Nick,
So you are saying that in a jet engine, it is the turbine blades that accelerate the air?
added as more time to answer the specific point
Newton's first law is a special case of the second law, and was intended to counter the commonly held belief that all things return to a state of rest over time. It states that a "body" i.e. a point item of constant mass remains in constant motion unless acted upon by an external force.
I hate to use cliches, but take the example of a skater rotating arms outstretched. Upon bringing their arms in, their angular velocity increases without any external force. This appears to contradict Newton's first law, however upon closer inspection we see that the skater has reduced their moment of inertia, and hence to conserve angular momentum the angular velocity must increase. Within the rotating frame of reference, a force is experienced due to the apparent acceleration.
I still maintain the force and momentum change are instantaneous and inseparable. Further to this, whether or not there is a force or a momentum change depends upon which frame of reference you are observing from.
Back to the boy with the 3 mile long arm. Momentum change of an individual item is possible within the boundary of the system as long as momentum is conserved throughout the whole of the system.
Last edited by waspy77; 13th Jul 2007 at 16:28.
Reason: More time to respond