

Rolling Ball Size
Name: Lee S.
Status: student
Age: 10
Location: N/A
Country: N/A
Date: 20002001
Question:
Does a small ball roll faster than a larger ball? If so
why or why not? I think that a small ball rolls faster because it turns
over quicker? Is that right?
Replies:
The answer depends upon what you mean by "faster".
If you mean that a small ball and a large ball are moving at the same number
of revolutions/second, then the larger ball will move a greater distance
than the small ball in the same length of time, so you would say the large
ball moved faster  greater distance in the same time.
If you mean that a small ball and a large ball move the same distance in the
same time, the small ball will have to make more revolutions in that time in
order to travel the same distance as the large ball so it travels at more
revolutions/second  same distance in the same time, more revolutions per
second.
Of course the time did not have to be seconds, it could have been minutes or
what ever.
Vince Calder
This question is incomplete. There is no definite speed at which a ball
rolls. How hard and long the ball is pushed, as well as the mass of the
ball, will be important. Let us assume all things are equal (starting speed,
weight, both balls solid, path the ball follows) except for size. One ball
is larger than the other. On Earth, it is true that the large ball slows
more than the small ball. This is because the larger ball runs into more
air molecules. For a ball to move, it has to push the air out of the way.
This results in air resistance, the same force that slows a man with a
parachute. The larger ball feels more air resistance. On the moon, where
there is no air, both balls would roll the same.
Kenneth Mellendorf
Hello Lee.
I am not sure what the strating conditions of your balls are.
However, one of the most famous experiments ever performed was by Galileo,
some 400 years ago. He wanted to know whether heavy objects fell faster
than light ones. The famous story is that he went to the top of the leaning
tower of Pisa and dropped two objects, one heavy and one light. He saw that
they fell at the same rate! It has been shown that this was just a story
because Galileo did not have any way to measure the rate at which the
objects fell (he did not have a camera or other high speed recording
device). What he did do is make a ramp out of wood, and roll balls of
various weights down this ramp. The ramp allowed him extra time to measure
how long it took for the balls to reach the bottom. It took the same length
of time, no matter how heavy the balls were. This was the basis for a
wellknown law of acceleration due to gravity.
If you mean which ball rolls faster along a flat surface, that is
a bit different. If the two balls turn over at the same rate, then the
bigger ball will move farther in the same length of time. Let us say that
the radius of the small ball is r, and the radius of the large ball is R.
The circumference of a ball is (2 times Pi times radius). This means that
each time a ball turns over, the small ball will move (2 times Pi times r),
while the big ball will move (2 times Pi times R). Since R is bigger than
r, the big ball will go farther.
Jim Rubin
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Update: June 2012

