Our grade 4 students recently completed a unit on simple machines and I have compiled a few great resources available to teach students about machines and how they work.
Edheads is an essential resource to teach students about machines. It has everything you need to do a unit on simple machines, including interactive activities and teachers resources and lesson plans.
Algodoo (formerly know as Phun Physics) is a great software for students to interact with objects and learn about the physics that make them work. The previous version - Phun - was free but the new updated version isn't. I would say that it is worth the money however, but you may want to download the trial version to test it out.
Crayon Physics is a great software which is much like Algodoo but aimed at younger learners. I would recommend this for students below grade 4 and Algodoo for students who are a little older.
Students need to get an object to move to hit the star. It starts very easy but gets more difficult as you move to higher levels. I have also seen similar apps available on the App Store such as Touch Physics which sells for $1.99
Parkworld Plot by Engineering Interact is a fun activity to teach students all about forces. They are told that one of the rides in the theme park park has been tampered with and need to use their knowledge of forces to determine which ride to close down. The graphics are a little out of date but it is still a great website. At the end, students are given a printable certificate which shows them how they did.
Both Brainpop and Brainpop Junior are great resources for schools. Brainpop is more suitable for higher elementary/middle school whereas BrainpopJr is aimed more at students in grades 4 and below. You will find great resources on both related not only to simple machines, but virtually any topic you need to cover. The animated videos are entertaining and both sites include printable teacher activities. Brainpop is now integrated with Google Docs and you can download their free app from the App Store to get a video sent to you each day. There is also BrainPOP ESL to focused on second language learners.
These are just a few great resources I have found to help students learn about machines. If you are doing a unit on this and need more, contact me and I will send you a list. Please share any resources that you have used as well which you would recommend.