Physics 11–12 · Year 11

Relative motion: two toy cars on video, a boat crossing a river and an aeroplane in a crosswind

Module 1: Kinematics (Motion on a Plane)

Practical, model not builtLow risk

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The idea

Velocities add as vectors, so the motion of an object relative to the ground is the vector sum of its velocity relative to the medium and the medium's velocity.

What you need

Two motorised toy cars that run at different constant speeds, 2 m of floor or bench marked every 10 cm
Phone video at 30 or 60 frames per second and Tracker video analysis (the NSW Department of Education Module 1 guide points to Tracker's 'Relative motion of two carts' video in its moving reference frames library)
Metre rule for the video scale

How to do it

Film the two cars running side by side in the same direction, then towards each other, with the metre rule in view for scale.
In Tracker, measure each car's velocity in the ground frame from the gradient of its position-time graph.
Compute the velocity of car A relative to car B as v_A - v_B for both runs, then switch Tracker's reference frame to car B and read the relative velocity directly.
Simulation: set the boat's speed relative to the water and the river's speed; aim the boat straight across and read the ground velocity, the downstream drift and the crossing time.
Steer the boat upstream at the angle that cancels the drift and confirm that the crossing time increases; repeat in aeroplane mode with a crosswind and compare the heading needed to hold a track.

What you should see

Cars at 0.30 m/s and 0.20 m/s in the same direction: car A moves at 0.10 m/s relative to car B; running towards each other, the relative speed is 0.50 m/s, and in car B's frame the ground itself is seen moving backwards at 0.20 m/s. With boat speed 3.0 m/s across a 100 m river flowing at 2.0 m/s, the ground speed is 3.61 m/s directed 33.7 degrees downstream of straight across, the crossing takes 33.3 s and the boat lands 66.7 m downstream. Heading upstream at 41.8 degrees to the perpendicular cancels the drift and the crossing takes 44.7 s.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Aiming straight across gives the shortest crossing time and no drift; it does give the shortest time, but the current still carries the boat 66.7 m downstream.
Speeds along different directions can be added as numbers; only components along the same line add arithmetically.

Safety card

Low riskLearners carry it out

Hazards

cars running off the bench

Controls

film on the floor or put a stop at each end of the bench

Note

Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH11-8 PH11/12-6
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-11-01 PY-11WS-06
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities