Demos: 1F-06 Inertial Ball

This demonstration illustrates the effects of inertia. If the lower string is stretched sufficiently rapidly, it will reach its breaking point before the ball has a chance to move appreciably.

Directions: Using one of the loops of thin string, hang the heavy iron ball from the support as shown. Hang an identical loop from the lower hook on the ball. Place the aluminum rod in the lower loop and pull SLOWLY downward (keep your hands clear). The upper string should break first because the tension in that string will be the weight of the ball plus the tension in the lower string. Set up the apparatus again, but this time use the wooden mallet to strike a sharp blow to the aluminum rod. This will cause the tension in the lower string to increase so rapidly it will break before the ball moves any appreciable distance. Thus the tension force is not transmitted to the upper string.

Suggestions for Presentation: To illustrate that the tensions in the strings are as described, replace the strings with rubber bands, which more readily show the actual stretching. If you pull slowly, the upper band should stretch more than the lower. A sharp blow, however, should stretch the lower band a lot, but little stretching will be observed in the upper band.

Applications: Towing a truck or car with a strap requires that you start very slowly to avoid stretching the strap beyond its breaking point before the car starts to move. Relate to other demos involving inertia.

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