Confusion Over Collision: Which Formula is Correct? Urgent Help Needed!

[rootX]

need urgent help

Homework Statement

Still working on the question I posted few days ago, here's the link for that question:
https://www.physicsforums.com/showthread.php?t=164889

I kinda got confident over my answer, but I discovered another formula, and it screwed up everything.
It is:
$$V_{1,f} = \frac{m_{1}-m_{2}}{m_{1}+m_{2}} v_{1,i} + \frac{2 * m_{1}}{m_{1}+m_{2}} v_{2,i}$$
$$V_{2,f} = \frac{m_{1}-m_{2}}{m_{1}+m_{2}} v_{2,i} + \frac{2 * m_{1}}{m_{1}+m_{2}} v_{1,i}$$

Hope, I put the tex code right. Anyhow, it gives me
$$x_{1,f}=-4.37$$
$$x_{2,f}=10.03$$
$$y_{1,f}=7.01$$
$$y_{2,f}=4.48$$

But using my that previous way(that relativity method) I get
$$x_{1,f}=-1.20$$
$$x_{2,f}=9.40$$
$$y_{1,f}=13.20$$
$$y_{2,f}=5.85$$

Now, I am confused over mine(I tried that applet, but it is pretty cumbersome
to set values.

Anyhow, if anyone can tell me which answer is right?
I would be really thankful. I have this thing due tomorrow

 

[hage567]

I didn't really look that closely at your relativity method, but did you convert back to the "non-relativity" frame after you found your values? Just an idea.

 

[rootX]

I didn't really look that closely at your relativity method, but did you convert back to the "non-relativity" frame after you found your values? Just an idea.

Yes, I did.
And if I use my answers, then they give total final kinetic energy same as before the collisions.
However, the answer that that formula gives provide a different total kinetic energy(But I saw it in like two Physics books since then)

 

[rootX]

just one last more question

A metal tube is 2.40 m long and has a mass of 800g. It contains 200g sliding mass at its front end. There is no friction between any surface. When the sliding mass reached the end of the tube, it sticks to a magnet attached there. How long would it take the tube to travel a distance of 6.00 m, if the metal tube has an initial velocity of 6.00 m/s?

I calculated time it took before the sliding mass reached the end, and then found the distance left to cover when this happens, and find time for that distance. And, I got 1.16 s.
Anyone can check my answer?

 

[rootX]

oops, that was supposed to go in a new thread

 

[hage567]

I think those equations you posted are intended for one-dimensional collisions.

 

[rootX]

I think those equations you posted are intended for one-dimensional collisions.

yes, the book mentions that. However, I did break the velocities into components. So, shouldn't that make the collision 1-dimensional for each axis?