From the University of Colorado at Boulder comes PhET Physics Simulations.
Here’s a screen shot from the Gravity Force Lab, which demonstrates the universal law of gravitation:
By increases or decreasing the mass of the two objects or by changing the distance between them, the force on each object is updated. My students struggled with recording the correct force. Some would write 396 N and ignore the decimal places, so I’m careful to address that.
Each simulation has lesson plans and work sheets associated with it created by other teachers. These are a mixed bag. While some of the files provide a quick lesson or a good starting point, others are too advanced or upper level to be useful with my students.