Do not move the tray too fast before hitting the wall; both threads may
breake if you are not careful. And do not count on friction, unless you
want to demonstrate that it is also proportional to m (both block either
slide or not slide). I used lead and aluminum spheres as "blocks". The
table cover demo does depend on friction and students do not know that
frictionl F is proportional to m (to explain the difference between slow
pulling and jerking).