Conservation of Momentum
In an isolated system, total momentum stays constant.
About this topic
In a closed system with no external forces, total momentum before an interaction equals total momentum after. In a collision, m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂. Momentum is a vector, so direction matters.
It explains rocket propulsion, recoil of a gun, a skater pushing off a wall and billiard-ball collisions. It follows from Newton's third law: the forces between two objects are equal and opposite and act for the same time, so the impulses cancel. It holds even when energy is not conserved, such as in sticky collisions.
Example: a 2 kg trolley at 3 m/s hits a stationary 1 kg trolley and they stick together. Total momentum is 6 kg m/s, so the combined speed is 6/3 = 2 m/s. In a gun, a 0.01 kg bullet at 300 m/s gives a 3 kg rifle a recoil speed of 1 m/s backward. Two ice skaters pushing apart move in opposite directions with speeds inversely related to mass.
Tips: pick a positive direction, write the momentum sum before and after, and watch for external forces such as friction that may break the conservation. Over very short collision times, these external forces are normally small enough to ignore.
Key ideas
- Applies to all collisions
- Explosions also conserve it
- Vector sum before = after
Quick check
Use cases: where Conservation of Momentum helps
- Homework: review the key idea behind Conservation of Momentum before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Conservation of Momentum without notes.
- Visual learning: play with the interactive lab to see how Conservation of Momentum changes when you adjust its inputs.
- Teaching a friend: explain Conservation of Momentum in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Conservation of Momentum?
In an isolated system, total momentum stays constant.
What will I learn in Conservation of Momentum?
Applies to all collisions. Explosions also conserve it. Vector sum before = after.
Is Conservation of Momentum part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Conservation of Momentum lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Conservation of Momentum?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.