Download Newton`s Third Law of Motion – Action and Reaction 6.1 Forces and

Newton’s Third Law of Motion – Action and Reaction
6.1 Forces and Interactions:
•
In chapter 4 we learned that a force is a push or a pull. Newton takes the concept of a force one step further and says
that it is a part of an interaction between one thing and another. When you hit a nail with a hammer, the force of the
hammer on the nail is only half of the story because the nail is also pushing back onto the hammer as it resists the
change in its motion.
6.2 Newton’s Third Law:
•
•
•
Newton’s Third Law states: Whenever one object exerts a force on a second object, the second object exerts an
equal and opposite force on the first object.
The first force, the one initiating the action, is called the action force. The second force, the one responding to the
action, is called the reaction force.
Labeling the forces as action and reaction is not nearly as important as understanding that they are an inseparable
pair. You cannot create an action force without a reaction force happening in response to it.
6.3 Identifying Action and Reaction:
•
•
Sometimes action reaction pairs are hard to see, a good rule to follow is:
o The action force is a force that object A is exerting directly on object B
o The reaction force is a force that object B is exerting directly on object A
Good example question for this section on page 76, also good illustrations to look at on page 76
A good rule of thumb for identifying action and reaction forces is to look at the wording of the initial force.
o Action: The force that A exerts on B
o Reaction: The force that B exerts on A
This means that if the ground pushes on the boulder, then the boulder pushes back on the ground –
never change the objects or the action verb!!!!
6.4 Action and Reaction on Different Masses
•
•
•
The action and reaction forces are equal. In section 6.3 of the book it gives and action/reaction pair example of a
boulder falling to the earth. The action force was the earth’s gravitational pull on the boulder, and the reaction force
was the boulders gravitational pull on the earth. These two forces are of equal magnitude.
In the example with the boulder and the earth it may be hard to understand how the boulder can be exerting the same
force on the earth as the earth is exerting on it if the boulder is falling towards the earth and the earth is not moving
toward the boulder. Remember that Force = mass ⋅ acceleration and so the same force has a much different effect
on the boulder than on the earth due to the large difference in their mass.
This is why a gun or cannonball “kicks” when you fire them. The cannon or bullet travels very far very quickly
because it is very light. The same force that caused that movement in the bullet or cannonball is exerted on the gun
or cannon, however it does not cause as much movement because the cannon is so much more massive than the
cannonball and the gun is so much more massive than the bullet.
6.5 Do action and Reaction Forces Cancel?
•
•
Action and reaction forces would cancel if the masses were the same and there were no outside forces, however
most action reaction systems are not this simple, and either the masses are different or there is more than one
action/reaction interaction going on. On pg. 79 the book it gives a good example of why the forces of an apple
pulling an orange on a cart do not cancel out. In this system there are at least 3 different action and reaction pairs,
only one of which is the pull between the two pieces of fruit.
One way to simplify action/reaction problems is to isolate each pair and look at it individually. If this is done other
forces action on the object are coming from outside the “system” and are considered differently.
*It is important that students READ section 6.5, as this is one of the more complicated concepts that
students have been introduced to during this unit*A good example problem for this section is the one on pg79
6.6 The Horse Cart Problem:
• This section describes the classic horse-cart problem, it is essential that students read and understand how the
different action/reaction pairs in this system affect one another.
Good example questions about the horse-cart problem are on page 80
6.7 Action Equals Reaction:
•
In this section of the book it helps to explain why it is possible to hit a wall much harder than it is possible to hit a
piece of paper. This goes back to what we said about the same force hitting two different masses would cause a
different amount of movement. Some objects due to their mass are not capable of exerting enough force back to
make an action possible (like punching a piece of paper, the paper will break if it is hit with enough force because it
cannot create the appropriate reaction force. Make sure that students read this section and understand that concept.