

Gravity's Pull and Field Strength
Name: Sherry B.
Status: educator
Age: 60s
Location: N/A
Country: N/A
Date: 2/26/2003
Question:
Does the Earth have the same pull on objects regardless
of size, weight or mass?
Replies:
Shel:
Force is a push or pull on something. Gravity does not have the same force
on all objects. Gravity pulls on me with about 850 Newtons of force, while
it pulls on my daughter with only 530 Newtons of force. The local
gravitational force is given by the relationship, F=mg, where F is the
force due to gravity, m is mass, and g is the local gravitational field
intensity. At the surface of the earth, it is about 9.8 N/kg (to make
calculations simpler, many round to 10 N/m).
But this leaves you wondering about the claim of sameness.
According to Newton's Second law,
a=F/m
F is the net force, m is the mass, and a is the acceleration. The
ACCELERATION is the same for all objects (neglecting air resistance). How
is this so?
Let us say that we have a 10 kg and a 20 kg object. The force on the 10 kg
object is about 100N and for the 20 kg object, it is about 200
N/kg. These calculations are made from the first equation above, the force
due to gravity.
When we look at Newton's second law:
a = F/m
we find that the acceleration is the same for each object!
a = 10 F/10 m = 20 F/20 m
If you were to do this experiment in a vacuum, or in a way where the air
resistance is negligible, you would get absolutely consistent results.
Since you probably do not have a large vacuum chamber available, I proposes
you do the following experiment. Place the paper on top of a book (the
size of the book should exceed the size of the paper), and drop it. Do the
same with the aluminum. The book plows the air out of the way, to let the
paper or aluminum drop without air resistance. Just to show that the paper
and the aluminum were not pushing the book down, drop each from UNDER the
book. The results should be the same.
A good conceptual physics source that is easy to read and will help you is:
Paul Hewitt "Conceptual Physics" AddisonWesley
I hope this helps.
Nathan A. Unterman
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Update: June 2012

