Some physics help, please

[Peter B]

I'm not sure whether any non-Australians here are familiar with the "drip rifle", a modification of a standard British rifle developed by an Australian soldier at the end of the Gallipoli campaign in December 1915.

The drip rifle involved tying one end of a piece of string to the trigger of the rifle, and the other end to a can. Water dripped from a second can into the first, and when the first can filled up enough, it fell, pulling the string and thus the trigger. By varying the rate at which water dripped into the first can, the amount of time before the rifle fired could be varied.

The modification was applied to a couple of hundred rifles set up along the trench line. The objective was to maintain a low level of rifle fire through the night of the final evacuation, to fool the Turkish soldiers into believing there were still soldiers in the Australian trenches.

Anyway, in one video online, some people are saying the device wouldn't work as claimed. They say that the weight required to pull the trigger normally was several pounds, and it wouldn't be possible to have cans hold that weight of water.

Instinctively, that doesn't make sense to me - the trigger was pulled by the weight of the can of water falling, which seems different to the pressure exerted by a finger. But I don't know how to explain the difference.

Can someone please explain the different forces involved to numpty me, and why a falling can of water doesn't need to have the same mass as the force needed to pull the trigger?

Thank you!

Cheers

Peter

[rocketman]

I'm not sure whether any non-Australians here are familiar with the "drip rifle", a modification of a standard British rifle developed by an Australian soldier at the end of the Gallipoli campaign in December 1915.

The drip rifle involved tying one end of a piece of string to the trigger of the rifle, and the other end to a can. Water dripped from a second can into the first, and when the first can filled up enough, it fell, pulling the string and thus the trigger. By varying the rate at which water dripped into the first can, the amount of time before the rifle fired could be varied.

The modification was applied to a couple of hundred rifles set up along the trench line. The objective was to maintain a low level of rifle fire through the night of the final evacuation, to fool the Turkish soldiers into believing there were still soldiers in the Australian trenches.

Anyway, in one video online, some people are saying the device wouldn't work as claimed. They say that the weight required to pull the trigger normally was several pounds, and it wouldn't be possible to have cans hold that weight of water.

Instinctively, that doesn't make sense to me - the trigger was pulled by the weight of the can of water falling, which seems different to the pressure exerted by a finger. But I don't know how to explain the difference.

Can someone please explain the different forces involved to numpty me, and why a falling can of water doesn't need to have the same mass as the force needed to pull the trigger?

Thank you!

Cheers

Peter

I’m trying to work out the details here.  How does the can fall?  Is it supported by some weak material that breaks when the can reaches a certain weight?

E.g., what, maybe hang the can with a short, weak string that breaks at some point, but then also have a second, longer string attached to the trigger?

In any event, the force is the weight of the can, plus the force needed to decelerate the can from its falling speed to zero.  That force will depend on the physical properties of the string.  If it’s really stretchy and elastic, the deceleration will take place over a longer period of time, and the force won’t be that great.  But if it’s a very non-stretchy string, the deceleration will be very fast, and the force will be high; in this case, the weight of the can would not have to be that much.  But the force needed to pull the trigger would have to be less than the force that breaks the string.

[JayUtah]

Potential energy becomes kinetic energy during the fall. The longer the fall, the more kinetic energy. If the can is initially at the same height as the trigger, the first can reaches the end of a string of length L at a velocity



where g is the gravitational acceleration on whatever planet you're on, in whatever system you prefer. Potential energy is



where m is the mass of the water and can. At the end of the fall, all that potential energy is converted to an equivalent amount of kinetic energy



thus



Mass cancels, and algebra lets you solve for velocity in terms of height. There are, of course, more straightforward ways of determining the velocity of an object at the end of a fall. But using the energy formulation helps you understand where the energy comes from and therefore why work is required to stop a falling object.

The exact force imparted to the trigger depends on the elasticity of the string and therefore the stopping distance. But the "extra" force beyond the mere static weight of the can is necessitated by the kinetic energy obtained in the fall.

A half-liter can falling 10 cm and stopping in 1 cm produces an average force across that distance of around 50 N, which is about twice the nominal trigger pull force of a 1903 Springfield rifle such as those issued to American soldiers in World War 1.

[Peter B]

Brilliant! Thank you both.

I was missing the effect of kinetic energy.

[bobdude11]

Found this documentary on the 'Drip Rifle' - looks like they tied a rock to the lower string a little above the lower can and when the lower can had sufficient weight, it pulled the rock down that in turn pulled the trigger:

EDIT: changed 'the' to 'they' and corrected spelling of 'pulled'

[JayUtah]

It looks like the can and rock drop together, so their combined mass goes into the equations to determine the force applied to the trigger.

However the video illustrates an important principle that we try to use whenever necessary in mechanical designs. Where we must apply a large amount of force under precise control, we don't necessarily try to couple the control input and the actuating force directly. We'll often do something like just move the fulcrum of a lever slightly or—as in this case—just tip the balance. The actuating force that's thereby turned loose can be provided by a strong spring, a gas accumulator, or (here) gravity. This solves the inherent problem of controlling large actuating forces with precision. You generally can have lots of force or energy, or precise control, but not both if you try to apply direct control.

[Peter B]

This was also the campaign in which Australian soldiers developed the periscope rifle. Sticking your head above the parapet to shoot was obviously dangerous, so they developed another modification of their rifle. In this case it involved attaching a framework of wood to the butt of the rifle which incorporated two mirrors aligned like the mirrors in a periscope, with the top mirror also aligned with the rifle's sight.

Ingenuity in the service of warfare, but still ingenious.

[bobdude11]

It looks like the can and rock drop together, so their combined mass goes into the equations to determine the force applied to the trigger.

However the video illustrates an important principle that we try to use whenever necessary in mechanical designs. Where we must apply a large amount of force under precise control, we don't necessarily try to couple the control input and the actuating force directly. We'll often do something like just move the fulcrum of a lever slightly or—as in this case—just tip the balance. The actuating force that's thereby turned loose can be provided by a strong spring, a gas accumulator, or (here) gravity. This solves the inherent problem of controlling large actuating forces with precision. You generally can have lots of force or energy, or precise control, but not both if you try to apply direct control.

I ALWAYS learn something when I read your posts! Thank yo so much for sharing your expertise!

[bobdude11]

This was also the campaign in which Australian soldiers developed the periscope rifle. Sticking your head above the parapet to shoot was obviously dangerous, so they developed another modification of their rifle. In this case it involved attaching a framework of wood to the butt of the rifle which incorporated two mirrors aligned like the mirrors in a periscope, with the top mirror also aligned with the rifle's sight.

Ingenuity in the service of warfare, but still ingenious.

I remember reading about the periscope rifles ... forgot they were from Aussie ingenuity!