Scope, Sequence, and Coordination
A Framework for High School Science Education
Based on the National Science Education Standards
Impulse, Momentum and the Conservation of Momentum
Conservation of Momentum
During the time of an interaction between two objects, the forces acting between the objects may be of various kinds: contact, gravitational, electrical, or even nuclear. Contact forces are fundamentally electrical in nature. Similarly, elastic forces, like those exerted by a rubber band or spring, are electrical. Molecular forces are also electrical.
During an interaction, Newton’s third law requires that the impulse delivered by one system be equal and opposite to the impulse delivered by the other system. This behavior requires that the magnitude of the change in momentum of each system be the same. Since they are oppositely directed, their sum is zero. In two or three dimensions, where there are three or more interacting systems, the sum must be a vector, where the vector represents the total of the momenta of the interacting systems.
The law of conservation of momentum is one of the major laws of nature, and for any closed system that includes all interacting objects it always holds. Thus, regardless of the nature of the force and of other constraints on objects in the system, like energy considerations, the total momentum is conserved. If there is a net external force on the system, then the total momentum of the system changes at a rate equal to that force. This law is of great interest in applications like sports, safety devices for cars and planes, rockets, and jet propulsion.
Observationally, the total momentum after an event occurs in a closed system equals its total momentum before the event, but only within the uncertainty of measurement.
Interaction, action at a distance
Action and reaction pairs, impulse and momentum
Vector momenta and impulse (two dimensions), system
Free-body diagrams, three-dimensional vector momenta and impulse
Newton=s third law, law of conservation of momentum
Physical systems and free-body diagrams