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 Newton's Third Law in Motion
Name: Evan
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A

It is easy to explain to students what is happening with respect to Newton's Third Law when you stand on a surface and you are not moving. You press down, the surface presses up, and the net force is zero. What happens when you step onto a surface with a rotten plank and you fall through? Clearly the forces are not balanced. If the action and reaction forces are equal and opposite, does this mean that, regardless of your weight, that only the portion of your weight that was sufficient to break the plank is the action force, and, therefore, the reaction from the plank is just not sufficient to stop your descent?

The action force is constant; it is the pull due to gravity on your body. The plank provided a short term and insufficient reaction force. Unfortunately, as the plank broke, the reaction force from the plank disappeared and you are now falling with the force due gravity accelerating you until you hit the ground. On the way down, the reaction force is you pulling Earth up. Nothing has changed in the action force, but reaction force is different.

Bob Avakian

Oklahoma State University Institute of Technology.

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 (, or at Argonne's Educational Programs

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

Argonne National Laboratory