Equipment: A 3kg spherical weight with hooks on the top and bottom, two pieces of thread, a rigid frame, an aluminum rod and a piece of heavy-duty string used as a safety line.

What it does: A weight is hung from a frame by a piece of thread. An aluminum rod is attached to the bottom of the weight by a second piece of string. The aluminum rod is pulled down. If the rod is pulled down slowly, the top thread will break first. If it is pulled down quickly, the bottom thread will break first.

Concepts Demonstrated: Acceleration on an object.

Classroom assembly:

1. Attach the weight to the frame using the safety line.
2. Hang the weight from the frame using a piece of thread. The thread should be shorter than the safety line so that the thread, not the safety line, is taught.
3. Attach a second piece of thread to the bottom of the weight.
4. Attach the aluminum rod to the bottom piece of thread.
5. Pull the aluminum rod down slowly. The top thread will break. Note that the weight will be prevented from hitting the table (or your feet!) by the safety line.
6. Repeat steps 2-4.
7. Pull the aluminum rod down quickly. The bottom thread will break.

Cautions: Improper use of this demonstration can result in injury. Ensure that the safety line is firmly attached to the weight and the frame.

Setup Time: Short.

Difficulty: Straightforward.

Visibility: Medium. This demonstration is appropriate for small classrooms.

Related demonstrations:

References: PIRA 1F20.10
Video Encyclopedia 2-13
Sutton M-100; DHP Mc-2; DaR M-250

Original Construction: All parts were assembled from surplus lab equipment.