i am trying to make that using atan2 to calculate the angle and get the animation but i am a little confuse how can make that using the blocks of the ball

The atan^2 function returns an angle, not the length of a side.

so i need to store the angle and next apply the angle to the ball?

tan(BAC) = BC/AC

=> BC = AC * tan(BAC)

Since it's a right-angled triangle, AB^2 = AC^2 + BC^2.

r^2 = AC^2 + AC^2 * tan^2(BAC) [where r is the radius of the circle]

r^2/(1+tan^2(BAC) = AC^2

AC = sqrt(r^2/(1+tan^2(BAC)))

Plug in this value of AC into the definition of tan(BAC) to get BC. AC + the x-coordinate of the center is the x-coordinate, and BC + the y-coordinate of the center is the y-coordinate (assuming the diameter line drawn is perpendicular to the base).

thanks its working but i have other problem with that

when i use the drawline every movement mark drawline are draw the line with black but my problem is that only i will like to get one line no every position with a line its possible make that? that only draw one line ?

using other examples i am trying to make that like this but its not working

movimientopelota.aia (3.3 KB)

Do you want to draw just the arc the ball follows or the complete half circle ?

i like to draw only one line rotating using 2 balls. i am trying using the block drawline but the drawline is drawing in every point in the rotating a new line getting a half life painting black like a half circle painted in black. no just one line display rotating. how can get only one line?

two balls because i like to get a line between two balls. no the half circle drawing, i will like get like gauge line

Draw me a picture of what you want.

You have deviated considerably from the original question...

i will like the black arrow. a line between two balls

I have got it this far (it completes the half circle)

Are you trying to make a gauge of some kind? If so, the approach is quite different, and why not say so in the first place, there are already many examples available.

ok thanks but its possible make that? using 2 balls and draw only one line? an animate a gauge using the line between the balls?. and where is there other examples?. please help.

Some of them:

but using 2 balls its not possible?. because i am trying to make a animation movement legs body using lines between balls like joints

Why a half circle ?

Why a gauge type approach?

Explain and show what you are trying to achieve...