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Need help on a physics question!
Whitetail Deer
Contributors to this thread:
David A. 17-Jan-19
David A. 17-Jan-19
sticksender 17-Jan-19
moosenelson 17-Jan-19
Lost Arra 17-Jan-19
David A. 17-Jan-19
GF 17-Jan-19
Bob H in NH 17-Jan-19
elkstabber 17-Jan-19
BuzAL 17-Jan-19
skookumjt 17-Jan-19
Ambush 17-Jan-19
GF 17-Jan-19
Franzen 17-Jan-19
rgb 17-Jan-19
GF 17-Jan-19
Ambush 17-Jan-19
GF 17-Jan-19
Ambush 17-Jan-19
Woods Walker 17-Jan-19
BTM 17-Jan-19
GF 17-Jan-19
PTArcher1 18-Jan-19
elkstabber 18-Jan-19
elkstabber 18-Jan-19
Muskrat 18-Jan-19
Amoebus 18-Jan-19
ben yehuda 18-Jan-19
Woods Walker 18-Jan-19
Ambush 18-Jan-19
PTArcher1 18-Jan-19
Hunter II 18-Jan-19
Hunter II 18-Jan-19
bear bowman 18-Jan-19
GF 18-Jan-19
BuzAL 18-Jan-19
lawdy 18-Jan-19
PTArcher1 18-Jan-19
BTM 19-Jan-19
David A. 19-Jan-19
Ambush 19-Jan-19
LINK 19-Jan-19
Ambush 19-Jan-19
GF 19-Jan-19
1boonr 20-Jan-19
Vids 20-Jan-19
BTM 20-Jan-19
Vids 20-Jan-19
djb 20-Jan-19
David A. 20-Jan-19
David A. 20-Jan-19
David A. 20-Jan-19
David A. 20-Jan-19
BTM 21-Jan-19
HDE 21-Jan-19
From: David A.
17-Jan-19
Anyone smart enough to figure out what the deer drop calculations would be for an arrow speed of 180 fps at the yardages in this table using the experimental data shown? This would be very helpful for trad. bowhunters! I already know to keep my shots close, but specific data would be helpful to many. Thanks in advance for anyone smart enough to do actual calculations based on this data!

This data was obtained from this research: https://tinyurl.com/yb5eah26

From: David A.
17-Jan-19
Actually, if you're smart enough to do the calculations, could you maybe do two arrow speeds of 170 and 200 fps instead of 180 fps? That would pretty much cover the range of better longbows and recurves. Thanks in advance for any help!

From: sticksender
17-Jan-19
That's a great video. I think you'd want to do actual testing at those slower trad-bow speeds, in order to get useful data.

From: moosenelson
17-Jan-19
Drop of 7......Too easy.

From: Lost Arra
17-Jan-19
Thanks for posting David. Very good videos of research and hunting shots.

From: David A.
17-Jan-19

David A.'s embedded Photo
David A.'s embedded Photo
Here's the table I alluded to.

From: GF
17-Jan-19
You can calculate distance traveled as time in free fall, but the deer that get clear of the arrow aren’t just “dropping”; they’re getting the hell out of the way.

If a MLB pitcher rips a heater at your head, you’re not going to just let your legs go limp and let gravity to the work, are you?

I’m going to guess that if the answer to that is “No”, it would be more accurate to say “Not twice”...

If you know what I’m sayin’....

From: Bob H in NH
17-Jan-19
Deer drops by gravity, easy enough However arrow is travelling in an arc so a bit harder.

Also you need to kow when the deer starts dropping

From: elkstabber
17-Jan-19
The physics is simple. 10th grade stuff.

What GF said is true, the deer aren't just letting gravity do the work. When a deer's head is down it pulls its head up, which drops its body. This occurs much faster could gravity could have done it.

All of the deer that tried to duck my arrow in the last couple of years had their heads down when I shot. It's better to shoot when a deer's head is up because then it has to wait for gravity to load up its legs. That's what I'm doing from now on.

