One online game I recommended to fifth grade science teachers last year for their force and motion unit was Coaster Crafter. It doesn’t require any downloads or installations, it’s free, and kids can sign up for an account without an email account.
The game has a Design Challenge section that asks kids to fix some poor roller coaster designs. In the process, Brunette introduces relevant science vocabulary: velocity, gravity, acceleration, friction, momentum, etc.
Once Level 1 is completed in the Design Challenge, students can try the Coaster Challenge where they get to design their own coasters that meet certain requirements. Completing the Design Challenges also makes different design elements available in the Free Play section of the amusement park.
As a teacher, I might introduce the site by having us complete the Level 1 of the Design Challenge as a class. Then I’d let the kids spend some time working/playing through the site either individually or in pairs. I can imagine some kids getting stuck and a classmate helping them out. Eventually, I might have them do a screencast that shows off their favorite coaster design with a voice recording that explains the design using force and motion vocabulary.
By the way, I discovered this game site through Common Sense Media, a web site that offers reviews of books, movies, video games, web sites, etc. for kids, parents, and teachers. It’s a great resource!