If that physical property is weight, you can use what is known as a load cell. [DegrawSt] uses four 50 kg load cells to create a bathroom scale using an Arduino.

Some projects are both educational and useful. We believe that [Jasper’s] Arduino based electronic load is one of those project. [Jasper’s] electronic load can not only act as a constan… ...

In this quick Arduino tutorial I will explain how you can control a relay using the Arduino Board, one 1K and one 10K resistors, 1 BC547 transistor, one 6V or 12V relay, one 1N4007 diode and a 12V fan ...