7.18 - Summary
An object's momentum is the product of its mass and velocity. It is a vector quantity
with units of kg·m/s.
Like energy, momentum is conserved in an isolated system. If no net external force
acts on a system, its total momentum is constant.
A change in momentum is called impulse. It is a vector with the same units as
momentum. Impulse can be calculated as the difference between the final and initial
momenta, or as an average applied force times the duration of the force.
The conservation of momentum is useful in analyzing collisions between objects,
since the total momentum of the objects involved must be the same before and after
the collision. In an elastic collision, kinetic energy is also conserved. In an inelastic
collision, the kinetic energy is reduced during the collision as some or all of it is
converted into other forms of energy.
A collision is completely inelastic if the kinetic energy is reduced as much as
possible, consistent with the conservation of momentum. The two objects "stick
together" after the collision.
The center of mass of an object (or system of objects) is the average location of the
object's (or system's) mass. For a uniform object, this is the object's geometric
center. For more complicated objects and systems, center of mass equations must
be applied.
Moving objects behave as if all their mass were concentrated at their center of mass. For example, a hammer thrown into the air may rotate as
it falls, but its center of mass will follow the parabolic path followed by any projectile.
p = mv
Conservation of momentumpi1 + pi2 +...+ pin = pf1 + pf2 +...+ pfn
Center of mass