Rods, ropes and connected masses
Before you start
Why does pulling the right block accelerate the left one? Can a rope push like a rod?
The setupA is left of B on a frictionless horizontal rail, initially at rest. Only B receives the external force; right is positive. Choose a light rigid rod or an initially taut light rope.
What to do and noticeWhy does A move without a direct external pull? · Why do equal opposite orange forces not keep each block still? · Reverse the force: does A still follow?
Predict firstWhy does A move without a direct external pull?
Why does pulling the right block accelerate the left one? Can a rope push like a rod?
Prerequisites: A is left of B on a frictionless horizontal rail, initially at rest. Only B receives the external force; right is positive. Choose a light rigid rod or an initially taut light rope.First consider the pair
Predict firstWhy does A move without a direct external pull?
The taut rope transmits the pull to A. For the pair, internal forces cancel and F = (mA + mB)a. For A alone, T = mA a. Tension is not simply equal to F: B obeys F − T = mB a.
How to observe
- Why does A move without a direct external pull?
- Why do equal opposite orange forces not keep each block still?
- Reverse the force: does A still follow?
Model notes
Massless rod or inextensible rope, no friction. A slack rope transmits no force; playback stops before the blocks meet. Vertical weight and support balance. Rope sag illustrates slack rather than a solved catenary.