Crane’s Balance

6- Study of the base and its ballast.
Authors : Hélène Merle(plus d'infos)
Muriel Guedj(plus d'infos)
Summary :
The pupils study the role of the crane’s base.
Publication : 1 February 2004
Objectives :
Procedure goals: build an experimental protocol from a hypothesis; take into account possible variations; reason from a scale model; conclude from a set of experimental results. Language aspect: write a report. Vocabulary: bracket, lever arm, base.
Material :

For each group of pupils
-   A wooden bracket 50cm in height and length (can be made from a round rod of 1.5 cm in diameter in the vertical portion and a square section (1.5 cm square) for the horizontal portion. Fixing of the two parts by a wood screw.
-   Two pieces of wood 15cm by 15cm and 20cm by 20cm, 2cm thick, drilled with one or more holes with a diameter of 1.5 cm, on the axis, which allow the insertion of the bracket.
-  Objects to ballast the base (e.g. small mosaic tiles)
-   Clothespins (see sequence 2) to act as the counterweight and the load of the jib.

Pedagogical approach

Preparatory phase

-           Initial question or hypothesis of the pupils: the ballasted base of the crane has a role in the overall balance;

-           Present the material;

-           Ask pupils as a group to think of an experimental protocol to test this hypothesis.

Implementation phase

-           Firstly, let the pupils research freely;

-           Ask each group to write in the experiment notebooks the possibilities they found and to describe how they are different while giving the same result;

-           Or possibly suggest opportunities that would not have appeared in the groups.

Sharing phase

-           Sharing the experiences of groups;

-           Discuss the possible alternatives, especially with the various holes of the base boards and what it brings;

-           Ask for a conclusion.

The teacher’s word

To emphasize the role of the ballasted base in the balance of the crane, imbalance the model with a non-ballasted base. This is achieved  by adding clothepins on the horizontal rod of the bracket. The base is then ballasted by gradually adding small tiles.

The variants, small or large base plate, positions of the bracket in the balance, lead to introducing more or less useful ballasts. The concepts of center of gravity and support of the base are outside the program. The complexity of a crane’s balance allows the children to familiarize themselves with the role of the distribution of mass as well as the lever arm in balance with a device.

Difficulty: Little or no difficulties if the pupils have the time to experiment all the possibilities. Do not hesitate to spread these activities over two sessions.