Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Does Baloney violate Newton's 3d law?

Learn more physics!

Most recent answer: 04/11/2011
Q:
http://www.newton.dep.anl.gov/askasci/phy05/phy05244.htm
The question says that when you step on a rotten plank and fall through "the action and reaction forces are not equal." The answer says "Nothing has changed in the action force, but reaction force is different." Is it true that Newton's 3d law only applies to good-quality carpentry?
- Mike W.
A:
Oy. They've mixed up two different forces on one object (gravity and the force from the floor on you) with a single force between two objects. Newton's 3d law says absolutely nothing about whether the combination of forces on one object is zero or not. It does say that the force between two objects (you and the floor) is equal in strength and opposite in direction on the two objects. The floor pushes up on you exactly the same amount you push down on the floor. Likewise (as they sort of grasp at one point) you pull up on the earth exactly as much as the earth pulls down on you.

This is one of those mistakes which beginning students often make. To see  two educators (the questioner was also a teacher) reinforcing it is sad.

Mike W.

(published on 04/11/2011)

Follow-up on this answer.