Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne 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" Addison-Wesley

I hope this helps.

---Nathan A. Unterman



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory