Chapter 4
Newton’s Third Law of Motion Action
and Reaction
Presented by April Senger
Forces





A speeding baseball does not have a force until it
collides with something and interacts
Every interaction requires a minimum of 2 forces
Nonliving and living objects can both produce forces
Two objects that collide exchange the same force and
the same time in opposite directions
You can not possess of force but can possess
momentum and kinetic energy
Presented by April Senger
Concept Check



A car is accelerating along a horizontal road.
What is pushing the car along?
What direction is air drag acting?
What direction is road friction acting?
Presented by April Senger
Newton’s Third Law



Whenever one object exerts a force on a second
object, the second object exerts an equal and
opposite force on the first
Thus there is an action and reaction force
More simply stated…to every action there is an
equal and opposite reaction
Presented by April Senger
Concept Check



Which exerts more force, the Earth pulling on
the moon, or the moon pulling on the Earth
When a heavy football player and a light one run
into each other, does the light player really exert
as much force on the heavy player as the heavy
one exerts on the light one?
Is the damage to the heavy player the same as
the damage to the light one?
Presented by April Senger
Action and Reaction Forces




Stating action and reaction forces is simple
If I push on the wall I am the action force
The wall pushing back is the reaction force
To show this interaction, pictures are commonly
drawn with two arrow indicating the direction
of the action and reaction forces
Presented by April Senger
Concept Check

Can you identify the action and reaction forces
in the case of an object that is falling in a
vacuum? (A vacuum is a region of space that is
completely empty with no air)
Presented by April Senger
Mass and Actions



If you feel a recoil or kick after firing a gun, you are
feeling the push back on the weapon in reaction to the
action of the weapon pushing forward
The mass of the weapon and bullet are very different
so you see different results depending on their mass
A musket would have a different purpose than today’s
hunting rifles
Presented by April Senger
Example






a = F / m of bullet is a large acceleratcion
Any time a large number is divided by a small
number you get a big number
Ex: 10 / .1 is 100
a = F / M of weapon is smaller acceleration
Any time a large number is divided by a larger
number you get a small number
Ex: 10 / 1 is 10
Presented by April Senger
Rockets





A space shuttle or rock
pushes the Earth on ignition
The Earth pushes back
The rocket’s mass shows a
greater impact and moves up
The rocket doesn’t push
down on the atm to go
higher
It climbs as a result of the
first push



At one time, travel is space
was thought to be impossible
because the rockets couldn’t
push against empty space
It is actually easier for rocket
travel in space because there
is not air drag to create
friction
The rocket will continue to
move forward unless acted
on by an outside force
Presented by April Senger
Concept Check


A high-speed bus and an innocent bug have a
head on collision. The force of the bus on the
bug splatters the poor bug all over the
windshield. Is the corresponding force of the
bug on the bus greater, less, or the same?
Is the resulting deceleration of the bus greater
than, less than or the same as that of the bug?
Presented by April Senger
Adding It Up




We usually don’t notice action reaction forces that are
equal, such as a bike leaning against a tree, because they
are not moving
If you kick a football, your foot does slow due to the
interaction but the football accelerates in the opposite
direction
The only way to have motion result in zero force is to
have them be equal, in the opposite direction and at the
same time
Ex: Two people kick a football at the same time and
force from opposite sides
Presented by April Senger
Concept Check

Why does a flower pot sitting on a shelf never
accelerate “spontaneously” in response to the
trillions of inter-atomic forces acting within it?
Presented by April Senger
Tug-A-War



If we had a tug-a-war between boys and girls on
a slippery polished floor with the boys wearing
socks and the girls in rubber soled shoes…who
would win
The winning force has more to do with the
greatest net force with the ground than force
pulling the rope
Why can a horse pull a heavy cart?
Presented by April Senger
Concept Check


We said that a car accelerates along a road
because the road pushes it. Can we say that a
team wins in a tug-a-war when the ground
pushes harder on them than the other team?
Does the scale read 100 N, 200 N, or zero?
Scale
100
100
Presented by April Senger
So there’s a bird, plane and
helicopter…





What do all of these objects have in common?
They are all specially shaped so that they force air
molecules down (action) and the reaction is air pushing
up (reaction)
When the lift up is greater than the lift down, the object
takes flight
Why do birds and sometimes planes fly in a V pattern?
They have an updraft that a second bird/plane can
utilize for lift in flight
Presented by April Senger
Follow Through


If a person is boxing and hits a punching bag it
is easy to exert a force of 20 N (about 4.5 lb) on
the bag
It is impossible for a boxer to hit a piece of
paper hanging in the air with the same punch
and exert 20 N of force…Why?
Presented by April Senger