Physics Photo of the Week

November 6, 2009

Galileo's Two Mass Drop.  Video done by the Physics section taught by Jason Zumstein. 
Discussion by David Habib

The Physics Class at Warren Wilson each year repeats Galileo's famous demonstration of proving that two drastically different objects (a bowling ball and a golf ball) fall together with identical accelerations.  A physics student was very careful to let both balls roll off his finger tips at the same time.

From the video it can be seen that the bowling ball and golf ball fall at the same rate even though the bowling ball is 160 times more massive than the golf ball. The velocity of both balls also increase as the balls fall. Rate of change in velocity is known as acceleration and is expressed in Newton’s second law by the formula:
accel = force/mass.
 
This states that an object’s acceleration is proportional to the force on the object and inversely proportional to the object's mass. However, if Newton’s second law is true, why then do objects with different masses accelerate towards the earth at the same rate?  The force of gravity on the bowling ball is 160 times greater than the force of gravity on the golf ball.  To answer why acceleration due to gravity is the same for all objects we simply use Newton's second law of the above formula.
 
In the formula, both the force and the mass are 160 times greater for the bowling ball.  When calculating the acceleration from the ratio of force to mass, the 160 factor cancels out both in the numerator and denominator.  Therefore the acceleration due to gravity is independent of the mass of the object in question.  When air resistance is ignored all objects at the same altitude and latitude will fall at the same rate.



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

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