A door on hinges and a door closer
Three doors get the same push. With no closer the door stays open, with a spring only it rushes back and bangs, and with a spring and a damper it closes smoothly.
Drag to orbit the scene, scroll to zoom. Press “Run”.
What the experiment shows
The closer’s spring pulls the door to its closed position and the damper (like a shock absorber) keeps it from speeding up. Without the damper the door hits the frame at full speed.
How to run it
- Press “Run”: each door gets the same push.
- Watch the first door: nothing brings it back, so it stays open.
- Compare the other two: which door closes gently and which one bangs the frame?
Questions to discuss
- Why does the door with only a spring reach the frame at high speed?
- What happens if the damper is made too strong?
The science
The spring gives a restoring torque M = −k·φ and the damper a torque proportional to the angular speed. A door of 10 kg and 0.9 m width has a moment of inertia I = m·w²/3 ≈ 2.7 kg·m². The motion dies out without repeated bangs when the damping coefficient c is at least 2·√(k·I): about 15 N·m·s here.
Try it at home
Look at the doors in an apartment building or a shop: many have a closer on top — a spring and an oil damper in one case. Open the door and let go: it closes by itself without a bang.
What to change in the editor
- Select the middle door’s hinge and raise the damping d to 6.
- Select the third door’s hinge and make the spring three times stiffer (k = 60).