1. Sep 15, 2008

### i_island0

How would you define head-on collision? I am having confusion in definition itself
Among the following, which one would you call head-on collision:
-central impact
-eccentric impact
-direct impact
-oblique impact

2. Sep 15, 2008

### mathman

A head on collision means that the point of impact is on the straight line connecting the center of gravity of each of the objects. Which term describes it best is more of a language problem rather than a physics question. My off-hand guess would be central or direct, but these terms require precise definitions.

3. Sep 16, 2008

### i_island0

if i take any spherical ball, no matter what be the direction of velocities, i can always connect their centers with a straight line. So shouldn't i call all collisions to be head-on in that way?

4. Sep 16, 2008

### Shooting Star

Evidently then, there is at least one frame of reference (actually infinitely many) in which a collision is always head on. One of these is the centre of mass frame, which is frequently used to simplify calculations, and then the results reverted back to the lab or outside frame.

Is there any particular reason for your asking? The Physics remains the same how ever you term a collision.

5. Sep 16, 2008

### mathman

With further thinking, I can see my original comment was incorrect. For a collision to be head on, three things must be parallel (sticking to spheres), the center of mass line and the velocity vectors of the two objects.

6. Sep 16, 2008

### Zizy

Also, velocity vectors should point in the opposite directions and need to be nonzero.

7. Sep 17, 2008

### i_island0

Yes, there is a reason why i am asking this. Books like irving shames and Jhonston never actually defined head on collision. When i read resnick halliday walker.. or krane.. he also never defined head on collision. But in their exercise problems.. suddenly he started making statements on head on collision.
So my problem was how do i start solving a question without knowing the definition of the terms they are making in the question.

It says something like this: "A head-on collision is one where the front ends of two ships, trains, planes or vehicles hit each other, as opposed to a side-collision or rear-end collision".
So, if i were to accept wikepedia, then in that case i shouldn't worry about the velocity direction of the balls at all; as it is not saying anything about the velocity direction in its definition.

8. Sep 17, 2008

### Shooting Star

I think this calls more for common sense than Physics. Use the term head on collision as you would in real life. Wiki does not quite give the picture.

As has been outlined in the other posts, in the lab frame, the centres of mass of the two bodies should be travelling in the same straight line and approaching each other to call it a head on collision. You can’t make it simpler than this. (In general, in collision problems, rotations of the individual bodies are not taken into consideration explicitly, but there is no reason why it cannot be considered when the problem demands it.)

Do remember, though, that every oblique or otherwise collision is head on in the centre of mass frame when two bodies are involved. This allows us to confine the calculations to only one dimension.

9. Sep 17, 2008

### Staff: Mentor

What if he doesn't use English in real life?

An important part of learning physics at the introductory level is learning how to translate the description of a problem from everyday language to mathematical language. That's why textbook authors write exercises that don't use mathematically precise descriptions. This causes problems when the student doesn't understand the description because English is not his native language, or he's never had real-life experience with objects or devices that are used in the description, etc.

For example, exercises about conservation of energy (in particular kinetic energy and gravitational potential energy) often involve roller-coasters. I once had a student ask me for a detailed clarification of such an exercise because (amazingly) she had never ridden a roller-coaster! (and hadn't experienced how you speed up going downhill and slow down going uphill, etc.)

However, if the student does have the appropriate language skills and everyday-life knowledge, then... if the textbook doesn't explicitly define "head-on collision" in a mathematically precise way, I think it's safe to assume that the authors want the student to figure out the correct mathematical description himself, as part of the solution to the exercise.

Last edited: Sep 17, 2008
10. Sep 17, 2008

I think it's an interesting query. I would say that the language semantics, for at least English, imply the following definition:

- Initial points of contact on both objects are on a point along their respective motion vectors as drawn from their respective COGs. Or, if there are multiple instantaneous points of contact, the center of resistance of those collated points lies on the motion vector of the COG.
- The respective motion vectors of the two objects are, when summed, negative. (I don't see why it would have to be 180 degrees in all planes).

The usage of "motion vectors" implies a frame of reference without explicitly saying so. For a ball, from its reference frame any collision is always "head-on" and it is trivial to say so. But not from another reference frame.

11. Sep 17, 2008

### Shooting Star

Did I mention English anywhere? Of course, you meant something broader, but if language is the problem, then I sincerely don’t think this is the right place for this discussion. But you have raised a good point and I’ll remember it in the future, noting that this confusion can also occur to a student whose native tongue is English.

I do agree with you that at this introductory stage, every new term has to be explained to the student, but he also must play ball some day in real life. The student will learn very shortly to compare gas molecules with billiard balls, and then he will have to fall back on these elementary concepts. But he or she also has to learn to correlate words from everyday-life to problems in Physics, and as you point out, this could be the reason that some authors are not so explicit.

I did mention “common sense”, which I admit maybe rather vague. However, that is exactly why immediately afterwards I have given the precise and the simplest way of explaining the concept in question to the student.

12. Sep 17, 2008

### stewartcs

Additionally, I always found it good to explicitly state my assumptions when giving the solution so as to avoid any ambiguities in the meaning of my answer. By clearly defining what you mean by the terms you use in a problem, the reader can correctly interpret the meaning of your answer.

CS

13. Sep 17, 2008

I don't think common sense is so common... besides, if we're honest, many phrases which we use in every day life can seem obvious, but upon closer inspection it's found that we're not actually 100% sure what we're talking about...

14. Sep 17, 2008

### i_island0

Wow... so much philosophy.. but what is the final conclusion??
I will just write what i understood:
- head-on collision is one in which velocity vectors are directed towards each other.
- in COM frame all collisions are head-on

15. Sep 17, 2008

### Shooting Star

Unfortunately, that will be true for any science too, up to any degree of sophisticaton you choose to reach. However, this is not really the thread to discuss it.

In the frame in which the system is being studied, the CMs of the two bodies should be travelling in the same straight line. (Velocity vectors may be parallel or anti-parallel without lying on the same st. line -- Post #8.)

16. Sep 18, 2008

### i_island0

ok.. i got it this time.. thx to all