Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

You probably know that if you try to draw a line using a piece of chalk on a blackboard , under some conditions (for example, $\alpha<\frac{\pi}{2}$ in the picture below) you will have a dashed line pattern instead of a continuous line.

My question is :

(how) Can you model this special movement of the chalk ,and specially , find the length of line segments and the distance between them(which are the visible characteristics of the motion)?

Note:gravity is present. (I think it affects the solution, at least in some models)

the chalk

The pattern:

enter image description here

share|cite|improve this question
Well, the first thing I would do it figure out why that happens? It seems like the chalk gets stuck (doesn't overcome static friction). This strains the chalk, causing some stress to push against static friction. At some point the response to the strain overcomes friction and the chalk suddenly jumps to a new position. The distance would be given by how fast you are moving across the blackboard. – levitopher May 7 '13 at 19:58
Similar dynamics to the ones @levitopher describes occur when bowing a violin string. This has been extensively modelled because of its applications in sound synthesis, so it might be worth looking into the literature surrounding that. (I'm not familiar enough with it myself to attempt an answer to this question.) – Nathaniel May 8 '13 at 6:44
See a truly expert in action, here (Walter Lewin) – Eduardo Guerras Valera May 10 '13 at 17:16
up vote 5 down vote accepted

This is know as the Stick-Slip Phenomenon

For the chalk pattern see this.

share|cite|improve this answer
The report is great! thank you for sharing! – Zorich May 8 '13 at 12:49

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.