From: BuzAL
17-Jan-19
LOL! Unless a deer or batter can clutch the ground with his toes to jerk hisself downward, gravity alone is causing any and all dropping.

From: skookumjt
17-Jan-19
Deer ducking is both gravity and the deer using muscles to duck. As said above, you don't just fall out of the way when someone throws a ball or a punch at you.

From: Ambush
17-Jan-19
Any downward movement is strictly gravity. However, if the deer has its head down the weight of its head coming up can eccelarate the body’s drop in a counter affect. Same for legs. Any lateral or roll movement is muscle.

From: GF
17-Jan-19
So Buz...

Why do you think they’ve evolved hooves??

(Hint - it’s the same reason that batters dig in their CLEATS),,

Head up or down; a deer that wants to get out of the way is going to contract it’s abdominals and hip flexors - HARD - to bring its front end downward.

Their center of gravity is to the rear, so the get plenty of traction.

Human example: Martial artists smashing bricks with their foreheads. They’re not tied to the ground, and they’re not just letting gravity do the work.

Extreme example, but those are most easily grasped.

From: Franzen
17-Jan-19
Oh boy, not this again. I guess it's been a minute since this argument was hashed out.

We would need more than the simplistic table posted to really be of any help. Is the deer "drop" based on linear or non-linear regression?

From: rgb
17-Jan-19
Isn't the deer tightening its muscles and bending its legs to push against the ground when it is crouching to spring away? This has nothing to do with gravity! It is leverage instead. If the deer folded its feet out of the way, allowing its torso to free fall, now that would create a situation of gravity forcing it downward.

Or am I missing something here?

From: GF
17-Jan-19
OK, let’s make this simple.

Stand with your feet shoulder-width apart. Knees bent. Bend at the hip ‘til your torso is parallel to the floor. Now pretend you’re trying to smash a fly that is 6” below your forehead.

Can you do it?

From: Ambush
17-Jan-19
rgb, watch the video and watch the actual deer shots a few times. Freeze frame through them even.

GF, did you watch the video and if not, then please do. Carefully watch the slo-mo's and pay special attention to the feet and head position.

From: GF
17-Jan-19
I did see it. Or one just like it.

Head down gives them a counterweight that could help them drive their forequarters downward (equal and opposite reactions and all), but having it up does not immobilize the abdominals OR the hip flexors.

From: Ambush
17-Jan-19
I'm going back to the garage and for another plywood slab to-the-forehead treatment.

From: Woods Walker
17-Jan-19
I always sucked at math word problems. "If a train going at 45 MPH during the full of the moon in a leap year.......etc..."

From: BTM
17-Jan-19
Physics 101: Draw a free-body diagram showing the forces acting on that body and go from there. If the head is up and the hooves aren't stuck in mud, there's only one downward force acting to move the deer down. (Hint: That force starts with a "G".)

From: GF
17-Jan-19
Doesn’t matter what you know about Physics if you lack even a basic grasp of anatomy, reflexes, etc.

And just because you can only explain part of what’s happening doesn’t mean that that’s all that’s involved.

We’re not dealing with dead weight in a vacuum, we’re talking about critters that have evolved over millions of years to dodge predators which are trying to land on the shoulders of their prey, and the survivors all have one thing in common: they made the predators MISS.

The abs and hip flexors are ABSOLUTELY capable of driving the shoulders downward, regardless of head position. But like Ed Koch used to say: I can explain it to you; I can’t understand it for you.

From: PTArcher1
18-Jan-19
Sorry folks, BTM is right on this one. The squatting motion a deer performs on the "ducking" is all about eccentrically loading those muscles required for it to then spring forward or out of the way. This is the same thing we do when jumping. This eccentric loading is accomplished by the only downward force available to it, gravity. GF, you are right that the hip flexors and abdominals are capable of driving the shoulders downward. But in order for that to happen, the feet would have to be fixed to the ground to provide a solid base to pull from. That is not the case normally in nature. I believe most are underestimating the acceleration effect of gravity. It will take only 0.15 sec for a deer weighing approx. 110 lbs to drop 6 inches, all due to the effect of gravity. This is faster than an arrow traveling at 280 ft/sec, which would arrive to a deer 20 yds away in 0.21 sec from release. Granted I didn't account for the speed of sound to alert the deer, but you should get the gist of the situation. It's all about gravity!

