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Thursday, December 28, 2006

Strange wall-ball problem

Okay I get it - the title is not that imaginitive.
Hi there! The organic post that was to be never came up because I left my copy back in school but its some pretty interesting isomerism nuts that I was shown by my teacher. For now its some physics- collision style!
In fiitjee and other study mats there is always this obscure thing that concerns a ball with a wall. (Wall=infinite mass, ball=m)When the ball collides with the wall having an initial velocity u it returns back with the same velocity provided that the collision is elastic. Now fix the ball, let the wall strike the ball with a velocity u.Whatt happens now?

Contrary to all common sense the ball lurches forward with a velocity 2u.why??
A friend of mine kaustav sengupta (never mind the too erudite tone of the sentence) made sense when he said that consider the first case as in figure1, the frame instead of being stationary with respect to the ground is having a velocity u to its right. then?/Â…..we get the second case, fig2, minus the comment about not making sense.
Comment if u knew it
Comment if u did not
Comment if u think itÂ’s wrong
Comment anyway!


obiwankenoby said...

plz tell me how you like it.and tell me an alternate explanation in terms of common sense.of course the formulas do work out but i declare myself dissatisfied.

Anonymous said...

could you give any idea as to how one could prepare for the kvpy interview?
is there any specific branch or topic in any of the subjects that one should focus on?pls reply soon i put this comment it the last post also .

Gr81 said...

Well another interesting post!!

My simple knowledge of physics seems to defy my comman sense...

(M-> Mass of Wall etc.)

Well what I want to say is that this equation (and even by conserving Energy) can NOT help at the moment coz M->∞ and so there's no use trying to solve an eq of the form ∞=∞+k.

Also, since M is ∞, we find that its vel should not change (Dividing the above eq by M also gives this, but I'll have to think if this division (by ∞) is right..)

But seems like the way ur friend suggested, vel comes out to be 2V...

I'll think a bit and then comment again!

PS Some things that I must Clarify...:
- I'm in Narayana, but even I do not like the idea of Coaching Centers. But even those who say they are doing "self study" dont do so - they mostly join tutions. Anyway till u r being taught in the school, u cant really do self study, can u?
(It would be gr8 if someone is preparing for JEE WITHOUT attending any school classes too... but sad that they wont let him give the enterance!)

- As for Chem - seems like it DOES show how much hard work u r doing (Thus my Chem marks prove that I'm busy doing NOTHING!)

Gr81 said...


Well, First of all incase u would hav read some of the comments in the 3rd Post (The one with 20+ comments), DO NOT STUDY for the Interview. Infact never STUDY for any interview. (Dont think I'm a fool... atleast not coz I said this!)

Remember that the KVPY ppl have already judged your intelligence from the written paper, so they wont ask you to name uses of some weird compound or the history of Newton. They will test ur INTELLIGNECE not KNOWLEDGE. In my interview, I had questions which focussed mainly upon how much I can think, not on what I knew (ie what I had mugged up)

Also the main key to succeed in interview is to stay calm and cool and to say NO to a ques u dont know (yeah - instead of guessing) U could probably say - "Sir I'm sorry I dont know the answer but I feel that..."
This way u'll accept ur lack of knowledge and also show that u r not completly foolish either.

So better prepare for the environment and tension u'll be facing than for what to study and what not!

PS: Anonymous comments have been blocked, but not coz of u.. It was just that there were too many spammers. If u dont have an account on Blogger (And if u dont wanna make it - even though it hardly takes a minute) u (or anyone else) can mail me your queries to r.thegr81@gmail.com ( PLZ KEEP THE SUBJECT AS KVPY to prevent it from being considered SPAM)
But I'll be replying to them over here, so that others can also make use of it...

Arun Chaganty said...

@anom: "ditto" +
If at ANY point, you get worried that they think your not good enough or something, DON'T think about it. I thought my interview had gone to the dust when I made a small mistake. One of the interviewers even made a crack at me. STILL, I got in. Remember, as long as your cool, and have confidence in your ability, you'll do fine.

@Rash: I totally agree about the interview stuff.

@ObiwanKenoby (what's your real name again?):
It seems very intuitively right, and I have some things to offer an explaination.

First, at rash, using the inf = inf + k, you can say that the velocity of the ball after collision is indeterminate.

However, as these two bodies are inertial, and as they aren't moving with the speed of light, you can perfectly well change your perspective and solve the problem.

There was actually a problem I solved using this technique (and I'm proud of it because nobody else thought of it). I think it was like this:
There is a room full of dust uniformly distributed in air. The density is 'd'. You now have a board moving through with an initial velocity 'u', and mass 'm', area 'A'. Find the velocity after time 't' and blah blah (find everything you can about its motion).

Try it out.

And another thing I read somewhere, which was quite cool. If you have two bodies coming together for an elastic collision, just consider the frame of the centre of mass. In this frame, they just rebound. So you don't have to do any calculations, and calculuating velocity of CM is pretty much mental. Cool thing isn't it. (Prove it on your own).

Gr81 said...

Hey that CM thing was nice arun...

And yeah just 2 remind u, I think our blog can do with a bit more members... So mayb u shuld ask som of ur frnds (or ppl whom u think culd post som really good stuff here) to join the blog!

Arun, I reckon u shuld also b an admin, coz even u cum online often..