Action and Reaction Forces Versus Balanced Forces
Because action and reaction forces are equal and opposite, they may be
confused with balanced forces. Keep in mind that balanced forces act on
a single object, while action and reaction forces act on different objects.
If you and a friend pull on opposite sides of a backpack with the same amount of force, the backpack doesn’t move,
because the forces acting on it are balanced. In this case, both forces
are exerted on one object—the backpack.
Balanced Forces
As you drag a heavy backpack across a floor, you
can feel the backpack pulling on you with an equal amount of force.
The action force and the reaction force are acting on two different
things—one is acting on the backpack, and the other is acting on you.
Action and Reaction
The illustration below summarizes Newton’s third law. The girl
exerts an action force on the boy by pushing him. Even though the
boy is not trying to push the girl, an equal and opposite reaction force
acts upon the girl, causing her to move as well.
Newton’s Third Law
When one object exerts a force on another object, the second
object exerts an equal and opposite force on the first object.
1
One Skater Pushes
reaction
force
2
Both Skaters Move
action
force
The action force from the girl sets the
boy in motion.
Even though the boy does not do anything, the
reaction force from him sets the girl in motion as well.
How does the direction of the force on the girl
relate to her motion?
Chapter 11: Forces 379