From: elkstabber
18-Jan-19
WOW, just WOW PTArcher1. You chose a time period of 0.15 seconds and determined that a 110 lb deer would drop 6 inches?

First of all it is basic Physics that tells us that the deer's weight is irrelevant, so your choice of 110 lbs shows that you're not really good at Physics.

Here is a screenshot of the time that an object will freefall in 0.15 seconds. The second screenshot shows the conversion from feet to inches. The drop is a little over 4".

The good news is that your calculation for the arrow's speed was correct.

Now that we've got our Physics straight I still concur with GF in that the deer's body is not going limp and letting gravity take over. If the body went limp than we could use a free body diagram. The deer's body is moving faster than gravity, especially if the deer's head is down. When the deer's head raises rapidly the body drops rapidly, this is called Dynamics.

From: elkstabber
18-Jan-19

elkstabber's embedded Photo
Time versus Distance for a freefalling object
elkstabber's embedded Photo
Time versus Distance for a freefalling object
elkstabber's embedded Photo
Conversion to inches
elkstabber's embedded Photo
Conversion to inches
Here are the images that wouldn't post in the previous post.

From: Muskrat
18-Jan-19
IF the deer could grip something below it to pull itself downward, maybe it could increase the speed at which it descends due to gravity. Now, if the deer's feet and lower legs were of greater mass than the body they are supporting, the flexing of leg muscles to crouch could serve to add to the speed with which gravity moves the body downward, but that ain't the case. Gravity by itself allows unsupported objects to fall rather quickly, quicker than we at first assume.

From: Amoebus
18-Jan-19
You would also have to figure in the reaction time of the deer - which, presumably could vary from almost immediate to non-existent.

18-Jan-19
Let's round up a professional physicist for the next Bowsite interview ;)

From: Woods Walker
18-Jan-19
Don't forget the Coriolis force factor!

From: Ambush
18-Jan-19
Wouldn't "The Void" also add buoyancy? I realize not all deer have The Void and it seems it occurs more in some states than others and also may be broad head related. Might help explain why some deer drop faster. But still that hot air pocket would be like having a hot air balloon attached to the deer's back.

From: PTArcher1
18-Jan-19
Sorry elkstabber, was doing the math in my head at 3:30 am. I was off by .02 seconds. ( I guess that's worthy of a 'just wow') I mentioned the weight only as it relates to the amount of force impact of the fall. This is part of the Splat calculator and you're right, doesn't change the speed of the fall. However, if you want to talk straight physics, as you should, there is still no other force the deer can call upon to speed up its drop. The deer cannot simply pull itself downward!

From: Hunter II
18-Jan-19

Hunter II's Link
This is the buck I shot this fall. It appears that all four of his feet came off the ground as he reacted to the sound of the bow and began to bolt. Real time video

From: Hunter II
18-Jan-19

Hunter II's Link
Screen grabs showing reaction. Shot distance was 25 yards.

From: bear bowman
18-Jan-19
Are they reacting to the sound of the bow or the sound of the arrow in flight, or both?

From: GF
18-Jan-19
"Now, if the deer's feet and lower legs were of greater mass than the body they are supporting, the flexing of leg muscles to crouch could serve to add to the speed with which gravity moves the body downward, but that ain't the case. "

Where do you suppose the CG of a deer is located?

Mine has always been south of my navel, but maybe you guys are after bucks that really only work out on Bench.

Dave - I didn't see the right rear hoof leave the ground until really late i the sequence, but what I DID see...

Definite activation of the abs

Looks like he tried to roll away from the sound of the bow, at lest to a degree

Spine doesn't seem to lose any altitude until the arrow is past the left side of his rack (moving right-left only) across the screen.

