Track connected motion
Observe how the rope constrains the two objects’ acceleration magnitudes.
AP Physics 1 · Unit 2, Part 1 · Forces
Atwood Machines and Two-Body Systems
Connect two masses, compare their shared motion, and choose object or system equations to predict tension and acceleration.
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Learn by doing
Three ideas you’ll put into motion
Choose a system
Compare separate force diagrams with a combined-system model.
Change the mass balance
Predict how changing the connected masses affects acceleration, then check the motion.
The big idea
The key ideas, at a glance.
One rope links two motions
For an ideal taut, inextensible rope over a fixed pulley, the connected masses share an acceleration magnitude. Choose each direction consistently.
System boundaries simplify forces
Tension appears in each object’s equation. For the two-mass system, internal tension cancels when combining the equations appropriately.
A quick comparison
In an ideal hanging-mass system, the heavier side accelerates downward.
The other side accelerates upward with the same magnitude.
Teaching this topic?Classroom fit, AP connections, and support
- Classroom fit
- Use for a concept introduction, guided independent practice, or a projected class discussion.
- Before students begin
- Force diagrams, Newton’s second law, and algebra with two connected objects.
- Built-in support
- Prepared scenes, guided prompts, and feedback on lesson questions.
- AP Physics 1 connections
- Unit 2: connected systems, external forces, and Newton’s laws.
- Check for understanding
- Ask why tension in an accelerating system need not equal either object’s weight.
- Optional extension
- Explore the separate Atwood Machine lab.
Ready to test your thinking?
The first scene is set up. Start the lesson and explore.