Forces, Speed,...

Im looking for formula for calc impact force from speed and mass of flying object.
This i need becouse soon i gona start writing 2d physical engine.

the second law of newton should help you out in normal cases (no light speed involved)

you just need to invert it to get the force that's neccesary to stop the object

why don't you use the momentum equations? You can either have a completely elastic collision or an inelastic collision. Of course, there are some that are in between, where it's just a coefficient that is a property of the two materials involved multiplied by the final velocity.

How to check if line is crossed by another line and on what coordinates.

VB Code:

Type tPoint
    X As Double
    Y As Double
End Type
 
Type tLine
    B As tPoint
    E As tPoint
End Type
 
 
Function Intersect(L1 As tLine, L2 As tLine, Optional ByRef Pin As tPoint) As Boolean
'pin is used byref to return the point of intersection
Dim a1 As Double
Dim b1 As Double
Dim a2 As Double
Dim b2 As Double
Dim i As Boolean
Dim t As tPoint
 
    'determine the formula y = a * x + b for each line
    a1 = (L1.E.Y - L1.B.Y) / (L1.E.X - L1.B.X)
    b1 = L1.B.Y - L1.B.X * a1
    
    a2 = (L2.E.Y - L2.B.Y) / (L2.E.X - L2.B.X)
    b2 = L2.B.Y - L2.B.X * a2
    
    If a1 = a2 Then 'test if the lines are parallel
        If (b1 = b2) Then 'test if the lines start from the same point on the y axis
            If L1.E.X < L1.B.X Then
                t = L1.B
                L1.B = L1.E
                L1.E = t
            End If
            If L2.E.X < L2.B.X Then
                t = L2.B
                L2.B = L2.E
                L2.E = t
            End If
            If L2.B.X <= L1.B.X And L1.B.X <= L2.E.X Then 'pick the first point that is on both linesections
                Pin = L1.B
                i = True
            ElseIf L1.B.X <= L2.B.X And L2.B.X <= L1.E.X Then
                Pin = L2.B
                i = True
            Else 'the linesections do not match
                Pin.X = 0
                Pin.Y = b1
            End If
        Else 'no intersection at all
            Pin = t
        End If
    Else 'intersection in one point
        Pin.X = (b2 - b1) / (a1 - a2) 'calculate the intersection
        Pin.Y = a1 * Pin.X + b1
        i = (la.B.X <= Pin.X And Pin.X <= la.E.X) Or (la.E.X <= Pin.X And Pin.X <= la.B.X) 'test if the intersection is on one of the lines
    End If
    
    Intersect = i
End Function