Forces and torques that act so powerfully but so briefly that they produce finite changes in linear and angular momentum while the system undergoes negligible displacement are said to be impulsive.
Linear impulse: dp = Fdt,
Dp
= òFdt, Dp
= FavgDt.
The integral of force over time as Dt approaches 0
is called the impulse
of the force.
Angular impulse: dL =
tdt, DL
= òtdt, DL
= tavgDt.
The integral of torque over time as Dt approaches 0
is called the angular impulse
of the torque.
In collisions, it is assumed that the colliding particles interact for such a short time, that the impulse due to external forces is negligible. Thus the total momentum of the system just before the collision is the same as the total momentum just after the collision.
 | Elastic collision: momentum is conserved, mechanical energy is conserved |
| Inelastic collisions: momentum is conserved, mechanical energy is not conserved |