So essentially all of the vertical movement comes in what, maybe 5 frames? I believe that is faster than acceleration due to gravity...

But I'll let somebody else run the math...

And just something to bear in mind - the way that any given deer will be able to react does depend heavily on its weight distribution at the time, so you can't look at only one deer ducking one arrow and decide that that's how they all do it, all of the time.

JMO, just too bad that this particular shot didn't get off until the deer was locked onto the archer. With his head up, licking that branch, he would have had to have reacted very differently.

And FWIW, it'd be interesting to know what you got off of the trail cam on that tree....

From: BuzAL
18-Jan-19
Perfect example of someone taking out their ass. Sit down and listen to the grownups.

From: lawdy
18-Jan-19
As a former Physics teacher, we did an experiment back before guns were banned on school grounds. Longbow versus C-bow with overdraw, and a .22 versus a BB gun. If an arrow is shot at an angle, and all bullets and arrows are, you get into trig ratios. The most memorable thing about that lab was watching the limbs blow off that compound with the overdraw. All weapons were fired at angles of 0, 30, 45, and 60 degrees on a dead calm, cloudy day. Using data collected, the students were able to calculate flight times, projectile velocities, etc. My favorite lab was with vectors and a bomb. We replayed the system used to pinpoint which exact vehicle held the bomb in the first World Trade Center bombing. If I had more info, I would write this up and give it to the lady who filled my position at school and let the kids work it. Was the shot from a tree? How high? Did the deer drop and whirl at the same time? A lot of variables.

From: PTArcher1
18-Jan-19
Ok, my final post on this. After actually studying this more, I believe we are all accurate to some degree, but not fully. (me perhaps less than others) It is the center of mass of the deer than can and will only drop at the speed of gravity's acceleration. That is straight physics and cannot be refuted. However, when watching a film of a deer dropping, it becomes obvious that the vital area does indeed move downward faster than it's center of mass, which will be located more toward the rear of the animal. Meaning the front of the deer drops further than the rear of the deer. The deer is able to use its musculature to accelerate its vital region downward since it is stabilized somewhat by its distant center of gravity, located more to the rear. Thus the angular velocity of the vitals is indeed moving faster than the force of gravity acting upon it. I now consider myself more educated on this topic. Thanks! :)

From: BTM
19-Jan-19

BTM's embedded Photo
BTM's embedded Photo
BTM's embedded Photo
BTM's embedded Photo
BTM's embedded Photo
BTM's embedded Photo
BTM's embedded Photo
BTM's embedded Photo
These four pics from my stabilizer camera show how much a deer can drop and whirl in only half a second. Fortunately, the hit was still fatal and I recovered him 100 yards away.

From: David A.
19-Jan-19
Anyone dare to take a stab at my request and summarize your calculations?

"Actually, if you're smart enough to do the calculations, could you maybe do two arrow speeds of 170 and 200 fps instead of 180 fps? That would pretty much cover the range of better longbows and recurves. Thanks in advance for any help!"

From: Ambush
19-Jan-19
Sorry David, but protocol dictates that once a topic has been derailed it must not get back on the track. Rules are rules.

From: LINK
19-Jan-19
I’m definitely aiming 10” low on all my 40 yard shots in the future, next level stuff.

From: Ambush
19-Jan-19
So LINK, can that be accurately extrapolated to 30” at 120 yards? What other factors may play at that range? Can’t think of any, but you never know.

From: GF
19-Jan-19
So.... all the way back to the OP..... by the OP....

“From: David A.17-Jan-19: Anyone smart enough to figure out what the deer drop calculations would be for an arrow speed of 180 fps at the yardages in this table using the experimental data shown? This would be very helpful for trad. bowhunters!”

Lemme ask you this...

HOW is it helpful to know how much a deer can drop in whatever given period of time?

Sometimes they do. Sometimes they don’t. Sometimes they are positioned such that they can very likely move at speeds in excess of acceleration due to gravity, and at other times, maybe not so much.

