I have viewed and read several papers related to this subject but cant find any help to understand some seemingly a simple and a straitforward point, about whether a short impulse period compared to the wave propagation period in a rod may or may not fail to impart a motion on the rod. It seems that with a short duration impulse-contact between a ball and a long rod, no matter how powerful the impulse is , the struck/imparted long-rod would fail to accelerate/move due tue the rod´s wave propagation period being longer than the ball-rod contact impulse period(?)

To illustrate my point here is an example

A small ball hits one end of a long rod.(i.e a very short lived impulse on the rod where the ball "hits and runs away" from the rod.) let the two objects be made of the same material like steel, wood etc so that wave propagation speed is related only to length of rod or diameter of ball. Now we know that the mechanical longitudinal - force carrying- propagation wave starting right after the impact propagate through both the ball and rod it takes different times to propagate through each of them due Tue different lengths of the objects. This period, for the wave to propagate from the rod´s first end to the other rod´s end, and then to reflect back to the rod´s original first end is much longer then the period of the small ball´s wave travel.

Lets assume that the ball´s impact/ impulse duration is shorter than the rod´s wave propagation period but is longer than the ball´s wave propagation period, i.e the ball hits the rod, has time to have its wave transmitted/reflected and moves away, and right afterwards the ball is removed very quickly from the rod).

Now lets see if I understand this correctly:

First , in order for any object to move/accelerate or- the rod in this case-, the wave from the original ball -impulse at the first end of the rod must propagate all the way to the other end of the rod and then reflect back again to the first rod-end again. Only now, when the wave has completed a full reflection that the motion force on the "rod as a whole" can reveal itself and the rod as a whole can now accelerate forward away from its original place moving for miles and miles :). Before that instant, no matter how hard the ball hits the rod, the rod itself as a whole can not move/accelerate, i.e. can not displace itself away from its original location where it can reach the "end of the universe" - just exaggerating and clarify this very delicate simple point:). Of course, only local small compression and rarefaction of the rod + minor periodic cg shifts occur during the wave propagation through the rod and instantaneously with the impact..

Here is my main question:

Now, keeping in mind that the impulse duration was shorter than the rod´s wave period , i.e. the ball "hit and run away" from the rod before the propagation wave inside the rod had time to reflect back. The ball is now no longer in contact with the rod. The wave in the rod is finally "after a long time" reflected back to the original rod´s first end on which the original impulse occurred, but there is no ball there touching!. Will the rod now as a whole start to move/accelerate even if the ball is no longer touching the rod-end ? Or will the rod only stay there since there is no ball for the rod´s reflected wave to push on, and all motion we got is the compression wave going back and forth through the rod´s length? I know , I can already hear your thoughts now but that is why I am asking.


My point is actually not so complicated although it may very well sound so. If you are not understanding anything please dont hesitate to ask me to make anything clearer. I am surely making mistakes here so that is where your help comes in. Oh by the way, the ball rod example is just a rather inefficient bad example,. the best method to have a very short impulse( hitting an object detaching near instantaneously) would be electromagnetic, i.e a capacitor bank discharge into a coil.

looking forward for your replies. It is truly appreciated Thanks. Regards

  • 2
    $\begingroup$ What papers did you read on the subject? In any case, no matter how short the force, conservation of momentum would be violated if the ball hit the rod without transferring moment to it. $\endgroup$ Sep 27, 2014 at 1:42
  • $\begingroup$ Yes I understand that but I am wondering if you are absolutely positive about what you said, not whether $\endgroup$ Sep 27, 2014 at 13:02
  • $\begingroup$ Sorry, but here is the comment: Hi! I read many. Googling on wave propagation + rods show some lists. I understand what you are saying but I am wondering the "how". I am going try to put more clearly hopefully this way to check my understanding: if the impulse contact duration of the ball was much shorter than the propagation time in the rod, will the rod as a whole still translate forward? and by translating I mean moving ahead several times its length and not just slightly compressing and expanding? thx $\endgroup$ Sep 27, 2014 at 13:11
  • $\begingroup$ On first contact he elastic wave will force separation but a short time after another contact will happen and so on. So overall there are multiple impact events over a short period of time enough transfer the momentum needed. $\endgroup$ Sep 28, 2014 at 3:51

1 Answer 1


If we have two objects colliding in free space, conservation of momentum is required, regardless of what's happening inside each object. If the rod initially has no momentum in our reference frame, it will gain momentum after the ball hits it. It is possible that this momentum may initially be in the form of a wave propagating through the rod, with most of the atoms stationary except near the wavefront, but first, the momentum of the center of mass of the rod has increased because we're averaging over all the atoms, and second, once the wave hits the other end of the material it will rebound and the other stationary atoms will now start moving forward until the wave gets back to the front end. (It would be like a box with a ball bouncing back and forth between the sides. Even if the box is initially stationary when the ball is moving to the right, it'll start moving once the ball hits the wall and rebounds.) It is possible that we'd end up with a rod where most of the atoms only moves forward half the time, but nevertheless, it is on average moving forward and will continue to move forward (several times its length or whatever) for as long as we wait. The center of mass will move forward continuously the whole time.

Will the rod now as a whole start to move/accelerate even if the ball is no longer touching the rod-end ?


Or will the rod only stay there since there is no ball for the rod´s reflected wave to push on, and all motion we got is the compression wave going back and forth through the rod´s length?

No, the rod will move. The wave reflects off the ends of the rod and doesn't need the ball's help except to act as an initial impulse.

Now, whether an impact will cause all the atoms to move as one or if some waves are created instead (or if both happen) is an interesting question. For example, Mossbauer spectroscopy, which involves a gamma ray photon hitting an atom's nucleus, depends on the collision resulting in the atoms all recoiling in unison rather than a wave being formed.


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