Newton’s Third Law
The Most Misunderstood Concept in Science
LA W # 1
An object at rest will continue to
stay at rest or will continue to travel at a constant
speed in a straight line unless compelled to change by
forces acting on it.
“Law of Inertia”
LA W # 2
Fnet = 0
The ACCELERATION of an object is directly
proportional to the NET FORCE (unbalanced
force) acting on it and is inversely proportional to its
MASS where the direction of the ACCELERATION is in
the direction of the unbalanced force.
Fnet = MA
LA W # 3
When one object exerts a force on a
second object, the second object exerts
an equal and opposite force on the first.
“For every action there is an equal
and opposite reaction”
“Rule of Forces: ALL forces occur in PAIRS
where F1=-F2”
Law #1: All forces on object
balanced
Law #2: All forces are NOT
balanced
All motion is either Law #1 OR
Law #2
ALL motion is Law #3
Law #1 and #2 deals with forces
acting ON A SINGLE OBJECT
Law #3 deals with forces
BETWEEN TWO OBJECTS
LA W # 1
An object at rest will continue to stay at rest or will continue to
travel at a constant speed in a straight line unless compelled to
change by forces acting on it where the forces acting between
the object occur in equal and opposite pairs.
“Law of Inertia”
LA W # 2
Fnet = 0
The acceleration of an object is directly proportional to
the net force (unbalanced force) acting on it and is
inversely proportional to its mass where the direction of
the acceleration is in the direction of the unbalanced force
where the forces acting between the object occur in equal
and opposite pairs.
Fnet = MA
HW #4-1 (III) page 130
[MC] #9,10,11
#9. The action reaction forces of Newton’s third
law (a) are in the same direction, (b) have different
magnitudes, (c) act on different objects, (d) can be
the same force.
 Newton’s third law tells us that any
time two objects hit each other, they
exert equal and opposite forces on
each other.
The effect of
the force
is not
always
the same.
Force (on bullet)= -(Force (on gun))
F=ma (for both)
mbulletabullet = - (mgunagun)
#10. A brick hits a glass
window. The brick
breaks the glass, so (a)
force of brick greater
than force of glass, (b)
force of brick is smaller
than force of glass, (c)
force of brick equals
force of glass, (d) none
of these
#11. A freight truck collides head
on with a passenger car causing a
lot more damage to the car than
the truck. Which is true?
(a) The force of the truck on the car is bigger than
the force of the car on the truck.
(b) The force of the truck on the car is smaller than
the force of the car on the truck.
(c) The force of the truck on the car is the same as
the force of the car on the truck.
(d) None of the above
CQ #13.
Here is a story of a horse and a farmer…
One day, the farmer attaches a heavy cart to the
horse and demands that the horse pull the cart.
“Well,” says the horse, “I can’t pull the cart because,
according to Newton’s Third Law, if I apply a force
to the cart, the cart will apply an equal and
opposite force on me. The net result will be that I
cannot pull the cart, since all the forces will cancel.”
The farmer was very upset.
What is wrong with the horse’s reasoning?
The two forces colored yellow in the diagram are a Newton's Third Law force pair - "horse
pulls wagon" and "wagon pulls horse". They are equal in magnitude and opposite in direction.
The two forces colored blue in the diagram are a Newton's Third Law force pair - "horse
pushes ground" and "ground pushes horse". They are also equal in magnitude and opposite in
direction.
*only the forces ON the
HORSE cause the horse
to accelerate.
[E] #31, 32
#31. A book is sitting on a horizontal surface.
How many forces are acting on the book?
TWO:
Gravity and the table
#32. In an Olympic figure skating event, a 65-kg
skater (B) pushes a 55-kg skater (A), causing her
to accelerate at 2.0 m/s2. At what rate will
skater B accelerate?
A= B
F F
m a =m a
A
A
B
B
a)
(55 kg)(2.0 m/s2)=(65 kg)(
110 N /65 kg=
1.7 m/s2=
a
a
B
B
B
How does a rocket fly (according to Newton’s Laws)?