Galileo's Paradox

The blue bead falls straight down to the bottom of the circle along the vertical green wire.
The red bead starts at a lower point and slides without fraction diagonally, finishing at the same point as the blue bead.
Can you predict which bead will reach the bottom first?
Activate the trigger to find out!
Can you explain the result? (Click the hints for help.)


  • [Snapshot]
  • [Snapshot]
  • [Snapshot]


Falling objects seem familiar but their behavior can often be contrary to intuition. Here is a chance to see if you can predict the results of a "race" between two bodies falling along different paths towards the same point. Click the "Play" (right arrow ▶) button to start the race and find out! You can use the slider at the left to change the red ball's starting point. Does that change the outcome? Can you explain the results qualitatively? Can you calculate the times of descent for both balls and resolve Galileo's paradox? The snapshots show various starting positions for the red ball. This problem was first mentioned in 1602 by Galileo; see D. B. Meli, Thinking with Objects, Baltimore: Johns Hopkins University Press, 2006 p. 71.
Resolution of the paradox:
Both objects start from rest and are accelerated uniformly by gravity. Therefore the distances traveled (D and L), their accelerations and ), and the times elapsed and ) are related as follows:
The blue bead accelerates at . Therefore, .
Solving for yields: .
However, the red bead moves diagonally and its acceleration is, therefore, proportionally smaller. From the diagram, you can see that acceleration of the red bead will be the diagonal component of acceleration of the blue bead, that is, . Substituting into yields
Finally, you can see from the diagram that . Substituting this into the equation immediately above yields:
Solving for yields: .
This agrees with the time for the blue bead and resolves the paradox.
A faster solution is to divide the two equations and , which yields:
. Since , the two times must be equal. Physically, the blue bead's acceleration is larger than the red bead's acceleration by the same ratio that the distance the blue bead travels is greater than the distance the red bead travels.
Source: T. B. Greenslade, Jr., "Galileo's Paradox," The Physics Teacher, 46(5), 2008 p. 294.
    • Share:

Embed Interactive Demonstration New!

Just copy and paste this snippet of JavaScript code into your website or blog to put the live Demonstration on your site. More details »

Files require Wolfram CDF Player or Mathematica.

Mathematica »
The #1 tool for creating Demonstrations
and anything technical.
Wolfram|Alpha »
Explore anything with the first
computational knowledge engine.
MathWorld »
The web's most extensive
mathematics resource.
Course Assistant Apps »
An app for every course—
right in the palm of your hand.
Wolfram Blog »
Read our views on math,
science, and technology.
Computable Document Format »
The format that makes Demonstrations
(and any information) easy to share and
interact with.
STEM Initiative »
Programs & resources for
educators, schools & students.
Computerbasedmath.org »
Join the initiative for modernizing
math education.
Step-by-Step Solutions »
Walk through homework problems one step at a time, with hints to help along the way.
Wolfram Problem Generator »
Unlimited random practice problems and answers with built-in step-by-step solutions. Practice online or make a printable study sheet.
Wolfram Language »
Knowledge-based programming for everyone.
Powered by Wolfram Mathematica © 2018 Wolfram Demonstrations Project & Contributors  |  Terms of Use  |  Privacy Policy  |  RSS Give us your feedback
Note: To run this Demonstration you need Mathematica 7+ or the free Mathematica Player 7EX
Download or upgrade to Mathematica Player 7EX
I already have Mathematica Player or Mathematica 7+