I am currently working on the 2D Bouncing ball physics that bounces the ball up and down. The physics behaviour works fine but at the end the velocity keep +3 then 0 non-stop even the ball has stopped bouncing. How should I modify the code to fix this issue?

    ballPos         = D3DXVECTOR2( 50, 100 );
    velocity        = 0;
    accelaration    = 3.0f;
    isBallUp        = false;

void GameClass::Update()
{
    velocity += accelaration;
    ballPos.y += velocity;

    if ( ballPos.y >= 590 )
        isBallUp = true;
    else
        isBallUp = false;

    if ( isBallUp )
    {
        ballPos.y = 590;
        velocity *= -1;
    }

    // Graphics Rendering
    m_Graphics.BeginFrame();
    ComposeFrame();
    m_Graphics.EndFrame();
}