Stuff that works
Science & Technology of Tape
When children test tape, they are usually testing for ways in which it fails to do its job. Commonly they mean that the pieces are not held together, the poster falls from the wall, the package bursts open. This sort of failure results when the tape is unable to withstand the forces applied to it. The forces leading to tape failure are of two sorts - tension and shear:
A force (tension) of 150 grams, at an angle of about 30 degrees to the surface, was needed to peal the tape from the block. When we applied the force parallel to the block to which it was stuck (a shearing force), the tape never came unstuck from the block. Instead, at a force of 2300 grams the tape itself gave way, breaking in between the spring scale and the block to which the tape was stuck. The tape broke under tension just as a string or strip of paper might break under tension.
When the force lies in the same plane as the tape and the joined pieces, as the force of gravity on paper taped to a vertical wall, it is a shearing force. Here are some pictures of situations in which, if the tape fails, it will fail in shear.
When the force is perpendicular to the plane of the tape and the object to which it adheres, it is a tension force. In fact, if there is any angle between the direction of the force and the plane of the tape, then some component of the force will be perpendicular to the plane of the tape and object to which it is joined. Here are some pictures of tape that when it fails, it fails in tension.
Think about what you do to remove masking tape from a wall or to remove a self sticking stamp from its backing or from an envelope. If you are like most people, you will pull in a direction perpendicular to the object to which the tape is stuck. Tapes usually resist more force in shear than in tension. Here are pictures of an experiment we did to compare these two forces with masking tape.
A force (tension) of 150 grams, at an angle of about 30 degrees to the surface, was needed to peal the tape from the block. When we applied the force parallel to the block to which it was stuck (a shearing force), the tape never came unstuck from the block. Instead, at a force of 2300 grams the tape itself gave way, breaking in between the spring scale and the block to which the tape was stuck. The tape broke under tension just as a string or strip of paper might break under tension.