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:
Measurement
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
Gravity
Work and Energy
Momentum
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

#### Purpose:

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

#### Procedure:

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?

#### Hints:

For this demo, use:
• L=.67m
• s=.1m
• mg=2.5N
• k=4.9N/m
ans:
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!