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Hello and thanks for watching my video this video will an explanation and analysis of
my medium fiffer trebuchet if you haven't seen a fiffer trebuchet before check out First
in Fright they've got the big one that throws the farthest also I have some videos explaining
them on my channel if you haven't seen it move before here's a little bit of motion
counterweight comes down this scissors moves forward I've got a chain going over a cam
and.. that pulls the arm around and launches the projectile also note my full counterweight
stall as it comes down here we get a full stall before releasing the projectile which
is pretty good now I'm using a software here called tracker it's open source free if you
would old like to download it just Google physics tracker and you'll find it and it
works pretty well now to give you a little scale on this machine it's this blue line
from my throwing arm axle to the bottom of the machine 1.37 meters I've got about 2 meters
of counterweight fall my machines about, I'm going to switch units here, 8 feet tall this
is a 16 foot trailer my counterweight is 355 kilograms or about 800 pounds and my projectile
is 3.6 kilograms or a little over 8 pounds it's a 1 gallon milk jug filler with water
whenever you are analyzing a real world object you should check to make sure that your video
matches the real world results in some way I've done that three ways first we look at
the velocity here when we're releasing the projectile it says 23 meters per second if
we assume no wind resistance and a 45 degree launch angle that will get you about 50 meters
and I've stepped it off at 50 steps I've got about a one yard step so that's pretty close
there the second thing I noted the graph showing the counterweight acceleration you can see
that while it's free falling it's right around 10 meters per second squared and the acceleration
due to gravity is 9.8 so we're pretty close there I also in this frame the projectile
is 90 degrees to the arm and I know the acceleration there and the mass of the projectile the length
of the arm the length of the cam I can figure out the tension in the chain I got about 13.3
kilo newtons or 3000 pounds it also tells me the acceleration of my counterweight here
and through the geometry of the linkage I got a number within one percent so we can
assume that the video analysis is going to be accurate to our real world scenario the
main item I want to talk about in this video is the advantage that the fiffer design gives
you and it's all from this linkage here when the counterweight is all the way up you have
a lot of leverage the counterweight has a lot of leverage meaning it can start falling
and it falls a long way before the pull bar really starts moving which allows it to free
fall and gain the most velocity now as it comes down the leverage starts swinging the
other direction and if we were straight down we'd have infinite leverage pulling backwards
on this linkage so we can see that our counterweight has fallen over three quarters of the way
the pull bar has only moved half the and our arm has only moved you know a little ways
here so as we keep coming down the leverage gets so great that you can we actually end
up stalling the counterweight so a stalled counterweight means we've taken all the energy
out of the counterweight so we can see in this graph... we're released here we're falling
we've got negative velocity we're moving downward our velocities still negative and then in
between these two data points here we've switched from negative to positive so in between point
seven seconds and point seven three seconds we have stalled out our counterweight now
the counterweight does gain a slight upward velocity which means we're lifting it which
we don't want that's stealing energy from our projectile but that can be fixed with
tuning a lot of people talk about wanting a counterweight stall but it's not many machine
designs allow you to receive that along with the counterweight stall you can see the accelerations
of our counterweight so here here's free fall here it starts picking up pulling on the pull
bar is pulling on the chain and in here this is where our projectile start whipping around
and we get at the peak here 38 meters per second squared is our upward acceleration
so that four g's pulling up on the counterweight now that's happening at the same time our
projectile is accelerating and you can see here that from the start to oh half a second
you know not much has happened with our projectile but in this last .2 seconds our acceleration
goes way up now this is happening right in here we've started with the projectile right
in close to the arm and so it's not accelrating much but when it starts coming out away from
the arm it's accelerating the most pulling the hardest you can see here where its 90
degrees from the arm were it has the most leverage agaisnt the arm that's also where
we're getting the most upward acceleration on our counterweight and then we swing the
rest of the way around and release it now that's something that's not necessarily specific
to the fiffer um a lot fo machines these days will store there projectile to their throwing
