Physics · Interactive learning companion

Centre of Mass Simulation: Man and Plank

Use a weighted average of positions to locate a composite centre of mass.

Landscape activity · use fullscreen on a phone for larger controls

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📝 Description

Centre of Mass Simulation: Man and Plank helps students understand centre of mass in a frictionless system. Learners observe how the man and plank move relative to each other while the system's centre of mass remains consistent. It supports mechanics learning and classroom demonstrations.

📋 Instructions

Click start and use the sliders to change values. Observe the motion of the man, plank, and centre of mass throughout the simulation.

🎓 Learning Guide: Centre of Mass Simulation: Man and Plank

Learning focus: Use a weighted average of positions to locate a composite centre of mass.

Physics Interactive learning companion

Learning objectives

Find the centre of mass of two components using masses and positions.
Distinguish centre-of-mass motion from internal relative motion in a system.

Concepts and principles

For masses mᵢ at xᵢ, x_CM = Σmᵢxᵢ/Σmᵢ.
With negligible external horizontal force, the horizontal centre of mass stays fixed even while members move relative to one another.

Examples and worked solutions

A 60 kg man is at x=2 m and 40 kg plank’s centre is at x=5 m. Find x_CM.: Worked solution: (60×2+40×5)/100 = 3.2 m.

Try this investigation

Identify the reference origin and the positions of the man and plank.
If masses or positions are adjustable, predict how the weighted mean changes before moving a control.
Check whether friction with the ground is included before claiming strict centre-of-mass conservation.

Expected observations and explanations

Increasing one component’s mass shifts the centre of mass toward that component’s position.

Assessment questions and answers

Can internal walking move the total centre of mass without external horizontal force?: Answer: No, not in the ideal isolated horizontal system.