code for diagram

> aa:=[0,0]:bb:=[2,0]: cc:=[2,sqrt(12.)]:
tri:=plot({[aa,bb],[bb,cc],[cc,aa]},color=blue,
thickness=3, scaling=constrained):
txt:=plots[textplot]({[aa[1]+.4,aa[2]-.2,
`10 cos (60 degrees) m/s`],[.5*(cc[1]+bb[1])+ 1.2,.5*(cc[2]+bb[2]) ,
`10 sin (60 degrees) m/s`],[.5*(aa[1]+cc[1])-1/2 ,.5*(aa[2]+cc[2]) ,
`10 m/s`],[.8,.2,`sixty degrees`] },font=[TIMES,BOLD,12]):
circ:=plot([cos(t),sin(t),t=0.. .9*Pi/3],thickness=2):
circhd:=plot([ [cos(.9*Pi/3),sin(.9*Pi/3)],[cos(.9*Pi/3),.8*sin(.9*Pi/3)]],
thickness=2):
plots[display]({circhd,circ,txt,tri},axes=none);