The best alternatives to SimPhy are Khan Academy, Physical Web, and Kosmos School. If these 3 options don't work for you, we've listed a few more alternatives below.
What do you think of SimPhy?
AssemblyAI— Speech-to-Text API with superhuman accuracy
Kosmos is a project-based K-12 school that exists only in virtual reality offering classes (called Projects) where kids collaboratively complete objectives by learning and applying hard skills like math and physics, but also soft skills like critical thinking, creativity and collaboration.
Math fact fluency builds a strong foundation for future Math success. Fact Flow is an easy-to-use solution for parents to help their kids achieve Math Fact Fluency. Adaptive algorithms help parents ask the right question at right time.
Memorizing math facts like 9 x 7 allow the player to do complicated math problems more easily and with better speed and accuracy. Manta Math has no ads and works 100% locally all the time.
Sockhop is group video for socializing in shared virtual space. Customize your video, bump into each other and play games. Sockhop is your corner of the internet where you can create your room and invite the people you care about.
PhET provides fun, free, interactive, research-based science and mathematics simulations. The simulations are written in HTML5 and can be run online or downloaded to your computer. All simulations are open source. (from the product's site)