Buoyancy lab from this activity.
Choose the ball filled with marbles.
Before lowering the object, record its mass, volume, and liquid density.
Press the down-arrow button, wait until the motion has mostly settled, and record the final apparent mass/load reading.
Calculate displaced liquid mass as liquid density times object volume.
Estimate expected apparent reading as object mass minus displaced liquid mass.
Repeat for the ball filled with glycerin.
Add one floating object row as a limitation case. For that row, record mass, volume, liquid density, observed apparent reading, and a note. Leave the displaced liquid mass and expected apparent reading cells blank.
Apparent mass evidence table
Complete at least three rows. Use grams and cubic centimeters. The observed apparent reading is the dynamic Mass: ... g display after the object is lowered. Calculate displaced liquid mass and expected apparent reading for sinking rows. For a floating row, focus on the observed reading and interpretation note; leave the calculation cells blank. You may leave unused starter rows blank.
| Object | Object mass g | Volume cm3 | Liquid density g/cm3 | Displaced liquid mass g | Expected apparent reading g | Observed apparent reading g | Interpretation note |
|---|---|---|---|---|---|---|---|
For one sinking object, enter the expected apparent reading in grams. Explain how you calculated displaced liquid mass, subtracted it from object mass, and compared your expected reading with the observed reading from your table.
Connect to Archimedes' principle
20 min
Archimedes' principle says the buoyant effect depends on the liquid displaced. In this lab, the cleanest evidence comes from sinking objects that remain supported by the scale.
Compare your expected and observed readings for two sinking objects. Are they close enough to support the model? Use numbers from your table.
Why does a floating object make the apparent-reading model trickier? Use one floating object from your table.
Write a final claim: how does the lab support Archimedes' principle? Include one calculation, one observed reading, and one limitation of using gram readings instead of force measurements.