Physics at the University of Virginia
Academics People Research Announcements Facilities Adminstration Home Updates

Return to Demo Lab Homepage

Al Tobias (wat4y) - Office: Rm. 201, (434) 924-0538 - Lab: Rm. 202, (434) 924-6800

Physics Demo Manual

Demonstrations are cataloged according to PIRA Bibliography


Due to Physics Building renovations, the lead time to set up demo requests has increased due to the need to transport equipment across campus. Please be kind and let me know well ahead of time what you need.

Choose a Topic or Enter a keyword to search:
I cannot find what I want!

You have selected the following Demos:
  • None Selected

Choose a subtopic:
(Clicking a green button will add that demo to your list)
 Motion In One Dimension
 Motion In Two Dimensions
 Newton's First Law
 Newton's Second Law
 Newton's Third Law
 Statics Of Rigid Bodies
 Applications Of Newton's Laws
 Work and Energy
 Rotational Motion
 Properties of Matter
 Hooke's Law
 Force Constant of a Spring
 Spring Scale Collection
 Strings and Springs
 Breaking Wire (Plasticity)
 Bending Beams
 Prince Rupert's Drops
 Shear Block
 Happy and unhappy balls
  video  - Coefficient of Restitution
 Crystal Models

Strings and Springs


A non-intuitive example of how spring constants add when springs are connected in series and parallel.


A ball is held up by two identical springs which are connected in the center by a small string of length s (see picture below). Two strings of length L are added, one connecting the bottom string to the support and the other connecting the top spring to the ball. There is no tension in either of these strings. The question is, when the small string "s" is cut, where will the ball end up after it comes to equilibrium?


For this demo, use:
  • L=.67m
  • s=.1m
  • mg=2.5N
  • k=4.9N/m
The net change in the distance of the weight from the top is:
L - leq - s - 3mg/2k
where leq is the equilibrium length of the spring, and a positive result means an increase in distance from the top. Disregarding leq, we can already see that the result is negative so:
The ball rises!