Glossary
Closed System
A system where no mass, energy, or charge can enter or leave its boundaries.
Example:
Imagine a perfectly sealed thermos bottle; nothing gets in or out, making it a closed system.
Collision
An event in which two or more objects exert forces on each other for a relatively short time, resulting in a change in their motion.
Example:
When two bumper cars crash, it's a collision where momentum is exchanged.
Conservation of Momentum
The principle stating that the total momentum of a closed system remains constant if no net external forces act on it.
Example:
When a billiard ball hits another, the total momentum of the two-ball system before and after the collision remains the same.
Elastic Collision
A type of collision where both momentum and kinetic energy are conserved. Objects typically bounce off each other without deformation.
Example:
The ideal bouncing of billiard balls is often approximated as an elastic collision.
External Forces
Forces that act on a system from outside its boundaries. These forces can change the total momentum of the system.
Example:
Friction acting on a sliding box is an external force that slows it down.
Impulse
The change in momentum of an object, equal to the average net force acting on the object multiplied by the time interval over which the force acts.
Example:
A baseball bat hitting a ball delivers a large impulse, causing a significant change in the ball's momentum.
Impulse-Momentum Theorem
States that the impulse applied to an object is equal to the change in its momentum ($J = \Delta p = F_{net}\Delta t$).
Example:
This theorem explains why airbags reduce injury by increasing the time over which a force acts, thus reducing the force for the same change in momentum.
Inelastic Collision
A type of collision where momentum is conserved, but kinetic energy is not (some is converted to other forms like heat or sound). Objects often stick together or deform.
Example:
When two clay balls collide and stick together, it's an inelastic collision because kinetic energy is lost.
Internal Forces
Forces that act between objects within a defined system. They always occur in action-reaction pairs and do not change the total momentum of the system.
Example:
The force of a person pushing against the inside wall of a boat is an internal force to the boat-person system.
Momentum
A vector quantity defined as the product of an object's mass and its velocity, representing its 'quantity of motion.'
Example:
A heavy truck moving slowly can have the same momentum as a light car moving very fast.
Net External Force
The vector sum of all external forces acting on a system. If this sum is zero, the system's total momentum is conserved.
Example:
If a car is cruising at a constant velocity, the engine's thrust and air resistance create a net external force of zero.
Open System
A system that can exchange mass, energy, or charge with its surroundings.
Example:
A boiling pot of water is an open system because steam (mass and energy) escapes.
Vector Quantity
A physical quantity that has both magnitude (size) and direction.
Example:
Velocity, force, and momentum are all vector quantities, meaning their direction matters.