Teach lesson
Conservation of Momentum: collisions with real carts
Students use the University of Fort Hare Conservation of Momentum remote lab to compare elastic and inelastic cart collisions, calculate velocity ratios, test model assumptions, and explain real-data limitations.
Learning Outcomes
Use a real remote two-cart lab to collect before-and-after velocity evidence.
Distinguish elastic and inelastic collision outcomes.
Use equal-mass reasoning to test momentum predictions without inventing missing mass values.
Calculate and interpret velocity ratios from repeated trials.
Explain why kinetic energy is not conserved in an inelastic collision.
State a model limitation when real data do not match an ideal number exactly.
Student activity preview
Activity Content
Preview only. In a class session, students can fill in responses and submit their work to the teacher.
Predict what should be conserved
7 min
In this lab, cart 1 moves toward cart 2. The carts collide and the lab reports velocities before and after the collision. Your task is to decide what the data say about momentum, energy, and the limits of an ideal model.
For the required route, use equal extra masses first: 0 g on cart 1 and 0 g on cart 2. That lets you compare velocity ratios without needing the hidden base mass of each cart.
Collision modes in the real lab
Elastic collisions separate after impact. Inelastic collisions stick together and move as one pair after impact.
Momentum model
p = mv
\qquad
\sum p_\text{before} \approx \sum p_\text{after}
Before opening the lab, predict what should happen in equal-mass 0 g / 0 g collisions. Include one prediction for the elastic collision, one prediction for the inelastic collision, and one reason real data may not match the ideal model exactly.
Which setting should you choose for the equal-mass elastic comparison?
Use the remote lab deliberately
8 min
The lab lets you choose collision type and extra mass on each cart. For each setting, run three repetitions if available and read the processed velocity table in m/s. The table reports four velocities per run: cart 1 and cart 2, each before and after the collision.
Data-collection workflow
Keep the same collision type and mass setting for a set of repetitions. Do not mix elastic and inelastic rows when calculating a mean.
Open the Conservation of Momentum lab
Open the Conservation of Momentum lab from TEACH.
Set collision type to
elastic, cart 1 extra mass to 0 g, and cart 2 extra mass to 0 g.Run three repetitions if the lab offers repeated runs. Record the velocity table values.
Change collision type to
inelasticand keep 0 g / 0 g. Run three repetitions and record the values.If time permits, keep inelastic collisions and compare 0 g / 0 g with 0 g / 100 g or 0 g / 150 g.
Use velocities in m/s. If a table cell is blank, record it as 0 only if the lab clearly shows the cart was stationary; otherwise write a note.
Which data-collection choice makes your mean comparison strongest?
Collect before-and-after velocity evidence
18 min
Record at least six rows: three elastic 0 g / 0 g rows and three inelastic 0 g / 0 g rows. Add two more mass-effect rows if you have time.
Collision velocity evidence table
Use one row per repetition. For the required route, collect three elastic 0 g / 0 g rows and three inelastic 0 g / 0 g rows. Optional rows may use 0 g / 100 g or 0 g / 150 g. The 'speed ratio r' column is the inelastic common-speed ratio defined in the analysis phase; leave it blank for elastic rows.
| Collision type | Cart 1 extra mass g | Cart 2 extra mass g | Repetition | Cart 1 before m/s | Cart 1 after m/s | Cart 2 before m/s | Cart 2 after m/s | Useful speed ratio r | Observation note |
|---|---|---|---|---|---|---|---|---|---|
Check your table. Which rows form your best equal-mass elastic comparison, and which rows form your best equal-mass inelastic comparison? Mention collision type, masses, and repetition numbers.
Analyze equal-mass collisions
10 min
Use your table first. Choose one controlled elastic 0 g / 0 g comparison and one controlled inelastic 0 g / 0 g comparison. Keep the repetition numbers visible in your notes so your analysis is tied to the rows you actually recorded.
Using your elastic 0 g / 0 g rows, explain whether the carts approximately exchange velocity. Use at least two numerical velocity values.
For your inelastic 0 g / 0 g rows, calculate the common-speed ratio:
r = (shared speed after collision) / (cart 1 speed before collision)
Enter your best mean ratio and explain how it compares with the ideal equal-mass prediction of 0.5. That 0.5 comes from momentum conservation: when two equal-mass carts stick together, the combined mass is twice as large, so it must move at about half the speed to keep the total momentum the same.
Does the inelastic collision conserve kinetic energy? Use your ratio to support the answer. Hint: for equal masses that stick together, the kinetic-energy fraction after/before can be estimated as 2r^2 (two times the square of r). Any kinetic energy that disappears is transformed into heat, sound, and deformation, not destroyed; momentum is still conserved even when kinetic energy is not.
Why does this lesson not require an absolute total-momentum calculation using p = mv for every row?
Test mass effects
7 min
Now test how extra mass changes the pattern. Keep the collision type inelastic. Compare 0 g / 0 g with 0 g / 100 g or 0 g / 150 g. You are not calculating absolute momentum; you are asking whether a heavier target pair leaves with a smaller shared speed.
Use your own rows for this comparison. A good pair of rows keeps the collision type and incoming cart setting the same, then changes only the extra mass on the target cart.
Compare one inelastic 0 g / 0 g row or mean with one inelastic row where cart 2 has extra mass. What happens to the shared speed after collision? Explain using momentum ideas without inventing a base cart mass.
Handle real-data uncertainty
5 min
Real collisions are not textbook drawings. Friction, timing, sensors, impact details, and table reading can all move a value away from the ideal. The point is to explain the difference clearly.
Evidence table, graph, or annotated screenshot reference
Attach or describe one evidence artifact: a small velocity table, a graph you made from your table, or an annotated screenshot reference of the lab's processed velocity table. For screenshots, use an image reference or link; for file upload, use a table, report, or slide file. The artifact must support your final claim; it cannot be only an unlabeled screenshot.
Name two sources of uncertainty or non-ideal behavior in this lab and explain how each one could affect a velocity ratio or conservation claim.
Make the scientific claim
5 min
Write a conclusion that a teacher could grade from evidence. Use claim, evidence, reasoning, and limitation.
What does the Conservation of Momentum lab show about elastic and inelastic collisions? Include a claim, at least three pieces of numerical evidence from your table, reasoning using momentum and kinetic energy, and one limitation of the lab evidence.
Optional extension: absolute momentum with teacher-supplied mass
15 min
If your teacher gives the base cart mass from a verified source, add the extra mass to each cart and calculate total momentum before and after.
Optional absolute momentum
p_\text{total}=m_1v_1+m_2v_2
Using a teacher-supplied base cart mass, calculate total momentum before and after for one row. State whether the comparison is within a reasonable real-data tolerance.