Point being, you can measure and calculate, but you will almost never guess exactly right. What’s the exact range on the shot? Will the animal react to the shot or not? If it does, will the reaction be mild or volcanic?

You’ll never know.

All you can do is get deadly-accurate close to an unsuspecting animal and make the best shot you can make. 1/3 up the body is ideal because about the top 1/3 is all meat and bone anyway.

But FWIW, if they’re right underneath you, they can drop all they want and it makes no difference because they’re not getting out of the way anyhow.

From: 1boonr
20-Jan-19
You do not need to know the answer. Just get as close as possible and shoot deer that aren’t alarmed. I have shot a bunch of deer with a very slow recurve and never even thought of calculating anything other than how much meat I thought I was gonna get etc.

From: Vids
20-Jan-19
For those of you claiming that gravity is the only force dropping the deer: When you swing a hammer at a nail is gravity doing all the work or are your muscles helping to create a force greater than gravitational pull alone?

From: BTM
20-Jan-19
Vids: Any downward force you apply to the hammer must be offset by a slight lifting of your body. Forces can't be created out of nowhere; they must balance. (That's one of the first things you learn in Physics, Engineering, and many other scientific disciplines.) If you want proof, stand on a scale while you swing the hammer. The scale will register a slight decrease as you swing the hammer downward.

From: Vids
20-Jan-19
BTM: I agree, but that doesn't mean the hammer's downward speed is limited to the rate of acceleration that gravity provides. Another example: splitting wood. Try to split a piece by letting the axe drop using gravity alone. Doesn't work too well. Accelerating the axe more by using your muscles creates more velocity which translates to momentum (force) which helps split the wood.

We're talking about the velocity at which the deer's chest cavity drops, not the amount of force that is applied to the ground under the deer's hooves. The hooves may lift slightly to offset the forces, but there are muscles dropping the chest faster than what gravity alone would do.

From: djb
20-Jan-19
I saw this and thought it relates to the question about dropping faster then gravity. Is it possible to squat quicker than gravity? https://physics.stackexchange.com/questions/439511/is-it-possible-to-squat-quicker-than-gravity

From: David A.
20-Jan-19

David A.'s embedded Photo
David A.'s embedded Photo
David A.'s embedded Photo
David A.'s embedded Photo
I'm trying to quantify it for trad. bowhunters, vs. "just aim a little low".

From: David A.
20-Jan-19
Just to add, I hit a nice whitetail in the shoulder blade last year. Might have been because I tried to place the shot too close to the shoulder crease (a bad habit from all the whitetails I took when I was younger and a rifle hunter) or it might have been because the buck dropped enough so the shoulder blade was brought into play on an otherwise excellent shot...I'll never really know. Having these facts, however can help me and others. Sure, I know to aim a little low. I just want to have as many facts as possible.

From: David A.
20-Jan-19
oops, I wanted to delete the top pic....oh well.

From: David A.
20-Jan-19
N/T

From: BTM
21-Jan-19
Vids: I chop tons of wood every year, so I understand. However, the operation involved in swinging an axe is different from a deer relaxing his leg muscles to make himself drop when he hears a noise that panics him. The only way a deer can drop faster than the acceleration due to the force of gravity is if his feet are attached to the ground. In that case, he can PULL himself downward even faster than what gravity will do.

Of course, if his feet are attached to the ground, he has other problems, but I digress. :)

For those that still think a deer can make himself accelerate downward faster than the acceleration due to the force of gravity, swing by the nearest high school and spend two minutes with a physics teacher--or one of his students.

I'm done with this thread because I need to explain to my HGTV-obsessed sister-in-law why she can't remove a load-bearing wall willy-nilly without inserting a glu-lam into the attic. :)

From: HDE
21-Jan-19
and the members of Team Bowsite Wolf Pack say I'm clueless and post stupid stuff...

There are some posts on this thread that make the great "TBM" look like a genius...

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