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Falling Objects, Size, Timing
Name: Laret
Status: educator
Grade: N/A
Location: CA
Country: N/A
Date: 4/18/2005
Question:
If two objects of different weights are dropped form the
same height, which one will reach the ground first?
Why? What is the scientific explanation and how would I teach it to
children?
Replies:
This is an old question and often difficult to explain to the young mind.
First, we need to eliminate wind resistance from the discussion. Obviously
if two objects of the same mass are dropped simultaneously... the one with
the parachute lands last! Air resistance can make a great or small
difference, but either way, it messes up the answer to your core question.
Second, it is important to realize that weight and mass are related, but
different. Said another way, the greater the mass of an object, the greater
the force of gravity... the weight that is.
Now, consider that if an object has twice the weight, it will have twice the
force of gravity pulling on it. It would be common sense that if the 2X
object has twice the force acting on it, it would accelerate at the same
speed as the 1X object - that has half the mass and half the force acting on
it.
Whenever we change mass of an object, we change the force of the pull of
gravity proportionately. It requires more force to accelerate a larger
object at a given rate, but because the larger object has more weight (force
pulling on it) the extra force is available. It will - ignoring the effect
of the friction of the air - accelerate at the same speed when dropped.
They will reach the ground at the same time.
Have you seen the pence and the feather in the glass tube? It is a good
demonstration of this principle.
A classic question. I hope a helpful answer.
Larry Krengel
They both reach the ground at the same time, unless there is a difference
in air resistance between the two bodies (for example a feather and a
stone). Both would reach the ground at the same time if the dropping were
done in a vacuum where there is no air resistance. This is the famous
demonstation attributed to Galileo from the tower of Pisa. The reason is
the gravitational force exerted on a body depends upon its mass, not its
weight. There are many websites with explanations at differing levels of
sophistication, depending upon grade level etc. A 'google' search on the
term "gravity Galileo experiment" found the following, among many others:
http://vesuvius.jsc.nasa.gov/er/seh/feather.html
http://www.spartechsoftware.com/reeko/Experiments/gravity.htm
http://www.phy6.org/stargaze/Sfall.htm
The outcome is counter-intuitive, but is a good way to teach students about
not necessarily believing the "obvious" outcome without actually doing the
experiment.
A good conceptual physics source that is easy to read and will help you is:
Paul Hewitt "Conceptual Physics" Addison-Wesley
Vince Calder
As Galileo is often reported to have done (though he probably did not), he
dropped a 10 lb and a 1 lb weight at the same time from the top of the
leaning tower of Pisa and saw that they reached the ground at the same time
within the accuracy of his measurements. A similar experiment, perhaps from
a balcony at your school might be an excellent way to teach your students.
Use, perhaps, a bowling ball and a bocce ball.
You will be able to easily convince your students that the 10 lb ball does
NOT fall 10 times faster than the 1 lb ball, as Aristotle believed.
The physics reason is that, although the gravitational force accelerating
the 10 lb ball downward is 10 times as large as that accelerating the 1 lab
ball downward, the inertia of the 10 pound ball is 10 times as large as that
of the 1 pound ball. Inertia, as you know, is how bodies resist a change in
their motion.
It is still a mystery why gravitational mass is absolutely identical to
inertial mass, but it is true to incredible accuracy.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
Laret,
IF there is no air resistance, they will both hit the ground first. On the
moon, which has no air, a feather and an iron ball will fall together. I
believe it was Neil Armstrong that proved this. When you have air slowing
things down, shape and size are just as important as weight.
A good example requires a book and a piece of paper smaller than the book.
If you drop them side-by-side, the book reaches the ground long before the
paper. If you place the paper UNDER the book (firmly in contact) and drop
them together, they will fall together. Most can easily believe this. If
you place the paper OVER the book and drop them together, they still fall
together. The book pushes the air out of the way. Since there is no air
slowing down the paper, it falls with the book.
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
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