Tension in a String Connecting Two Hanging 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.

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.

PowerPoint for use with this demonstration (right-click, save link as): Tension.ppt.

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

NCEA & Science Curriculum

SCI 1.1, PHYS 2.4

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 or a personal response system (clickers).

Larger photo of apparatus

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

Dropped mass and scale