Easy Forces
Before moving on to more real-world physics, I think a word should be said about the quick and dirty way of simulating forces in 2D games. Most often, these forces are wind and gravity. If you are more interested in real-world physics than 2D game physics, you can skip this chapter.
In the last chapter, you were shown how to simulate the forces of thrusters on a ship in space with no other forces acting on it. The only difference between these simple thrusters and wind or gravity is that the player is not in control of them. Because all forces add up to "Net Force", there is no complicated math to deal with. You can simply add another force to the speed of the game object.
In the below code example, I have added the force of gravity to dot :
x# = 400
y# = 300
VertThrust# = 0.01 ; 1 cppl
HortThrust# = 0.01
gravity# = 0.005 ; define the force of gravity
xSpeed# = 0
ySpeed# = 0
SetFPS 60
Do
ySpeed# = ySpeed# + gravity# ; add gravity to the speed
x# = x# + xSpeed#
y# = y# + ySpeed#
Cls 0
Print "xSpeed : " + str$(xSpeed#)
Print "ySpeed : " + str$(ySpeed#)
Circle x#, y#, 2, 1
If RightKey()
xSpeed# = xSpeed# + -HortThrust#
DotC x#+3,y#, rgb(255,0,0)
EndIf
If LeftKey()
xSpeed# = xSpeed# + HortThrust#
DotC x#-3,y#, rgb(255,0,0)
EndIf
If UpKey()
ySpeed# = ySpeed# + VertThrust#
DotC x#, y# - 3, rgb(255,0,0)
EndIf
If DownKey()
ySpeed# = ySpeed# + -VertThrust#
DotC x#, y# + 3, rgb(255,0,0)
EndIf
Sync
Loop
Only two lines needed to be added to the previous code example for this :
gravity# = 0.005 ; define the force of gravity
ySpeed# = ySpeed# + gravity# ; add gravity to the speed
Adding the force of wind to the above example is not at all different from the above example. The only difference is that wind comes from a different direction.
x# = 400
y# = 300
VertThrust# = 0.01
HortThrust# = 0.01
gravity# = 0.005
wind# = 0.005 ; define the force of wind
xSpeed# = 0
ySpeed# = 0
SetFPS 60
Do
ySpeed# = ySpeed# + gravity#
xSpeed# = xSpeed# + wind# ; add wind to speed
x# = x# + xSpeed#
y# = y# + ySpeed#
Cls 0
Print "xSpeed : " + str$(xSpeed#)
Print "ySpeed : " + str$(ySpeed#)
Circle x#, y#, 2, 1
If RightKey()
xSpeed# = xSpeed# + -HortThrust#
DotC x#+3,y#, rgb(255,0,0)
EndIf
If LeftKey()
xSpeed# = xSpeed# + HortThrust#
DotC x#-3,y#, rgb(255,0,0)
EndIf
If UpKey()
ySpeed# = ySpeed# + VertThrust#
DotC x#, y# - 3, rgb(255,0,0)
EndIf
If DownKey()
ySpeed# = ySpeed# + -VertThrust#
DotC x#, y# + 3, rgb(255,0,0)
EndIf
Sync
Loop
Using this quick and dirty method of 2D game physics and Net Force, you can make any number of pulling or pushing forces to your game object.
