Download Newton`s Third Law/ Common Misconceptions

Newton’s Third Law: Common Misconceptions
What is Newton’s Third Law?
Formally stated, for every action, there is an equal and opposite reaction.
Meaning…?
The statement means that with every interaction (which involves two objects),
there is a pair of forces acting on the two interacting objects. These two forces
are called action and reaction forces and are the subject of Newton's third law
of motion. The size of the force on the first object equals the size of the force
on the second object. The direction of the force on the first object is opposite to
the direction of the force on the second object.
A common example
A variety of action-reaction force pairs are evident in nature.
Consider the propulsion of a fish through the water. A fish uses
its fins to push water backwards. But a push (force) on the water
serves to accelerate the water. Since the fish interacts with the
water, the water must also be pushing the fish forwards. The
size of the force on the fish from the water equals the size of the
force on the water from the fish, but in the forward direction,
thus allowing the fish to swim.
Makes sense? Don’t fall for the common misconceptions!
#1: “The two forces in an action-reaction pair can cancel out.”
In the diagram below, Blue pushing on Green is the action force and Green pushing on
Blue is the reaction force. If students are asked which of these forces is larger, they all
dutifully respond that the forces are of the same magnitude and in opposite directions.
But when students are asked why Blue and Green do not move, most say that the two
forces “cancel". Action reaction pairs can NEVER cancel under any circumstances
because the two forces are on different objects. The action is a force on Green and the
reaction is a force on Blue.
g
Blue
Green
b
The reason Blue and Green do not move is because of the force of friction. The force
balancing the force Blue applies on Green is friction between Green’s feet and the floor.
Same applies to Blue.
#2: “Newton’s Third Law is nullified when motion is initiated.”
EMMA
ALEX
Emma and Alex push on each other. Emma weighs less than Alex and Alex is able to
push Emma backwards. When students are asked which person exerts the greater
force, the common response is Alex, “because he was able to move Emma”.
This is also incorrect! Just because motion is initiated, does not mean the third law of
motion goes out the window! We know from Newton’s Third Law that interacting forces
exert a force of equal magnitude but in opposite directions! Consider Newton’s 2nd
Law…
The force on Alex from Emma is equal to the force on Emma from Alex just in opposite
directions. The forces do not cancel because they act on two different objects.
!
So if the forces are the same, why does Emma move? This is due to their MASS, not
the FORCE.
Newton’s 2nd law says that the net force is equal to mass times the acceleration.
!
Since the forces are equal, the mass of Alex multiplied by the acceleration of Alex must
equal the mass of Emma multiplied by the acceleration of Emma.
!
Since Alex weighs more, his acceleration will be less. Since
Emma’s mass is smaller, her acceleration is larger. This greater
acceleration causes Emma to fall backwards.
Our definition of Newton’s 3rd law:
For every action there is an equal and opposite reaction
on the other object.