Postby **ThinkerEmeritus** » Tue Nov 18, 2008 12:14 am UTC

Rgeminas gives the correct result, of course. BUT, you are going to see a lot of physics problems like this and even more complicated -- much more complicated. How do you approach them? You need a set of things to try when you get stuck.

Here is one very good set:

Find all the quantities and data mentioned in the problem. Here it is kinetic energy, momentum, mass, and velocity. The last two are to be determined, but they are quantities nevertheless.

Leaf through your mind, your notes, and/or your book for algebraic relations among the quantities. Especially don't forget definitions. Here you have KE in terms of mass and velocity, and momentum in terms of mass and velocity.

Are there enough relations to allow you to determine your unknowns? Here you have mass and velocity unknown, KE and momentum known, so two unknown quantities. You have two equations. Voila. If unknowns and relations hadn't matched, you would have to look for a relation you forgot to include or hope that one of the unknowns drops out. It is possible also that a relation may bring in another unknown quantity. If so, either you are going in the wrong direction or you should add that quantity to your list.

Figure out how to do the algebra.

On more complicated problems, it is best to find a check. One way is to plug all the knowns and now-determined values into your original relations (original meaning before doing the algebra) and make sure that the relations become identities.

"An expert is a person who has already made all possible mistakes." -- paraphrase of a statement by Niels Bohr

Seen on a bumper sticker: "My other vehicle is a Krebs cycle".