Physics Photo of the Week

Sept. 30, 2005

Galileo's Famous Experiment

A Bowling ball and a golf ball were dropped simultaneously from the 2nd floor of Spidel near the Physics Laboratory.  The massive bowling ball (7.3 kg) weighs about 160 times more than the 46 gm golf ball.  In other words, the earth's gravity pulls down on the massive bowling ball 160 times more than gravity pulls on the golf ball.  In the photo at right (a single frame from a video clip made by physics student Liina Laufer) another physics student - Veda King Blanchard - has just released the bowling ball and the golf ball simultaneously.  The two different-sized balls can be seen about half-way to the ground in the image - clearly falling together. Galileo, in his study of physics and motion over 400 years ago startled the world when he demonstrated a similar experiment in front of scholars at the Leaning Tower of Pisa. Click here to see an animated view of the falling objects.  The animated file is very large, so have patience...

Why do the two masses fall at the same rate?  Since the force of gravity is 160 times more intense on the large ball, shouldn't it fall 160 times faster?  Yet, they fall at the same rate: the acceleration due to gravity.  The answer can be explained by the concept of inertial mass.  Galileo had understood the concept of inertia, that is a mass in motion or at rest will remain in that state in the absence of forces.  Isaac Newton later formulated the law of forces and motion: the acceleration is given by  a = force/mass.

If the parameters for the bowling ball are substituded for the force and mass in the equation above, we have:

The factor of 160 (or whatever the mass ratio is) cancels out and we have the law of physics that all objects in the influence of Earth's gravity, in the absence of friction, fall with the same acceleration.


Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an 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.

Click here to see all Physics Photo of the Week for 2005

Click here to see all Physics Photo of the Week for 2004.