phy_speed_y;
返回： Real（实数）
This variable can be used to get or to change the y component
the instances linear speed vector and is defined in pixels per step
(for pixels per second, see phy_linear_velocity_y).
Altering this for a static instance (ie: an instance with 0
density) will turn it into a kinematic instance.
if phy_speed_y != 0
{
phy_speed_y = 0;
}
The above code will check the y component of the linear speed vector and if it is anything other than 0 it will set it to 0.