Paul Lulai, physics instructor at Saint Anthony Village Senior High School in Saint Anthony Village, MN, developed an activity to measure the stress limits of printer ribbon. Paul’s students use a Force Sensor and Motion Detector to find the yield point of a single loop of the ribbon. Paul uses printer ribbon, since its yield point is typically within the range of the Force Sensor.

Students create two loops on either end of a small length of printer ribbon. (Each loop is taped so there is no false weakness caused by tied knots.) One loop goes over the hook of the Force Sensor. The second loop is used to suspend a small plastic cup. A Motion Detector is placed under the cup in order to measure the position of the bottom of the cup. Small masses are placed within the cup to increase the load on the printer ribbon. Data are collected using Selected Events generating a tensile strength graph.


The graph shows the somewhat linear stretch associated with the ribbon stretching elastically. Beyond the yield point, the ribbon is compromised, continuing to stretch without adding additional weight.

To test their results, the students must determine how many loops of ribbon are necessary to suspend a person on a board with an instructor-stated safety factor. As you can see, this group got it right.