phy_speed;
返回： Real（实数）
This read-only variable returns the speed as a value in
pixels-per-second of the physics enabled instance. Should you need
to change this value, you must do so by changing the x and y
vectors using the variables phy_speed_x and phy_speed_y.
if phy_speed > 10
{
phy_linear_damping += 0.01;
}
else
{
phy_linear_damping = 2;
}
The above code checks the speed of the physics enabled instance and then changes the linear damping based on the returned value.