Download Action / Reaction forces

AP Humor of the Day
What do you get
when you cross
an elephant and
a grape?
Elephant grape sine theta
More AP Humor
What do you get
when you cross
an elephant with
a mountain
climber?
You cannot cross an elephant with a mountain
climber.
A mountain climber is a scalar!
If something is moving, then
something MUST be pushing
on it. Things don’t move all
by themselves!
Aristotle
Galileo
Oh, baloney! Once an object is
moving in a straight line, it’s
going to keep right on moving,
unless it runs into something.
That is, of course, as long as
there is NO friction! Friction is
what slows things down.
Galileo…
Born in Pisa 1564
Galileo found that, ignoring
the effect of the air, all
freely falling objects had
the SAME acceleration,
regardless of their mass or
the height from which they
were dropped.
The Law of Falling Bodies
Galileo described HOW things moved, but
not WHY they moved that way.
Galileo wrote, “the present does not seem to
be the proper time to investigate the cause of
the acceleration of natural motion….”
Galileo died in 1642.
When Isaac Newton, born Christmas day,
1642, began his studies of motion in the
second half of the seventeenth century, that
statement was no longer appropriate.
Because Galileo had been so effective in describing
motion, Newton could turn his attention to
dynamics.
Dynamics is the study of why an object
moves the way it doeswhy it starts to move instead of remaining at rest,
why it speeds up or moves on a curved path, and
why it comes to a stop.
Newton’s most famous book was “Principia”,
published in 1687. This book first listed what
came to be known as
Newton’s Three Laws of Motion
Forces
Force: a push or a pull
Forces are vectors. ( you push or pull in a
direction!)
There are Four Fundamental Forces in our
universe
1. