Download Newton`s 3rd Law Notes

Newton’s 3rd Law Notes
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For every force there is an equal and opposite force.
For every action force there is always a reaction force.
Force involves the interaction of two objects and/or substances.
Identifying Action-Reaction Forces Pairs
Identifying action-reaction force pairs requires discipline and focus on the rules of the 3 rd law of motion:
 The action force and reaction force act on different objects.
 If A exerts an action force on B, then B exerts an action force on A
The sketch on the right shows a book resting on a table. The force of
gravity, Fg, and the normal force, Fn, are not an action reaction pair.
At first glance it may appear that these are action-reaction pairs
because the forces are equal and opposite. However, they are not
because both forces are acting on the same object, the book.
The sketch on the right correctly shows the
action and reaction force pair. The book
exerts a force on the table and the table
exerts an equal and opposite force on the
table. It does not matter which force is
designated the action force and which is
designated the reaction force.
In a closed balloon the air
exerts an action force, A,
outward on the walls of
the balloon, and the walls
of the balloon exert an
equal reaction force, B,
inward on the air.
Forces, Acceleration & Mass
What about the force of gravity and what is its reaction force? When the force of gravity pulls on an object it is an
Action force and the reaction force is the object pulling on
earth. It is a mutual attraction. In the example to the right,
the action force is the earth pulling down on the ball and
the reaction force is the ball pulling up on the earth with
the same force. We see the ball moving toward the earth,
but if the ball is pulling up on the earth with an equal force,
why don’t we see the earth moving toward the ball?
and
The mass of the earth is so large compared to the force that the acceleration of the earth toward the ball is too
infinitesimal for us to notice the earth moving toward the ball.
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More massive objects accelerate slower than less massive objects.
When a cannon fires a cannonball, the cannon pushes on the
cannonball and the cannonball pushes back on the cannon with equal
force. This is why the cannon recoils. The cannonball moves with much
greater acceleration than the cannon, because the cannonball’s mass is
much less than the mass of the cannon. Also, the acceleration of the
cannon’s recoil is reduced by the friction between the cannon and the
ground, whereas the cannonball’s acceleration is only reduced by air resistance.
How Can We Have Acceleration if the Forces are Opposite & Equal?
Newton’s 2nd Law tells us that in order for an object to accelerate there must be unbalanced forces acting on the
object. Why do objects accelerate when every force has an equal and opposite force?
 Identifying Forces Working on the Moving Object.
In the example to the right, the rocket pushes the exhaust gas downward and
the exhaust gas pushes the rocket upward with equal force. The force of the
rocket pushing on the gas does not balance or counter the upward force of the
exhaust gas because the downward force is acting on the exhaust gas not the
rocket. The motion of the rocket is determined by the forces acting on the
rocket. The forces acting on the rocket are the exhaust gas pushing upward,
the force of gravity pulling downward, and the force of air resistance pushing
downward. If the force of the exhaust gas (known as thrust) is greater than the
force of gravity (aka weight of the rocket) and air resistance the rocket will
accelerate upward.
 Defining the System.
When a horse pulls a cart the horse the cart pulls back on
the horse with the same force the horse pulls forward on
the cart. If these were the only two forces involved the
horse and cart would never move. However, we need to
define the system and identify the forces working on the
system. We can define the system as the horse only or as
the horse and the cart.
System = Horse
We need to look at the external forces acting on the horse. The wagon pulls backward on the horse. The horse
pushes backward on the ground and in accordance with Newton’s 3rd Law, the ground pushes forward on the
horse with the same magnitude. Therefore, the horse and the cart will accelerate forward if the horse can push
on the ground with more force than the cart pulls on the horse.
System = Horse + Cart
We need to look at external forces acting on the system. The action-reaction force pair created by the interaction
of the horse and the cart can be ignored because they are internal system forces. The horse pushes backward on
the ground and the ground pushes forward on the horse-cart system. Friction between the cart and the ground
causes the ground to push backward on the horse-cart system. If the horse can push backward on the ground
with more force than the force of friction between the cart and the ground, then the horse-cart system will
accelerate forward.