new
wall id 5 kn 1e8 nodes -5,0 5,0
ball id 1 x 0 y 1.1 rad 1
prop dens 1000 kn 1e8
set grav 0 -10
set dt max=2e-3
def down_force
down_force = b_yfob(ball_head)
end
hist down_force
cyc 500
plot his 1
set dt max=2e-3是什么意思?ball_head表示球的地址,那么down_force = b_yfob(ball_head)中为什么要加ball_head,不太理解,求高手指点啊?