How to get contact forces in Pymunk?

Question

I am having some trouble using Pymunk. Consider the following scenario, where a ball and a surface have been created and are touching:

import pymunk
# Create space
space = pymunk.Space()
space.gravity = (0, 900)

# Create the surface
surface_body = pymunk.Body(body_type=pymunk.Body.STATIC)
surface_body.position = (400, 600)
surface_shape = pymunk.Segment(surface_body, (-400, 0), (400, 0), 3)
space.add(surface_body, surface_shape)

# Create ball
ball_body = pymunk.Body()
ball_body.position = (400, 550)
ball_shape = pymunk.Circle(ball_body, 40)
ball_shape.mass = 1
ball_shape.friction = 0
space.add(surface_body, surface_shape)

# Run the simulation a few times in order to get contact between ball and surface
for i in range(50):
    space.step(1/60)

If I run shapes_collide, I get that a contact point exists:

print(ball_shape.shapes_collide(surface_shape))
> ContactPointSet(normal=Vec2d(0.0, 1.0), points=[ContactPoint(point_a=Vec2d(400.0, 597.5), point_b=Vec2d(400.0, 597.0), distance=-0.5)])

The question is: is there a way I can use Pymunk to get the force that the surface is exerting upon the ball? In this case it should be something like Vec2D(0, -900) applied at the contact point, but I need a function like that for some situations which are a bit more complex...

I tried searching the documentation but couldn't find anything (maybe the solution slipped past me). Any help is appreciated.

Thanks!

Answer 1

You might be able to use the total_ke or total_impulse on the Arbiter to get what you want. These two properties are documented here: http://www.pymunk.org/en/latest/pymunk.html#pymunk.Arbiter.total_impulse

Either you read it out from the arbiter with a post-solve collision callback, or you do it "manually" by calling each_arbiter on the body you are interested in. Impulse * dt is 900.

def f(arb):
    print(f"ke: {arb.total_ke}, impulse: {arb.total_impulse}")

ball_body.each_arbiter(e)

# will print ke: 225.0, impulse: Vec2d(-0.0, -15.0)