gravity simulation

Question

I want to simulate a free fall and a collision with the ground (for example a bouncing ball). The object will fall in a vacuum - an air resistance can be omitted. A collision with the ground should causes some energy loss so finally the object will stop moving. I use JOGL to render a point which is my falling object. A gravity is constant (-9.8 m/s^2).

I found an euler method to calculate a new position of the point:

deltaTime = currentTime - previousTime;
vel += acc * deltaTime;
pos += vel * deltaTime;

but I'm doing something wrong. The point bounces a few times and then it's moving down (very slow).

Here is a pseudocode (initial pos = (0.0f, 2.0f, 0.0f), initial vel(0.0f, 0.0f, 0.0f), gravity = -9.8f):

display()
{
     calculateDeltaTime();
     velocity.y += gravity * deltaTime;
     pos.y += velocity.y * deltaTime;

     if(pos.y < -2.0f) //a collision with the ground
     {
        velocity.y = velocity.y * energyLoss * -1.0f;
     }

}

What is the best way to achieve a realistic effect ? How the euler method refer to the constant acceleration equations ?

What happens if you just take the energy loss out of the equation? — redbmk
apart from Yochai's answer, you might consider setting pos.y to ground level in your collision case to avoid clipping errors — Tobias Kienzler
Without energy loss the point also stop bouncing after some period of time and it's very slow moving down. — Vert
In my, now deleted, answer, I suggested that your bleeding your energy off to fast. Upon reflection, and further working the math, I realized that if you simply change it to velocity.y -= velocity.y*energyloss/2 you'd be correct. — rcollyer
@rcollyer: is that inside or outside the collision block? i.e. do you suggest friction [instead of] or [additionally to] a lossy bounce? — Tobias Kienzler

Yochai Timmer Yochai Timmer · Accepted Answer · 2011-08-10T15:44:37

Because floating points dont round-up nicely, you'll never get at a velocity that's actually 0. You'd probably get something like -0.00000000000001 or something.

you need to to make it 0.0 when it's close enough. (define some delta.)

gravity simulation

2 Answers