Demos: 1F-01 Inertial Balance


Because the gravitational force acts vertically downward, any change in motion along the horizontal direction can be affected only by a force having a component in this direction. The inertial balance consists of a horizontal bar on which an object is placed. The bar is attached to the stand by strap steel bands that act as "springs." As the bar is displaced horizontally, a restoring force from the springs alternately slows down and speeds up the mass on the bar. The system oscillates at a natural frequency that is inversely related to the mass, i.e. a larger mass will oscillate at a lower frequency. Since the system is not influenced by gravity, the oscillation frequency gives a qualitative measurement of the MASS of the object rather than its weight.

Directions: Two objects of identical size and appearance represent the masses to be tested. Visually one cannot distinguish between the two. By placing the objects one at a time on the bar and setting the system into oscillation, it is seen that one object has considerably more mass than the other.

Suggestions for Presentation: Obviously the difference could be determined by allowing the student to hold the objects and sense the weight difference, but if this apparatus were in a space station in a "weightless" environment, the inertial balance results would be unaffected, whereas holding them in the hand would not allow a difference to be detected. Ask the students if there is a way other than holding the objects that would detect the difference. It is not likely to happen, but a student might respond that by moving it quickly back and forth while holding it, the difference could be noticed. This would then set the stage for actual demonstration.

Applications: The most obvious application is the determination of mass of an astronaut in orbit. A special chair holds the astronaut while it is allowed to oscillate back in forth under the action of calibrated springs. The mass can be determined from the oscillation frequency.

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Last Updated: May 9, 2016 11:44 AM