Physics Photo of the Week

March 2, 2012

Galileo's Famous Experiment - Falling rate of two highly unequal mass objects.
Discussion by Allison Giles

When a bowling ball and a golf ball are dropped from the same height, at the same time, which do you expect to hit the ground first? Many assume that the bowling ball will land before the golf ball; after all it is heavier. Because it has more mass, shouldn’t it land first? Wouldn’t it fall faster? This is not the case, however. In fact, the balls both accelerate at the same rate.

This is all thanks to Sir Isaac Newton’s second law of motion. This law says that when you calculate acceleration, you use the formula A=F/M, which means acceleration is equal to the force divided by the mass of the object.  A bowling ball has about 160 times the mass of a golf ball. This means that the Mass in the denominator of the formula should be 160 times greater than the mass of the golf ball. If this were the only factor that was changed, it would mean that the bowling ball would accelerate at a much slower rate than the golf ball, and hit the ground much later. However, since the mass is 160 times larger the force of gravity on the bowling ball is also 160 times greater. Both the numerator and the denominator are multiplied by 160, the ratio remains the same because both 160 factors in the numerator and the denominator cancel out.

                               A=160F/160M = F/M

The acceleration of the bowling ball becomes equal to the acceleration of the golf ball, so they hit the ground at the same time if they are released at the same time.

In the video clip, Zazie Tobey happened to release the golf ball a tiny bit earlier and the golf ball actually lands a little bit ahead of the bowling ball.  It is difficult to control the letting-go times.

Both Zazie and Allison are students enrolled in Earth, Light, and Sky at Warren Wilson College.


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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