arm axle and then allow it so swing around and I think that really helps take energy
it's pulling harder it's really taking nergy back away from the counterweight and putting
it into your projectile so LDVance would want us to talk about where is the energy in our
machine and how much are we getting into the projectile so if I count from where the counterweight
if I raise it from here the lowest point to the top I'm only getting about 25 percent
of my energy into my projectile now I've got my counterweight stall at this height if I
this here is rubber bumper with wood underneath if I move that up 4 inches then that would
probably make me 10 percent more efficient simply because this is where my stall is happening
and my projectile is releasing and it wouldn't count this here if I look at my peak velocity
of my counterweight so the peak velocity is right here we're at 3.8 meters per second
if I view that as kinetic energy I'm getting about 40 percent of the kinetic energy from
my counterweight into my projectile now I did say that my counterweight starts moving
up we can see the velocity of the counterweight right here somewhere between the point seven
and pint seven three second mark is when our counterweight starts going up now in our video
that's between these two frames and in the very next frame we've release the projectile
now some of this is from the trailer bouncing up when we look at the graphs of the projectile
we can see that the projectile has positive acceleration the entire time and the velocity
is going up the entire time so that lets us know that we actually haven't taken any energy
away from the projectile it's all in there the bouncing up I think is coming from the
trailer so that's a little misleading on our data here I don't think we'd get this positive
velocity if the machine was mounted on the ground instead of on a trailer so if I got
40 percent of my energy into my projectile where did the remaining 60 percent end up
well I think the biggest hog of my energy is my trailer I'm not really sure what the
capacity of my trailer is but it takes a fair amount to squat the shocks down and you can
see here that I squish the trailer down oh three inches maybe so that has taken a lot
of energy out plus the video started these were in the same location so my whole machine
had jumped backwards oh about 4 inches or so how would I fix these problems well first
at the competition I normally block the trailer so it doesn't squish I didn't do that here
because I was just hurriedly testing my machine so either block my trailer or set the machine
on the ground plus I could stake it or strap it so that the machine didn't move backwards
and I think that would gain a fair amount of efficiency just by doing those two little
things. Energy lost due to friction is also a big factor all of my axles are just steel
axles going through wood I don't have any bushing or bearing in those plus some of my
axles aren't very well aligned I had some drilling inaccuracies or measuring inaccuracies
when I made a couple of my axle holes so I am loosing some there the traditional fiffer
would have a pin in the back and an arm coming out that would guide the counterweight path
instead I've chosen the use of vertical guide similar to an F2K or a FAT trebuchet and at
the bottom here it's really pulling pretty hard like I said about 13 kilo newtons 3000
pounds pulling back now I get to divide that half of it's going up here half of it's going
down here I looked up the coefficient of friction steel on wood is about 0.2 now I waxed the
wood I couldn't find a coefficient for that I think that's helping me quite a bit compared
to the .2 if it was .2 that would be about 5 percent of my energy would be lost just
right there rubbing so I figure probably 2 percent I think the wax is making it twice
as slippery Finally what would I improve in this machine well the first thinking as I
mentioned before is that either block my trailer or put the machine on the ground so I don't
lose the energy there I also was thinking when I first ran this machine I was actually
getting I had the chain and cam set up so my arm instead of being straight down here
my arm was horizontal with the projectile sitting on top of it. The problem with that
was when my arm was here is where I was getting my counterweight stall. My counterweight had
down a little bit farther the arm was straight back and I could only get it to throw up or
backwards. So when I realized the stall was happening in the wrong spot I shortened this
chain that put the arm straight down and the I set the projectile as a temporary measure
on this log here now I've realized that maybe what I should have done instead of shortening
the chain would have been to shorten the arm so that it would have had less leverage and
let my counterweight fall just a little farther you know every couple inches down I gain is
more energy so I'm not sure where the diminishing returns would be my arm is already pretty
short you know put that's something I'd like to test as well. Well I think that's about
it for this video if you have any questions or comments please leave them below and good
luck chuckin'.