Okay, print this out and show it to your teacher:
Why in the world did you choose to post a poll? Especially when you don't have a "poll" question. I have to choose to check either "Do you use physics in flight" or "How are you"?
As for your question:
"a 1g car going 5 m/s on a frictionless track colides with another car that weights 2.5g and is motionless after the colision the cars stay together what forces are keeping them together "
(Have they told you about question marks in school yet?)
I have no idea what forces are keeping them together and the information given in that question certainly won't tell me. I could calculate their common speed after the collision (it is 1/7 m/s) but there is no way to know "what forces are keeping them together".
Since I can also calculate that if this were a perfectly elastic collision, the smaller car would rebound sharply, this is not an elastic collision. Perhaps the two cars got locked together in the collision, perhaps there was glue on one or both cars! There is no way to know WHAT is holding them together.
To the teacher: This is an excellent place for students to post questions about their homework. People responding to posts are very good about giving hints without simply doing the homework themselves.
It is NOT a good idea to post things just for the sake of posting!