Gap shooting uphill

oceanjaws

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Can someone explain the physics of this to me?
Why is it that when we are shooting uphill, we have to compensate with the gap, and aim a good deal higher, even at short distances (like 20yds).
I don't quite get it. Is there a sight line change we have to consider, or is it just harder for arrows to go 'up'?
 

Ian

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First port of call is to check that your draw length stays the same.
Shooting steep up shots can shorten your draw length if you do not get your stance right.
Shooting uphill should only make a minimal change to your gaps.
 

wully

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Shooting up or down hill you should be aiming low...if not you are doing something to your form when lifting or lowering your bow.
The short answer as to why this is that the arrow is less exposed to gravity when shooting a vertical angle off horizontal, counter intuitive but “Ye Cannae argue with the laws of physics Captain”...
 

oceanjaws

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Thanks. Yers it makes sense actually. I found this article which says
"The key to understanding an arrow’s trajectory on uphill and downhill shots is realizing that gravity only acts perpendicular to the earth’s surface. Thus an arrow is only acted on by gravity for the distance that it travels parallel to the earth, or rather only the horizontal distance. This results in having to aim low for both downhill and uphill shots!"

I'll have to assess my form when shooting up.
I mean shooting arrows upwards. Just in case the police are monitoring this thread.

 

geoffretired

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When we shoot arrows, they drop towards the ground under the force of gravity. gravity pulls them down but doesn't really slow them down.( wind resistance does that.) If you shot an arrow vertically upwards, gravity will slow it down but not change its direction, until it turned over to fall. If you shot vertically down wards, the arrow would speed up but would not change direction. Shooting steeply up or down means gravity tends to change the speed of the arrow more than shooting level; while pulling it off course, less. Our aiming is dependent on how steeply up/down we are shooting because gravity has a greater/lesser effect on the way the arrow's path curves down
 

oceanjaws

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Right....
This also makes sense!
So now I'm thinking that if I'm shooting up at a target which is 20 yards away, at a 45º angle, I'll have to aim slightly higher than I would shoot for a target 20 yards away on my horizontal plane, because gravity will slow the arrow down by pulling it down on the vertical. Thus the vertical velocity of the arrow is reduced, affecting its overall speed.
This morning in the woods I was shooting up at a target about 30 yards away and about 20º up from me. I guess this sort of practice is valuable too. I was starting to get a feel for this kind of shot, and I'll do more on it.
Thanks Geoff :)
 

oceanjaws

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In the article I quoted above, the writer says
"Thus an arrow is only acted on by gravity for the distance that it travels parallel to the earth, or rather only the horizontal distance. This results in having to aim low for both downhill and uphill shots!"
but now I'm thinking he should have said "for the time it travels through the air'. After all, if an arrow is stationary mid-air, gravity will still act on it, even though it isn't travelling any distance parallel to the earth.
 

Geophys2

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The article is perfectly correct, for both uphill and downhill shots it is the horizontal distance that counts, not the direct distance to the target. Gravity only operates on the horizontal plane. Imagine shooting on a hill with a target at the top and another at the bottom. No matter if you are shooting from the bottom to the top or visa versa, the horizontal distance would be the same and would be shorter than the direct path from the shooting position to the target. As the slope of the hill is the hypotenuse of the triangle it is greater than the horizontal distance, so your sight would need to be set for the shorter horizontal distance both shooting up and down the hill. There are a number of archery sight apps that will calculate it for you. Trust me I'm a physicist!
 

oceanjaws

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But if the arrow is in the air for longer because the hypotenuse is bigger, then doesn't gravity have more time to act on the arrow, even if it only acts on the horizontal plane.
To take it ad absurdum - a target 10 yards away on the horizontal plane, compared to a target 10 yards in front of me, but 70 yards up, on top of a cliff. To my eye, I'm aiming a good deal 'below' the first target. Surely this can't be true of the second target.

I do believe you are right, because you are a physicist and I am but a humble musician, but I'd love to know why. I'm shooting TBH, so I have no sights. Just shooting off the shelf with a recurve. I'm only saying this in case your explanation is framed by a particular set-up.
Thanks :)
 

geoffretired

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The issue, I think, needs to take into account how far is the arrow travelling, from archer to target. That is the usual way of measuring its distance over the ground. If the target happens to be much higher than the level we usually experience, then the distance( hypotenuse for the arrow) is longer than the horizontal distance( base of triangle) That would lead us into aiming lower; not higher. Higher would seem logical because we feel the arrow has a harder journey ; as we would feel if walking.
 
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