SimPhy VS Physion

You can create different types of bodies inside its physics world with different parameters like restitution, friction, velocity etc. attach them with different types of Joints like spring, rope, chain, pulley etc. Due to its native Physics engine the accuracy in solving is great.

One can visualize the motion with the numerous built in tools like tracers of points on body or Body ghosting, Graphs between different parameters( like KE, speed, velocity, momentum, etc), FBD of grouped and ungrouped objects, Camera tool ( to set frame of reference) etc.

It supports gravitational , electric, magnetic and buoyancy fields. One can even set variable fields ( time dependent ) and can easily change the fields as well using sliders.

One can create their own GUI elements in it like buttons , sliders , checkboxes , List , dialog etc. and even can write scriptable codes in them for different events in its in-built powerful scripting editing tool.

Physion is a web application which allows you to design and simulate physics experiments. You can think of it as a "CAD-like" application combined with a 2D physics simulator where the objects you design can be instantly simulated. Physion provides a rich set of tools with which you can design physics experiments for educational or fun purposes.