Do the internal forces affect the motion of a system under the effect of some external force?

Answer 1

No, to alter the path of a system, the force vector must be external. F = ma, which says, the net force has to act on the whole system mass (m) in the direction of the acceleration (a) of that system. Now, having said that....

The source of that external force vector can come from inside the system. Internal gyroscopes, for example, can be used to alter the path of a guided missile.

When a gyroscope is nudged inside the missile, the gyro remains fixed in space because of the gyroscopic effect; so to accomodate that nudge, the missile itself changes the direction it is pointing.

It would be something like you pushing on a wall (the nudge), but the wall does not move...you, the system, do. You push yourself away from the gyro fixed in space. (The real force vectors of a gyro are not quite that simplistic, but this serves to illustrate how the forces can transfer.)

The forces that reorient the missile act on the whole missile and, therefore, are external to it. In other words, I could draw force vectors external to the missile to show how the direction would be altered. In a very real sense, that internal force (the nudge) was transferred to an external force that "affected the motion of [the] system." But it wasn't the internal force itself that caused that change.

A shifting mailbag in an airplane can alter the flight path of that airplane. At first glance, that might appear to be an internal force changing the motion of the plane. But weight is the result of a gravitational pull on a mass. The force of gravity is external to the aircraft...once again, despite the source being internal, the force exerted on the system (the aircraft) is external. Again, I can draw a force vector (an arrow) on the outside of the system.

Answer 2

Well i believe it would depend on what type of system ur talking about....but yes as a general rule, there are several internal forces that could increase or decrease the impact of the application of an external force....

Answer 3

Yes. This is exactly why satellites use gyroscopes.