Transcript Newton’s Third Law:

“How many
horses had to
die for this
stupid, itchy
wig?”
Newton’s
Third Law:
Impulse &
Momentum
a momentous topic
“For every action, there exists an equal and
opposite reaction.”
- Isaac Newton, Principia Mathematica
F  ma
v
a

t
so
and
v f  vi
t
v
F m
t
Ft  mv
“For every action, there exists an equal and
opposite reaction.”
- Isaac Newton, Principia Mathematica
Ft  mv
Impulse
Is known as the
“Impulse-Momentum”
theorem, where
Change in
momentum
“For every action, there exists an equal and
opposite reaction.”
- Isaac Newton, Principia Mathematica
Inertia in motion**
Product of mass
& velocity
**For subatomic-sized particles moving really, really fast (~90%
of light speed), momentum is defined differently. It’s impossible to
determine BOTH position and momentum at the same instant in
time because of some freaky mathematics; hence, we have
Heisenberg’s Uncertainty Principle.
“For every action, there exists an equal and
opposite reaction.”
- Isaac Newton, Principia Mathematica
Heisenberg may
have slept here
. . . but, for relatively large objects (like molecules) that are moving
fairly slowly (<0.9c), the classical definition of momentum works just
fine, and is much less complicated.
“For every action, there exists an equal and
opposite reaction.”
- Isaac Newton, Principia Mathematica
For momentum
to change,
p = mv
p
the mass can change,
the velocity can change,
Or BOTH may change!
 m*v
 m*v
p = mv + mv
These two
canoeists (each of
mass 60 kg) propel
their canoe (30 kg)
at 5 m/s towards
an unseen rock on
this pond. After
the 0.5-s collision,
the canoe moves
back at 1 m/s.
Big rock just
This canoe can
under here
withstand a force
of 1500 N without
puncturing.
What is the change in momentum of the canoe/passengers?
Will the canoe break?
“For every action, there exists an equal and
opposite reaction.”
- Isaac Newton, Principia Mathematica
m asses  150kg
changein velocity  v f  vi
-5 m/s
m  m
v  1    5 
s 
s
p = mv
p = (150 kg)(6 m/s)
p = 900 kg m/s
1 m/s
“For every action, there exists an equal and
opposite reaction.”
- Isaac Newton, Principia Mathematica
Ft  mv
mv
F
t
kg m 

 900

s 

F
0.5 s
F = 1800 N
“For every action, there exists an equal and
opposite reaction.”
- Isaac Newton, Principia Mathematica
A 75-kg driver is traveling at 100 m/s
when his car comes to a sudden stop.
The driver is not wearing a seatbelt.
He collides with the steering column
for 0.005 s. The human body can
withstand about 9 times the weight of
the body as impact force. Has this
driver committed suicide-sansseatbelt?
“For every action, there exists an equal and
opposite reaction.”
- Isaac Newton, Principia Mathematica
A 75-kg driver is traveling at
100 m/s when his car comes
to a sudden stop. The driver
is wearing a seatbelt. He
collides with the steering
column for 1.2 s. The
human body can withstand
about 9 times the weight of
the body as impact force.
Has this driver committed
suicide-sans-seatbelt?
This crash occurred in Oahu; yes,
the driver was wearing a seat belt
AND WALKED AWAY!
Many mooses’ mass may be
more than your Maserati’s
mass. This moose’s mass is
680 kg, and it is initially
moving at 9 m/s. If the
collision lasts for 0.5 s, how
much force is sustained by the
car?
Water leaves a fire hose at a rate of 125 kg/s with a speed of
200 m/s and strikes a wall, which stops it. (We’re ignoring
any splashing back.) What is the force exerted by the
water on the wall?
FYI: Fire hoses were used in the 1960’s
to break up peaceful civil rights
demonstrations.
Calculate the force exerted on a rocket, given that the propelling
gases are expelled at a rate of 1000 kg/s, with a speed of 60,000
m/s (at takeoff).
Your assignment
for next time:

pp. 83+:
RQ 1-8;
T&E 1,3-12,15