Physics Photo of the Week

March 4, 2011

The New Big Swing
Recently the Warren Wilson Landscape Crew with professional help from John Parmenter of Pro Tree Service installed the Big Swing on campus for general recreational use.  Dr. Mark Brenner of the Environmental Studies Department is trying it out.  Many students and staff have a lot of fun swinging on the 31 foot swing.

A standard physics problem is to calculate the tension in the suspension cables that run between the two large oak trees that support the weight of the swing and rider.  The tension in each of the vertical ropes is easy to calculate: simply half the weight of the rider when stationary.  The tension in the horizontal cable is more complicated, but "easily" solved using vector algebra.  Similar problems appear in many physics textbooks in sections on static forces and equilibria.

The forces depend on the inclination angle of each end of the supporting cable emphasized in the photo below.  The arrows represent the tension forces along the length of the inclined supporting cable between the two trees.  The two vertical components of the tension forces must add up to the weight of the rider.  The two horizo
ntal components of the tension forces must cancel becaus they pull in opposite directions.  The angles of the cables are easily measured using a digital camera and trigonometry. These two angles average to about 10 degrees inclination.  When a typical male rider is sitting on the swing, and not swinging, the static tension on these suspension cables is about 500 pounds, considerably more than the weight of the rider!  The trees and cables and cable bolts easily support that 1/4 ton force.

When someone is swinging on the swing, the centripetal force will increase the tension.  For this we will need to make a video of the swing in its arc - a future PPOW topic.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu.

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.