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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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