1. ## Half Life 2 Question about physics...

...OK...to preface this...let me say that as good as I am at games like Q3 and UT2004, I will never be as good as a 12year-old that doesn't think like a physicist. You can figure out why...

I keep hearing how Half Life's physics engine is so great. So, I have a simple question:

When moving left, sliding (strafing) and firing a projectile straight ahead, does it track to the left, consistant with Newton's first law, or does it (like every other FPS) move only in one direction, straight ahead, ignoring it's initial momentum?

2. That's a good question... but all I can say is, you are definitely more accurate when you stop, crouch, concentrate and aim.

In other words I'll have to check when I get home.

Then again... when I think about it... when you're aiming at something (in the game) you are already compensating for the angular momentum, because you are adjusting your aim as you strafe left and right, so I don't know if you can really tell if the bullets are moving left or right... so I guess the test will be strafing to the left and not aiming at anything.

3. If you wanna get into specifics, it SHOULD move to the left if ur strafing left, but don't forget, you're probably "running" at a rate of probably no more than 10 mph while a bullet travels how fast? (i don't know) Do you really think the addition of the vectors is going to make that much of a difference?

4. Maybe if you were shooting at very long ranges it would matter, but in a small room how much could it matter?

5. so there is no rocket launcher or grenade launcher in half life 2?

(the bullets don't concern me...the slower projectiles do.

6. I would probably guess they didn't program that into the physics engine but they set it up that if you are moving your aim error changes. by a certain factor for running, jumping, straffing, standing and crouching. That is how most games before it have done it, I don't see them going to the work since you wouldn't really notice it unless it is over a long distance.

Cause mostly in real life if you are running sideways and firing your gun the other way like you describe. You aren't goign to be accureate, but if you are say walking/jogging you are more. Then again if you are moving sideways and trying to fire accurately you are probably not moving at 10mph

7. I was playing with the crossbow tonight on DM and it does appear the bolt will move slightly to the left if you shoot it while strafing left. Hard to do extensive testing since you die so fast though .

BTW. Yes there is a rocket launcher in HL2.

8. The rocket launcher is guided so it won't matter, heh.

9. Originally Posted by Cantacuzene
The rocket launcher is guided so it won't matter, heh.
I was going to say the same thing...

I just tried it using the gravity gun, and I don't see any noticeable tendency for fired objects to continue moving in the direction you are strafing...even with big, heavy items. What I did was pick something up, like a barrel, and then strafe left or right until the weapons sight reached a target in the distance. After firing (while still moving), I stopped and watched the path of the fired object. In every case, it impacted exactly where I was pointing when I fired. If it kept its sideways momentum, it should have missed by several feet. However, I think the biggest factor is how slowly you move when strafing--even if you hold the "sprint" key, you don't strafe any faster.

Now if you could use the gravity gun from a moving vehicle or something, maybe that would be a better test.

All in all, the physics of this game are impressive...I'll probably be done with it this weekend. I decided to finish Medal of Honor: Pacific Assault first instead, so I could enjoy HL2 without distraction...

10. I am no Physicist but I would have to agree that at the speed you are straffing and the quick firing of a weapon, the amound of left/right trend would be very minimal, even at a small distance. The bullet leaves the barrell so quickly, that in relation to your running speed, there would not be that much a push to either side.

11. Right, even if you are running at 5mph, the bullet is only going to veer to the side at 5mph, so when compared to the 600mph+ that bullets travel at that is really insignificant except at the longest distances.

12. I'm not referring to bullets. I'm referring to low-velocity projectiles and dumb-fire rockets, like grenade and rocket launchers.

13. Originally Posted by Bires
I'm not referring to bullets. I'm referring to low-velocity projectiles and dumb-fire rockets, like grenade and rocket launchers.
That's why I tested it by throwing big heavy things with the gravity gun--even throwing a barrel as far as I could didn't result in any noticeably curved flight path.

14. It wouldn't be curved, it'd just be traveling at an angle.

15. Ya...it's really too bad they missed this. I get kinda irritated every time I hear someone describe HF2 as "having great physics" because bodies fall over like rag dolls instead of blocks, but projectiles still don't act like they should.

I figure they must have thought about it, but imagined the gaming world is not ready for it, as it would be a considerable shift in FPSs.

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