Physics Photo of the Week

November 7, 2008

Thrust Reaction - Discussion by Lauren Parker
According to, Thrust is a “linear reactive force exerted by a propeller, propulsive gases, etc., to propel a ship, aircraft, etc.” Warren Wilson student Alex Chambers decided to outfit himself in a helmet and ride a rolling cart to demonstrate this principle. In this case, thrust comes from the propulsion of carbon dioxide from the fire extinguisher, causing Alex’s cart to accelerate in the opposite direction of the gas.

Thrust is a vector quantity that relates to Newton’s
Large animation file - please be patient!Second and Third Laws. The Third Law states that “every action has an equal and opposite reaction.” The fire extinguisher ejects the gas toward the left.  The reaction of pushing the gas to the left results in the gas pushing Alex and the cart to the right.  Newton's Second Law states that acceleration is inversely proportional to mass and directly proportional to force, a = f/m. The mass of the expelled gas is defined as the mass per unit time, or the mass flow rate. The amount of gas that is accelerated and the difference in velocity of the gas in the extinguisher versus the gas exiting the nozzle determine the magnitude of the thrust and thus Alex’s 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 

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