Leaning Ladder
addition of forces - equilibrium of forces or moments - friction
What it shows:
A small ladder is leaned against a vertical wall. A weight can be moved
up succeeding steps to find when the ladder will slip and fall down.
The forces holding the ladder in equilibrium are in consideration here.
How it works:
A short (4 foot) step-ladder is equipped with wheels or casters at its
upper end so that as it leans against the wall, the reaction of the wall
may be considered horizontal, tangential friction being negligible. The
ladder is leaned against the wall at such an angle that it just does not
slip. What are the forces holding the ladder in equilibrium? Now a large
weight (14.5 kg = 32 lb) is placed on the first step. What are the forces?
The weight is moved up to succeeding higher steps until the ladder begins
to slip and equilibrium has been lost.
Setting it up:
Use one of the side lecture hall walls between the blackboard and the
side door. The plain cement floor is usually O.K. but if it's a
problem, a sheet of thin aluminum or galvanized iron will make the
friction between the floor and the base of the ladder more dependable.
Comments:
This is one of those demonstrations where a little showmanship flair will
liven it up considerably. The weight is quite heavy and the audience's
anticipation of the Great Crash can be capitalized on. On occasion, the
lecturer has substituted for the weight by cautiously climbing the ladder
until it slips. While much more dramatic, it is of course more dangerous
and is only recommended for the nimble footed. A board, butted against
the lecture bench and ending about 15 cm from the foot of the ladder,
can be used as a "stop" to prevent the ladder from crashing all the way
down and preventing a serious fall. Rating **