What causes a chain to slip on a new mountain bike when peddled hard?

Question

What causes a chain to slip on a new mountain bike when peddled hard?

Answer 1

Your cables have probably stretched. This is very common. Take your bike back to the shop and let them readjust it. The shop that sold you the bike should do this for free. If you didn't buy the bike from a local shop, they will rightfully charge you for the service, but it shouldn't be much. My local shop charges about $15 to adjust the front or rear derailleur.

HTH

Answer 2

It sounds like it's not tuned up properly. I would take it back from where you bought it and tell them the problem.  There should be a 30 day warranty on it (at least) , but they should fix it at no cost to you, unless you've had it for a while.  It's a minor adjustment that can be made in minutes.