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So assuming an open field and there are targets set up away from you. Now you are shooting magic spears or spears that defy gravity(I don't know what I'm talking about). You have the option to have them fly straight at the target or you could throw them at an angle so that the spear would arc and hit the target. Why would someone conceivable throw it so that it flies at an arc then?

I have read one or two novels where someone would fire off projectiles so that they fly in an arc to hit the targets instead of just having them fly straight(for example, a fireball) and it just didn't make sense. Having them fly off in a straight line would allow them to reach the target faster and with more velocity instead as 1) in order for the projectile to arc, it would mean the projectile slowed down and 2) there's probably a much higher chance of missing than if you shot it straight. Which begs the question of why did they do that?

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    $\begingroup$ The most obvious reason is if there's a wall or low cover in front of the target. A gravity-defying spear would just fly off into space though, not arc over the wall. $\endgroup$ – Cyrus Sep 1 '16 at 12:54
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    $\begingroup$ With some slight deviation from air resistance, yes. You couldn't throw them in an arc without applying some force changing their direction. $\endgroup$ – Cyrus Sep 1 '16 at 13:06
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    $\begingroup$ I argue that if I rain spears above your head chances are you won't notice my fireballs coming straight for your face! $\endgroup$ – user6760 Sep 1 '16 at 13:16
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    $\begingroup$ If the magic spear defies gravity, how could it possibly fly in an arc? What force would bend its trajectory? $\endgroup$ – Polygnome Sep 1 '16 at 17:00
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    $\begingroup$ Launched objects don't arc because they slow down. They arc because they are accelerated by gravity. Theoretically, the object's horizontal velocity is constant once it is released (this neglects air resistance) and only the vertical component of velocity is affected by gravity. $\endgroup$ – MPW Sep 1 '16 at 17:01
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Also known as plunging or indirect fire, this technique is intentionally used to accomplish a few goals.

1) Target Background When you use plunging fire, even if you miss, you won't hit something far off in the background. This can be used to prevent collateral damage.

2) Proximal Fire Projectiles that explode, such as artillery shells or classic fireballs, will impact and detonate near the target, if you only miss by a little bit. This can vastly increase your chances of a successful attack.

3) Armour Mitigation For armoured targets, plunging fire can often hit more lightly armoured overhead areas like turret tops or open decks. In certain fantastic scenarios, plunging fire will pierce a target that is supported by the ground, instead of knocking it backwards to arise unharmed. Compare hitting a hanging piñata from the side to hitting it with an over hand swing when it's on the ground.

4) Overcoming Intervening Terrain It may seem obvious, but it's worth mentioning for the sake of completeness that walls or other objects can be avoided in this way. This includes scenarios where your target is at a higher altitude than you.

5) Remaining Hidden May not apply to your specific scenario, but sometimes an attacker will want to take advantage of the intervening obstacles to remain hidden themselves while still being able to attack.

EDIT: 6) Preservation Of Velocity I had to think hard to remember this one. With indirect fire, your projectile will be driven by gravity to a velocity determined by altitude to a maximum of the projectile's terminal velocity. In certain circumstances, plunging fire may impact at a higher velocity than direct fire.

EDIT: 7) Ranging Where visual feedback is required to help zero in on a target, and the projectile is not visible (high-speed artillery shells, invisible magic force ball, etc.), indirect fire will impact somewhere near the target, allowing one to adjust their aim. Direct fire that misses will likely provide little to no visual feedback with such projectiles.

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  • $\begingroup$ #2 here is the one I was thinking. Imagining a circular shockwave extending from the point of impact, if you hit a target straight on the shockwave will extend out vertically on the Y-Axis, not causing much effect on surrounding troops (assuming it literally traveled in a straight circle outward), but with an arc letting the projectile come down and impact the ground the shockwave would travel horizontally on the X-axis possibly leveling the whole battleground. $\endgroup$ – DasBeasto Sep 1 '16 at 16:53
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    $\begingroup$ If the enemy is trying to avoid the projectile (and for example with suppressive fire that's not just an added bonus, it's the main goal and any damage is a bonus), then you might find that a high-arcing projectile makes a large number of targets think it might be coming in their direction, and take evasive action, whereas with a low flat path it's easier to judge where it's headed and so more targets can safely ignore it. $\endgroup$ – Steve Jessop Sep 1 '16 at 17:34
  • $\begingroup$ @DasBeasto True, but the point is that if the shell/fireball/whatever flies past the target and hits something far off in the background, it won't have any kind of effect at all. The geometry of a hit is less relevant; it's a hit already, after all. $\endgroup$ – Lord Dust Sep 1 '16 at 18:00
  • $\begingroup$ @Steve Jessop Indeed, that is the guiding principle behind a "creeping barrage". Good point. $\endgroup$ – Lord Dust Sep 1 '16 at 18:01
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    $\begingroup$ @Sarfaraaz It's true, they do mention that. They also mention fireballs. They don't mention how an arc might develop without any kind of gravity effect. Perhaps not coincidentally, they also mention that they don't know what they're talking about. I interpret the question to mean that gravity is in effect, but the magic spear can be imparted with superhuman force, so as to describe a very tall ballistic arc. $\endgroup$ – Lord Dust Sep 2 '16 at 6:26
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To avoid other targets

Hit the guy behind the guy in front. Swoop it under his legs or over his head.

predictability

Curves are harder to predict than straight lines and if you switch between them this will increase your chances of hitting but it also requires your own understanding of arcs to be quite good. Imagine throwing the spear in a very high arc nearly vertical and then throwing a few straight spears (and maybe one more curved one for the heck of it) while the other spear is still performing it's arc.

