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 horizontal 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.
Click
here
to
see the Physics Photo
of
the Week Archive.
Observers are invited to submit
digital photos to:
