I had been wondering for some time just how fast a mouse trap takes to slam shut once it has been tripped. What follows is a description of a simple way to measure the closing speed, along with the results I obtained.
The trap I used was a classic Victor brand mouse trap. This is the kind that uses a spring loaded bar held back by a hook. The front of the hook rests on a small protrusion on a metal piece used as a bait holder. When the bait holder is moved even slightly, the hook releases, and the upward force on the bar from the spring pushes the hook out of the way and allows the bar to slam down on the front of the trap. Any mouse that is unlucky enough to have its neck in the way of the bar will be dead.
I’ve included timing results from both the mouse trap and the larger rat trap. I’ve also included a step showing how the closing speed can be estimated using a simplified model based on the physics of an ideal torsion spring.
Step 1: Trap Modification and Equipment Setup
I used my oscilloscope to capture the timing of the process. Two events need to be captured: The release, when the hook separates from the bait holder, and the closing of the trap, when the bar contacts the wooden base. The release of the trap was used as the triggering event for the scope.
The different parts of the mousetrap were used to create two switches which allow the release and closing events to be represented on the scope. To make electrical connections to the various parts of the trap, I soldered wires to the U shaped tacks that are used to fasten them to the wooden base. A 9 volt battery was used as a voltage source to produce a signal that represents the state of the switches.
The trigger event for the scope (the release of the trap) is generated by using the bait holder and hook as the contacts of a switch. This is effectively a single pole, single throw normally closed switch. It is in the closed state when the trap is set, allowing the positive terminal of the battery to connect to the bait holder. When the trap is released, the battery will no longer be connected to the bait holder and so the voltage on it will drop to zero. The scope was set to trigger on the falling voltage seen on the bait holder.
The second switch generates the signal representing the closing of the trap. This switch is single pole, single throw normally open switch. It is in the open state prior to the bar impacting the front of the trap. This switch uses the spring loaded bar as one contact. The second contact is a strip of aluminum tape on the front edge of the trap. The battery voltage is applied to the bar, and the metal tape contact is connected to the second channel of the scope. When the trap is set, the voltage on the metal tape front contact will be zero. When the bar impacts the front contact, the voltage on it will rise to the battery voltage. By examining the timing between the voltage drop on the bait holder and the voltage rise on the front contact, the time from release to closure of the trap can be determined.
The diagram shows how the different parts of the trap are used to make the two switches, and how the switch states change from the setting of the trap to its final closure. The schematic also shows how the two switches, the battery, and the oscilloscope connect together. The nodes numbered in the schematic correspond to the parts of the trap numbered in the picture of the actual trap. Node 1 is the hook, node 2 is the bar, node 3 is the bait holder, and node 4 is the metal tape contact added to the front of the trap.
Step 2: Mouse Trap Results
The plot shows the signals captured by the oscilloscope, representing the release and close of trap. Three sets of data were collected from the mousetrap, and the waveforms captured were all similar to that shown. The timing results between runs were fairly consistent.
The timing from release to first closure was always about 12 milliseconds. I performed the same measurements on both of the traps that were included in the package, and the results were again fairly consistent.
Note the “bouncing” visible as the bar slams down on the front contact. Anyone who has dealt with interfacing mechanical switches to digital inputs will be familiar with the need to “debounce” the inputs. Mechanical contacts generally don’t just make a solid clean connection when closed. Instead, the switch contacts actually open and close several times before settling on a stable state. Notice that the timing between the initial closing and the second closing is fairly large, about 4 milliseconds. The subsequent closings are more closely spaced, until the bar finally comes to rest and switch stays closed.
I don’t have a high speed camera, but there are high speed videos of mouse traps on YouTube. When I viewed some of them, you could clearly see the bar bounce back up from the wooden base about half an inch after the initial contact. This agrees with the bouncing seen on the graph.
To put the 12 millisecond close time into perspective, one cycle of 60 Hz power is 16.7 milliseconds. So the trap slams shut in about ¾ of the time it takes the voltage on your AC outlet to make a complete cycle. I looked up how fast a humming bird’s wings move, and the general range stated in various sources was between 50 and 80 beats per second. So, the trap will slam shut about as fast as a single flapping of a hummingbird’s wing.
The 12 milliseconds represents the time for the bar to impact the wooden base. If a mouse were present, eating food from the bait holder, then the bar would not have to even travel that far to hit the mouse. So, if you are a mouse eating from a trap and you accidentally trip it, you actually have less than 12 milliseconds to back out and escape. I’m pretty sure that no mouse can move that quickly.
You could say that the bar of a mousetrap literally travels at “break neck” speed!
Step 3: Rat Trap
I repeated the same exercise using a Victor rat trap. The rat trap is pretty much just a scaled up version of the mouse trap. The picture shows how it compares in size to the mouse trap. The trap was wired up in the same fashion as the mouse trap. A all the schematics shown earlier for the mouse trap apply here as well.
The rat trap close time was approximately 23 milliseconds. Several runs were made, with the results being fairly consistent between them. The graph shows the timing. Again, note the bouncing of the switch closure before settling on a stable state.
Step 4: Analysis Using Basic Physics
I wanted to examine the behavior of a trap using a model based on physics, to provide a comparison against the observed timing.
I put together an analysis of the system using a simplified model based on the physics of ideal springs and simple harmonic motion. In this simplified analysis, many things have been idealized. Frictional forces, air resistance, and gravitational forces have been neglected. The energy used to push the hook up and out of the way has not been considered. The spring is assumed to be ideal, where the torsion constant of the spring does not change with the angle.
The attached PDF document contains the equations which describe the simplified physical model of the rat trap. It shows how the equations of motion are determined, and it includes a spreadsheet of the system parameters and the performance for the rat trap. All the equations and concepts contained in the analysis apply for both the mouse trap and the rat trap.
I’ll just list a couple of the more important results for the rat trap here, and leave it to you to look at the PDF for more details.
The main thing I wanted to get from the analysis was a calculated value for the time to close, for comparison with the measured value.
Measured and Calculated Results for the Rat Trap:
T_close_measured = 23ms
These results agree reasonably well with the observation. It makes sense that the calculated value would predict a faster closing time, as it does not include any frictional forces that would reduce the speed.
The potential energy stored in the rat trap when set was calculated to be 3.16 Joules. That doesn’t seem like very much energy, but it is apparently enough to break a rat’s neck. I didn’t perform the calculations for the mouse trap, but the torsion constant of the spring would be much lower than that of the rat trap, so the energy needed to kill a mouse would be even less.