Physics Photo of the Week

May 11, 2007

Thrust from a fire extinguisher - By Eri Watanabe

In this experiment, the rider holds a CO2 fire extinguisher and discharges it towards the back while he rides on the cart.  The rider should continuously hold the fire extinguisher as the cart glides past the camera.  The camera recorded a video of the motion and we analyzed it. 

The graphs of the motion of the cart are shown below.  On the left is the position vs. time.  In this graph we can see that as time increases, the distance the cart moves also increases, meaning that the cart travels with greater and greater speed.

The position vs time shows that as time increases, the incremental distance increases, indicating that the mass accelerates
The velocity vs time shows a steady increase in the velocity indicating that the acceleration is constant

In addition, we made another graph.  This new graph (right) shows the velocity of the cart vs. time.  Both of graphs are not completely accurate, because of my marking the points.  On the right-hand graph of velocity vs. time, the velocity increases as time increases.  Note that the slope of the velocity vs. time graph is a constant.  The slope of the velocity equals the acceleration, and this means that the acceleration is constant.  In this experiment, Newton’s second law is seen: with constant force (thrust), acceleration is constant.  Acceleration = thrust/mass.

Many thanks to Terry Payne of the Public Safety Department for recharging the fire extinguisher.

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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