The software you likely remember (the blue interface, the gravity controls, the "air resistance" checkboxes) was kept alive and updated.
In the late 90s, Knowledge Revolution released Interactive Physics 2000. This is the most stable "retro" version that works on Windows (up to Win 7/8, sometimes 10 with compatibility mode).
Eventually, MSC Software bought the IP. The direct descendant of this software is MSC Working Model 2D. It is the industry-standard tool for engineering dynamics and looks/feels very similar to the classic Interactive Physics.
Where to Download (Safe & Official):
To understand why people are hunting for an "updated download" of a 34-year-old program, you must understand the context of 1989. The Macintosh SE and Compaq 386 were state-of-the-art. Most educational software was either text-based drills or crude "draw-and-click" adventures.
Interactive Physics changed the landscape overnight.
The Core Features (Circa 1989):
In 1989, running a rigid body dynamics solver on a home computer felt like magic. It was the first time a student could drop a ball on a see-saw, watch a catapult launch, and instantly see the vector arrows change length.
The Internet Archive (Archive.org) legally hosts old software for preservation.
IP.EXE file. Right-click > Troubleshoot compatibility > "Troubleshoot program" > Select "Program worked in earlier versions but won't install or run now." Select Windows XP (Service Pack 3).If you are determined to run the actual 1989 executable: interactive physics 1989 updated download
If you are specifically looking for the classic interface for educational or retro-computing purposes, you generally want to search for "Interactive Physics 2000" rather than 1989.
The "deep" aspect of this software lies in its physics engine, which was revolutionary for consumer hardware.