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Falling Objects, Same Time
Name: Nanette
Status: educator
Grade: K-3
Location: CA
Country: N/A
Date: 5/31/2005
Question:
Our class did a science project to determine which object
would land first if two objects were dropped from the same height at the
same time: a heavier object or a lighter one. We were surprised to find
that both objects landed at the same time. Why is that?
Replies:
Nanette,
In an ideal situation, all objects dropped from the same height will land at
the same time. Gravity causes the same acceleration for everything.
Gravity pulls down on an object. The object's mass resists the pull. This
resistance is called inertia. In an ideal situation the heavier object gets
pulled harder, but resists harder as well. Both the heavier and lighter
object therefore move the same way.
In a real situation, the air has an effect. As an object moves through the
air, it must push the air out of the way. Small objects, such as marbles,
feel very little air resistance. The weight is much more than the air
resistance, so the air resistance does not matter very much. For a ping-pong
ball, the weight is very small compared to the air resistance. Air
resistance slows a ping-pong ball.
Very large objects are slowed by air resistance. A heavy man with a
parachute is slowed. Before the parachute opens, the man must push a small
amount of air out of the way as he falls. He feels very little air
resistance until moving very fast. After opening, the wide parachute must
push much more air out of the way. The greater air resistance slows down
the man with the open parachute.
As an example, drop a flat full-sheet of paper and a crunched up half-sheet
of paper. They will not fall together. The full sheet is heavier, but
falls slower.
Another good example is a large book and an index card:
1) When dropped alongside each other, the book hits the floor first. The
card is slowed very much by air resistance because it is both wide and
light.
2) When the card is against the underside of the book, they fall together.
The air tries to slow the card, but the book keeps the card moving.
3) When the card is lying on top of the book, they drop together. The book
pushes the air out of the way for the card. With no air resistance, the
card falls just as fast as the book.
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
You have repeated one of the most famous experiments in all of physics,
which Galileo is said to have performed at the leaning tower of Pisa around
1600 (though he probably did not). The experiment has been done many times
by others, always with the result you found.
The reason is clear from Newton's laws of motion and of gravity, which have
by now also been tested very many times and have always been in accord with
experiment.
Newton's law of gravity says the gravitational force pulling the balls down
is proportional to the masses of the balls. His second law of motion says
that for a given force acting on a ball, its acceleration is inversely
proportional to the mass of the ball. So the time to fall, which depends on
the acceleration, is independent of the mass, since it is proportional to
m/m (mass divided by mass).
Notice that this means the gravitational mass is identical to the inertial
mass, a rather remarkable result since the gravitational mass is what causes
the ball to accelerate and the inertial mass opposes the acceleration.
Nonetheless, that seems to be the way the universe is put together.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
Gravity affects all objects similarly. more massive objects, however,
experience a greater gravitational pull. This is balanced out by the extra
energy needed to accelerate that greater mass.
For a demonstration, have students try pushing two objects across a desk.
Such as a bowling ball and a large marble. The Bowling ball takes far more
effort to accelerate to the same speed! (so even though there is more
gravity pulling on the heavier object, the acceleration rate remains the
same, because it's harder to move.)
Ryan Belscamper
Hi Nanette,
What you have experienced with your experiment is that gravity affects
all objects with any mass equally. It does not matter if they are very
heavy or very light; all objects are accelerated toward the earth's
surface at the same rate.
The only concern with light objects is when the affect of air resistance
will take effect. If you drop a bowling ball and a feather, the ball
will land first because the feather has been slowed by air resistance.
In a total vacuum, they would both land at the same time.
Bob Hartwell
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