closed systemA group of objects with no outside force on it. Two carts on a smooth, level track are close to one: nothing outside the pair pushes them along the track.
collisionA brief meeting of objects that push on each other. In physics it is any event in which bodies exert forces on each other for a short time.
conservation of momentumThe law that a system's momentum total cannot change. The total before a collision or an explosion equals the total after it, as long as no outside force acts.
elastic collisionA collision whose kinetic energy total is unchanged. Atoms bouncing off one another collide elastically. Large objects, such as billiard balls, only come close.
external forceA push or pull from outside the system. Only an external force can change the total momentum of a system.
impulseThe product of a force and how long it lasts. A small force that acts for a long time can give as much impulse as a large force that acts briefly.
inelastic collisionA collision that turns kinetic energy to other forms. When large objects collide, some kinetic energy always goes into heating them and changing their shape.
Show all 36 terms (29 more)
Momentum and impulse, continued
massThe measure of an object's inertia, in kilograms. Doubling the mass of a cart that keeps the same velocity doubles its momentum.
momentumThe amount of motion in a moving mass, symbol p. Its SI unit is the kilogram meter per second.
net forceThe single force that all the forces add up to. It is also called the resultant force. The momentum of an object changes at a rate equal to the net force on it.
newton-secondThe unit in which SI measures impulse. It is the impulse of a force of 1 newton acting for 1 second, and it is equal to 1 kilogram meter per second, the unit of momentum.
velocityA vector that gives an object's pace and heading. Because velocity has a direction, momentum has one too. Two equal carts rolling toward each other at the same speed have a total momentum of zero.
Collisions and safety 12
airbagA gas bag that pops out to soften a crash. It spreads the passenger's change of momentum over a longer time.
coefficient of restitutionA bounciness number from 0 to 1 for a collision. It is 1 for an elastic collision and 0 for a perfectly inelastic one.
crumple zoneA part of a car built to crush in a crash. By crushing, it makes the collision last longer, and a longer collision means a smaller force.
deformationThe change of shape a force leaves in an object. Some of the kinetic energy of an inelastic collision goes into deforming the objects that collide.
explosionAn event in which parts of a system fly apart. If the system was at rest, the momentums of all the pieces still add up to zero.
kinetic energyHalf the mass times the speed squared. An object that doubles its speed has twice the momentum and four times the kinetic energy.
Newton's cradleA row of hanging balls that pass a swing along. Lift and let go of one ball at one end, and one ball swings out at the other end. Let go of two, and two swing out.
perfectly inelastic collisionA collision after which the objects move as one. The objects stick together. No other kind of collision loses as much kinetic energy.
reboundTo come back off a surface after striking it. A ball that rebounds changes its momentum by more than a ball that stops dead, because its velocity reverses.
recoilThe backward motion of a launcher as it fires. The thing thrown gains momentum one way, so the thrower gains an equal amount the other way.
seat beltA webbing strap that locks tight in a sudden stop. Seat belts, airbags and crumple zones all work the same way: they increase the time the collision takes.
total momentumThe sum of the momentums of every object in a system. Directions count: 6 units to the right and 4 units to the left make a total of 2 units to the right.
Laws, rockets and rotation 12
angular momentumThe momentum of something that spins or orbits. It is conserved too. A spinning figure skater who draws in her arms and legs turns faster.
axis of rotationThe straight line through the middle of a spin. A moment of inertia is always worked out about one chosen axis.
center of massThe point where a system's mass seems gathered. It is sometimes called the balance point.
gyroscopeA spinning wheel whose axis resists being tilted. Its axis keeps pointing the same way while the mounting turns, because angular momentum is conserved.
inertiaThe reluctance of matter to change its velocity. An object at rest stays at rest, and a moving one keeps moving, unless a force changes its velocity.
internal forceA force between two parts of the same system. Internal forces come in equal and opposite pairs, so they cannot change the total momentum.
moment of inertiaA measure of how hard it is to change a spin. It depends on how the mass is spread around the axis. Drawing the arms in makes it smaller.
Newton's second lawThe law linking net force to change of momentum. For an object whose mass stays the same it becomes force equals mass times acceleration.
Newton's third lawThe law that forces come in equal, opposite pairs. In a collision each object pushes on the other with the same size of force, in opposite directions.
propellantThe mass a rocket throws backward to move forward. A rocket carries it as fuel and oxidizer, burns them and throws the hot gas out of the nozzle.
thrustThe push a rocket gets from the gas it throws out. The engine speeds a gas up and throws it out of the back. The gas goes one way and the engine is pushed the other way.
torqueA twist: force times distance from the pivot. The farther from the pivot a force is applied, the greater its torque.
How this list was compiled: Term list chosen for this site on 2026-10-12. Each meaning is written in our own words and was checked that day against the article on the term in the English Wikipedia (https://en.wikipedia.org/), whose text is under the Creative Commons Attribution-ShareAlike license: Momentum, Mass, Impulse (physics), Collision, Elastic collision, Inelastic collision, Newton-second, Recoil, Kinetic energy, Coefficient of restitution, Crumple zone, Airbag, Seat belt, Newton's cradle, Deformation (engineering), Inertia, Thrust, Propellant, Center of mass, Angular momentum, Moment of inertia, Torque, Rotation around a fixed axis and Gyroscope; net force and axis of rotation against their entries in the English Wiktionary (https://en.wiktionary.org/). Momentum, the closed system, conservation of momentum, collisions and explosions, the resultant force and the car safety features were checked a second time against BBC Bitesize (https://www.bbc.co.uk/bitesize/guides/zytb8mn/revision/1 and its next two pages, Momentum; https://www.bbc.co.uk/bitesize/articles/zw3bjfr, Momentum). Newton's second and third laws, thrust, propellant, torque and the center of mass were checked against the Beginner's Guide to Aeronautics of the NASA Glenn Research Center (https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/newtons-laws-of-motion/, .../what-is-thrust/, .../rocket-thrust/, .../center-of-gravity/ and https://www.grc.nasa.gov/www/k-12/airplane/torque.html). The fact after each meaning is from the same pages. The worked figures (6 units and 4 units, the two carts) are our own arithmetic. Licence: own work
How it works
This momentum vocabulary worksheet lists momentum, mass, velocity, impulse, collision, conservation of momentum, elastic collision and five more terms down the left, with their meanings mixed up down the right. Its answer key prints last.
Read the full guide
Collisions and safety
Set opens on Momentum and impulse. The second set has perfectly inelastic collision, recoil, explosion, rebound, kinetic energy, coefficient of restitution, crumple zone, airbag, seat belt, Newton's cradle, deformation and total momentum.
Laws, rockets and rotation
The third set has inertia, Newton's second law, Newton's third law, thrust, propellant, center of mass, internal force, angular momentum, moment of inertia, torque, axis of rotation and gyroscope.
Momentum equals mass times velocity, written p = mv. A 2-kilogram cart moving at 3 meters per second has a momentum of 6 kilogram meters per second, in the direction in which it is moving.
What is the difference between an elastic and an inelastic collision?
Both conserve momentum. An elastic collision also keeps the total kinetic energy; in an inelastic one some of it becomes heat, sound or a change of shape. If the objects stick together, the collision is perfectly inelastic.
What is the impulse-momentum theorem?
It says that the impulse on an object equals the change in its momentum: force times time equals mass times the change in velocity. For the same change, a longer time means a smaller force.