Chapter 7 Newtons Third Law of Motion: Action and Reaction
7.1 Forces and Interactions
A force is always part of a mutual action that involves another force.
A mutual action is an interaction between one thing and another.
Ex. a hammer and a nail—A hammer exerts a force on the nail and drives it into a board. But this force is only half the story, for there must also be a force exerted on the hammer to halt it in the process. That would be the nail.
Newton reasoned that while the hammer exerts a force on the nail, the nail exerts a force on the hammer. In their reaction there are two forces, the hammer on the nails and the nail on the hammer. Such observations led to Newtons third law, the law of action and reaction.
7.2 Newton’s Third Law
Newtons third law describes the relationship between two forces in an interaction.
Newtons third law states that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
One force is called action force. The other is called the reaction force. It doesn’t matter which one we call action and which we call reaction. The important thing is that they are partners in a single interaction and that neither forces exist without the other.
They are equal in strength and opposite in direction. Newtons third law is often stated: “ To every action there is always an equal but opposite reaction.”
7.3 Identifying Action and Reaction
To identify a pair of action—reaction forces, first identify the interacting object A and B, and if the action is A on B, the reaction is B on A.
7.4 Action and Reaction on Different Masses
Newtons second law states that acceleration is not only proportional to the net force, but it is also inversely proportional to mass. Because Earth has a huge mass, we don’t sense its infinitesimally small acceleration. Although Earth’s acceleration is negligible, strictly speaking it does move up toward the falling boulder as the boulder is falling toward the earth.
Force and Mass
When a cannon is fired, there is an interaction with the cannon and the cannonball. The force the cannon exerts on the cannonball is exactly equal and opposite to the force the cannonball exerts on the cannon, so the cannon kicks.
A given force exerted on a small mass produces greater acceleration than the same force exerted on a larger mass.
7.5 Defining Systems
Action and reaction forces do not cancel each other out when either of the forces is external to the system being considered.
7.6 The Horse—Cart Problem
If the horse pushes the ground with a greater force than it pulls on the cart, there is a net force on the horse, and the horse-cart system accelerates.
7.7 Action Equals Reaction
For every interaction between things, there is always a pair of oppistely directed forces that are equal in strength.