NEW ZEALAND PHYSICS TEACHERS' RESOURCE BANK

Tension with Two Masses

Abstract
Tension in a string supporting a 1 kg mass, a 1 kg mass hanging over a pulley, and two 1 kg masses over two pulleys is always 10 N.
Portable Yes
Principles Illustrated
Tension in the rope is the same with one string fixed, both ends suspending weights, or fixed at an angle. Tension in the string is zero when the mass is dropped.
NCEA & Science Curriculum
SCI 1.1, PHYS 2.4
Teacher Guide
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Instructions

With the use of the video camera, computer running VideoPoint Capture or similar software, and a data projector, this demo is suitable for a large auditorium. The scale can be read from the back of a lecture theatre. The small mass attached to the scale helps keep the scale vertical for better viewing.

See PowerPoint below for one version of the demonstration that starts with a 1 kg mass suspended by a string, moves to a mass suspended over a pulley (left above) and then to two masses suspended by a pulley. One can then continue with a mass and force meter dropped, and a string at an angle. Ask students to predict answers using the ABC cards.

The answers all are 10 N except for the free fall example where the tension is 0 N.

Front view two masses || Front view one mass || Angle View || Side View || Close up || Details camera and scale || Computer screen

Safety

 

Watch your feet! The 1 kg masses are pretty high up.
Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.
Related Resources
References
PIRA 1J30.20
PowerPoint
Credits

This teaching resource was developed with support from
 
Copyright

Copyright and fair use statement