Equal Masses Collide
Lesson notes
Overview
Identical elastic masses exchange velocities completely — Newton’s cradle. This topic is rated beginner and is number 112 of 227 in Physics. Core ideas and first intuitions. No prior knowledge needed.
What you are seeing
The panel above shows an interactive animation of the idea. Every slider changes one quantity so you can see its effect on the outcome. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.
In depth
When two identical elastic masses collide head-on, they exchange velocities completely: v₁′ = v₂ and v₂′ = v₁. A moving ball stops dead and the stationary one leaves with the same speed. This is why Newton’s cradle works: the momentum and the energy pass along the line of balls and the last ball swings out. It follows directly from conservation of both momentum and kinetic energy.
Reading the relationship
The expression "v₁′ = v₂, v₂′ = v₁" is a compact summary of the idea. It uses the symbols v₁, v₂; name what each one stands for before you start. Then use the sliders to see how changing one quantity changes the others.
A guided exploration
1. Run the animation once with the default settings and describe in words what happens. 2. Change one slider at a time and note which quantity responds most. 3. Push each slider to its extreme and decide whether the result still makes sense.
Try this
Start by only moving one slider at a time. Say aloud what you expect, then check.
Common pitfalls
Check units and directions first. Many mistakes come from mixing up a scalar with a vector, or from using a formula outside the conditions where it holds.
How it connects
It builds on “Turning Around”, the topic before it in Physics. Once you are comfortable, continue with “Heavy Hits Light”.
Why it matters
Physics explains how the world moves, heats, shines and interacts, and the same principles power technology around you.
Where you might use it
- Revising “Equal Masses Collide” before a class, exam or certification
- Designing vehicles, bridges and devices where motion, forces or waves must be predicted
- Understanding how sensors, lasers and communication links actually work
- Explaining everyday phenomena such as sound, light, heat and orbits
Frequently asked questions
What is Equal Masses Collide?
Identical elastic masses exchange velocities completely — Newton’s cradle. It is a beginner topic in Physics.
How do I use the Equal Masses Collide simulation?
The panel at the top shows an interactive animation of the idea. Every slider changes one quantity so you can see its effect on the outcome. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression v₁′ = v₂, v₂′ = v₁ packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.
Who is this topic for?
Core ideas and first intuitions. No prior knowledge needed. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “Turning Around”, the previous topic in Physics. Each topic in a subject builds on the ones before it.
How do I track my progress?
Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.