 
Impulse of Force
The product of average
force
and the time it is exerted is called the impulse of force. From
Newton's
second law
the impulse of force can be extracted and found to be equal to the change
in momentum
of an object provided the mass is constant:
The main utility of the concept is in the study of the average impact
force during collisions. For collisions, the mass and change in velocity
are often readily measured, but the force during the collision is not. If
the time of collision can be measured, then the average force of impact
can be calculated.

Minimizing Impact Force
The process of minimizing an impact force can be approached from the
definition of the
impulse of
force:
If an impact stops a moving object, then the change in
momentum
is a fixed quantity, and extending the time of the collision will decrease
the impact force by the same factor.
Alternatively, the same scenario can be examined with the aid of the
workenergy
principle.
An impact which stops a moving object must do enough work to take away its
kinetic energy, so extending the distance moved during the collision
reduces the impact force.

Airplane and Duck
Estimate the average impact force between an airliner traveling at 600
mi/hr and a 1 pound duck whose length is 1 foot. This is an example of the
use of
impulse of force.

Airplane & Duck Force Estimate
For the
airplane and duck force estimate, the
mass of
the duck is needed, but the
weight
in the U. S. Common system of units is given. The mass is

Airplane & Duck Estimates
For the
airplane and duck force estimate, the mass of the duck is determined,
but the change in velocity and time of collision must be estimated in
order to estimate the average impact force.
The change in velocity of the duck is estimated to be 600 mi/hr = 880
ft/s by assuming a head on collision, assuming that the duck is riding
with the airliner after the collision, and assuming that the duck's
velocity is negligible compared to that of the airliner, the "hovering
duck" approximation.
The time of collision is assumed to be the time of transit of the duck's
dimension of 1 foot, so 1/880 second.

Airplane & Duck Collision Force
As a variation on the
airplane
and duck problem, change the mass of the bird and speed of the
aircraft. Enter a length of the bird for estimation of the impact time.

