Physics 11–12 · Year 11
Displacement, distance and the sign of velocity: a tumbling toy car and a motion sensor (syllabus practical)
Module 1: Kinematics (Motion in a Straight Line)
The idea
Velocity carries a direction, so a position-time graph slopes up while an object moves away from the origin and down while it returns, and a round trip has zero displacement but a non-zero distance.
What you need
- Motorised tumbling toy car that runs at constant speed, flips over on meeting an obstacle and drives back (the NSW Department of Education Module 1 guide activity)
- Ultrasonic motion sensor with data logger, or a phone video analysed in Tracker
- A wall or a heavy book as the obstacle, metre rule, masking tape to mark the start
How to do it
- Place the sensor at one end of the bench, pointing along it; mark a start line 0.40 m from the sensor and put the obstacle 1.40 m from the sensor.
- Start logging, release the car at the start line heading away from the sensor, and let it reach the obstacle, tumble and drive back past the start line.
- From the position-time graph read the gradient on the way out and on the way back, and the time spent tumbling.
- Compute the displacement and the distance for the trip from the start line back to the start line, then the average velocity and the average speed.
- Repeat three times and compare the out and back gradients.
What you should see
A car at 0.25 m/s sets off from the start line (0.40 m) and reaches the obstacle (1.40 m) after 4.0 s, tumbles for about 0.5 s and returns to the start line at t = 8.5 s. The position-time graph rises with gradient +0.25 m/s, flattens briefly, then falls with gradient -0.25 m/s; the velocity-time graph is a horizontal line above the axis, a short jump through zero and a horizontal line below it. Over the round trip the displacement is 0 m and the average velocity is 0 m/s, while the distance is 2.00 m and the average speed is 0.235 m/s. The learner knows it worked when the out and back gradients match in size and differ in sign.
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.
- A negative velocity means slowing down; here it means moving back towards the sensor at the same speed.
- Average speed and average velocity are always equal; over a round trip the average velocity is zero while the average speed is not.
Safety card
Hazards
- car running off the bench
Controls
- run on the floor or put a stop at the far end
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 2027PH11-8PH11/12-3PH11/12-4
- Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-11-01PY-11WS-03
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.
- www.nsw.gov.au/sites/default/files/noindex/2025-03/physics-stage-6-syllabus-2017.docx
- education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/physics/Physics-module-1-guide.docx
- physlets.org/tracker
- curriculum.nsw.edu.au/learning-areas/science/physics-11-12-2025/outcomes