Conservation of linear momentum, general law of physics according to which the quantity called momentum that characterizes motion never changes in an isolated collection of objects; that is, the total momentum of a system remains constant. Momentum is equal to the mass of an object multiplied by its velocity and is equivalent to the force required to bring the object to a stop in a unit length of time. For any array of several objects, the total momentum is the sum of the individual momenta. There is a peculiarity, however, in that momentum is a vector, involving both the direction and the magnitude of motion, so that the momenta of objects going in opposite directions can cancel to yield an overall sum of zero.
Encyclopaedia Britannica thistle graphic to be used with a Mendel/Consumer quiz in place of a photograph.
BRITANNICA QUIZ
All About Physics Quiz
Who was the first scientist to conduct a controlled nuclear chain reaction experiment? What is the unit of measure for cycles per second? Test your physics acumen with this quiz.
Before launch, the total momentum of a rocket and its fuel is zero. During launch, the downward momentum of the expanding exhaust gases just equals in magnitude the upward momentum of the rising rocket, so that the total momentum of the system remains constant—in this case, at zero value. In a collision of two particles, the sum of the two momenta before collision is equal to their sum after collision. What momentum one particle loses, the other gains.
The law of conservation of linear momentum is abundantly confirmed by experiment and can even be mathematically deduced on the reasonable presumption that space is uniform—that is, that there is nothing in the laws of nature that singles out one position in space as peculiar compared with any other.
The Editors of Encyclopaedia Britannica
This article was most recently revised and updated by Erik Gregersen, Senior Editor.
LEARN MORE in these related Britannica articles:
vector mathematics
mechanics: Lagrange’s and Hamilton’s equations
…conservation of angular momentum and linear momentum, respectively.…
conservation law
Conservation of linear momentum expresses the fact that a body or system of bodies in motion retains its total momentum, the product of mass and vector velocity, unless an external force is applied to it. In an isolated system (such as the universe), there are…
Bernoulli model of gas pressure
physics
Physics, science that deals with the structure of matter and the interactions between the fundamental constituents of the observable universe. In the broadest sense, physics (from the Greek physikos) is concerned with all aspects of nature on both the macroscopic and submicroscopic levels. Its scope of study encompasses not only…
newsletter icon
Explanation: