# Can't seem to figure out a trajectory for an object

**URL:** <https://forum.kirupa.com/t/cant-seem-to-figure-out-a-trajectory-for-an-object/231257>\
**Category:** programming\
**Created:** [July 3, 2007, 8:58am UTC](https://forum.kirupa.com/t/cant-seem-to-figure-out-a-trajectory-for-an-object/231257 "2007-07-03T08:58:45Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![Charleh](https://avatars.discourse-cdn.com/v4/letter/c/a9a28c/32.png) [@Charleh](https://forum.kirupa.com/u/Charleh)\
**Post date:** [July 3, 2007, 8:58am UTC](https://forum.kirupa.com/t/cant-seem-to-figure-out-a-trajectory-for-an-object/231257/1 "2007-07-03T08:58:45Z")

</div>

I’m using the old laws of physics to try and calculate a trajectory for an object in my game.

I’m using the old

launch vel = sqrt(range \* gravity) / sin(2\*theta)

where theta is the angle of launch.

Now my constant for gravity is the number (9) and the game runs at 30 fps. Am I right in thinking the acceleration due to gravity is 9 units a second if I divide 9 by 30? (i.e. 9/30 = amount to accelerate y by each frame)

I’m trying to plug gravity into this equation, as I’m launching at 45 degrees with a range of 200 and I seem to be landing about 48.2 units short of the target…(151.8 instead of 200)

I’m applying 9/30 to the y velocity of the object every frame which should yield an overall accel of 9 units a sec - does that sound right?

My launch speed ends up at 9.2 units/sec and the peak of the travel is around 38 units - I’m pretty sure this peak should be 50

I’m adding the calculated velocity to the velx and vely of the object at launch taking into account the launch angle (using cos/sin) but still it’s short…

any ideas?