Intimidation

Things above you are always more intimidating then things at shoulder height or eye level, especially if they are inherently dangerous. Some people can be overcome with fear in these situations watching their doom come from the sky. (Most people who are better than 2bit in a fight can overcome this). Also, looking up leaves your neck exposed.

gravity force (conditional)

If the spears are magical that normally means their wielders have some kind of control over them. if the gravity defying bit can be turned off and on then throwing the spear up to come harder down would have more impact.

It's an optical illusion

Perhaps the target perceived the throw as curved due to slight dips in the land so the spear is being thrown straight but the land gets further away from the ground and then closer again during travel line.

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    $\begingroup$ Curves are really easy to predict with a bit of practice, that's how you can catch a thrown ball. It's bouncers that are hard to predict. $\endgroup$ – Separatrix Sep 1 '16 at 13:14
  • $\begingroup$ I'm not too sure about the intimidation part, I have played one too many dodgeball games where some fool throws the ball real high up and then someone on the enemy team just catches it. Or just dodge it. Is it really that easy to be consumed by fear? $\endgroup$ – Skye Sep 1 '16 at 13:15
  • $\begingroup$ @sky that relies on the what goes up must come down principal where it slows down as it goes up, making it less intimidating. the spear would be more akin to a baseball curveball, table tennis curveball, soccer curveball or cricket curveball (i don't actually know the technical names ) $\endgroup$ – Sarfaraaz Sep 1 '16 at 13:16
  • $\begingroup$ @JDługosz thanks I noticed that now when I added another point $\endgroup$ – Sarfaraaz Sep 1 '16 at 14:11
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    $\begingroup$ @Separatrix Actually, it is quite difficult—but human beings are naturally wired to be absurdly good at it. To the point that being good at it clearly must have been a very strong advantage that influenced our evolution in the past; we’re much too good at it for it to be coincidence. Suggests thrown or projectile weapons very early on in human development. Anyway, even with the advantages we have in that area, it’s still true that an arc is harder than a straight line. $\endgroup$ – KRyan Sep 1 '16 at 18:13
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Why arcs?

-To avoid obstructions/ friendly forces

-To change where on the target it hits (i.e. not on the shield the target is holding that protects the front, but not the side)

-To force the target to have to pay attention and distract him from other attacks (if he's watching you to move his shield, someone else's attack that he's not watching may get through)

-If the spear travels at a constant rate, to allow time on target salvos - if the arcing spear takes longer to arrive, you may be able to get a straight path spear to arrive at the same time - or if not, at least get them to arrive closer together so the target has less time to react.

-To signal other members of your force (e.g. arrange for an attack from the rear to start when you when you arc a spear in from the left, or whatever.

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    $\begingroup$ If your shield is low to protect you from the enemy in front of you, you are vulnerable to spears coming from a high angle and you can't really raise your shield or you find your enemy's sword in your intestines. Longshanks did this in Braveheart when ordering his archers to fire at the melee battle, not really caring about own losses. He had reserves as he said. $\endgroup$ – BentNielsen Sep 1 '16 at 17:26
  • $\begingroup$ "time on target" requires either the thrower to be ridiculously fast, or the time to reach the target or the first spear to be ridiculously long (which would involve the thrower being ridiculously strong). Today we can get that effect only with long range artillery pieces that do the process in an automatic way; it is of no use for anything shorter/smaller. $\endgroup$ – SJuan76 Sep 1 '16 at 19:05
  • $\begingroup$ Today I learned the word "ridiculously" :-P $\endgroup$ – SJuan76 Sep 1 '16 at 19:06
  • $\begingroup$ I've seen an artillery demonstration of point 4 -- very cool when both rounds hit the target at nearly the same time. The demo was with a short range target (short for artillery) where the team would send a high arcing shot and then lower the barrel to fire directly. This was in the 1980s with a small towed piece, no modern computers. $\endgroup$ – David Rouse Sep 1 '16 at 20:07
  • $\begingroup$ "Ridiculously fast" may not be all that ridiculous after all - some years back I saw an archery demonstration at Warwick castle, and if I remember correctly, the guy managed to fire a dozen arrows in about 10 seconds - way faster than I was expecting :) If you've got your spears set ready, then throwing 2 in 3 or 4 seconds doesn't sound too unreasonable.And if you look at typical javelin flight times, that might be fast enough... $\endgroup$ – JerryTheC Sep 1 '16 at 23:17
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Anyone who has ever been in a snowball fight knows that you would do both.

First fire a arcing snowball, and when your opponent looks upward to avoid it you fire the laser "Kill shoot"! game over.

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A number of good answers but I think anther possibility is to avoid Interception Fire hitting you.

If it is possible in this world to hit one of these projectiles out of the air with another projectile / energy beam / energy blast, your opponent intercepting an arcing projectile in less likely to cause injury to yourself.

Them firing at a Arching Projectile and missing would result in the intercepting shot flying off into space or hitting a random location if effected by gravity.

Them intercepting a shot that is travelling in a straight line directly at them and missing would result in the intercepting shot hitting (or very close to) the person that fired the initial shot.

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