Physics Photo of the Week

October 13, 2006

Pop-up ball.  Discussion by Christopher Miller

Large file - please be patient!On Wednesday October 4th, the Physics Class at Warren Wilson College conducted the following experiment: A person (Eric Thiedich) was sitting on a wheeled platform and was given a push. Then while the cart was moving, Eric launched a small plastic ball into the air with a mechanical "popper-upper" device. To the surprise of much of the class when the ball was launched, it traveled the same speed as the cart and went back to the same relative place that it was launched from. Why didn’t the ball land behind the cart?

When the Cart was pushed, it was given an initial Velocity and, since there was little friction on the cart, it stayed at that constant Velocity. The plastic ball was attached to the cart when it was given the Initial Velocity, so the plastic ball was given the same Constant Velocity. For this experiment we are going to say that air resistance was negligible. So according to Newton’s I First Law of Motion, Objects in motion will remain at the same state of motion unless acted upon by an external force.  Since the Ball and the cart are going at the same velocity, and there are no forces affecting the Horizontal Velocity of either the Cart or the ball, The constant velocity should have no effect on the Horizontal change in distance between the Rubber ball and the cart. In the photo at the right you can see what happens if we subtract this Velocity from both the cart and the ball. The Ball seems to go straight up and straight down. The Fact that the Ball goes up and down has do with the Gravitational Forces acting on the ball, which gives the ball a vertical acceleration of -9.8m/s2

In the last photo (left) both the ball and the cart are plotted with circles and vertical lines respectively. You can see that the Cart and the Ball stay at the same Horizontal Velocity. This Velocity is independent of the Gravitational Force which causes the ball to have a negative vertical  acceleration. 

All animated photos were processed by Donald Collins with DV tape photographed by students.

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

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