Physics Photo of the Week
December 13, 2013
Ping Pong Cannon - Discussion by Rachel Weber.
Video assistance by Justin Hale.
morning, a gun-shot sounding bang rung throughout a science
building, coming from the 11:00 physics class, except it was
not a gun, but instead a ping pong cannon. Constructed from a
10 foot long clear pipe evacuated with a vacuum pump, this
cannon was propelling the small 2.4 gram ping pong ball at
speeds of up to 280 meters per second.
First, the ping pong ball is placed just centimeters within
the back of the pipes, followed by 3-inch wide packing tape
sealing off each end. After the pipe was evacuated,
requiring about 2 minutes, the tape by the end with the ball
is punctured, where a rush of the outside air
accelerates the ping pong ball with a "BANG!". Due to the
molecular speeds bouncing off the ball in random motion
obeying the kinetic theory of gases, the resulting pressure
propelled the ball through the pipe with a velocity of several
hundred meters/sec. At such a high speed, a special
high-speed video camera at a frame rate of 240 frames/sec was
used in order for the results to be even slightly measured.
The high speed ping pong ball, essentially invisible to the
human eye, penetrated through two empty soda cans.
The destruction of the empty soda cans can be seen in the high
speed video clip at right, being played back at one
frame/sec. Two destroyed cans from a previos shot can be
seen in the foreground of the pictures.
After seeing the simple ping pong ball decapitate two soda
cans, we wanted to determine just how fast the ball was going by using a
ballistic pendulum. After firing the ping pong cannon
into a suspended 550 gram cardboard box , we would measure the
speed of the box after it essentially "caught" the ball.
Instead of pushing the box, the ball had so much momentum that
it punched right through it! The ping pong ball broke through
several layers of corrugated cardboard, even after
reinforcements were made. Thankfully though, the high
speed camera was able to photograp the ball emerging from the
box after breaking out of the box. This is shown in the
second video clip. The video shows the blurred ball
emerging from the box (marked with the indicator in each
frame) along with flying shards of cardboard box material.
Through some of the pictures, we were able to determine the velocity
and momentum of the emerging ping pong ball and the velocity
and momentum of the box (seen in the third video clip
at right. Even when the ball gets crushed and the box is
penetrated, we know that conservation of momentum is true
because the only forces are "internal", between the box and
the ping pong ball, so we know the total momentum
is constant. By equating the total momentum of the ball
and box after the collision to the momentum of the ball before
the collision, we found that the speed of the ball emerging
from the vacuum gun was 280 m/s which means 630 miles an hour!
Through this experiment, kinematics, molecular kinetic theory,
conservation of momentum, thermodynamics, and aerodynamics are
Both Rachel Weber and Justin Gale are physics students at
Mars Hill University where this experiment was
conducted. The high speed video camera, vacuum cannon,
and a vacuum pump were on loan from the Physics and Chemistry
Department at Warren Wilson College
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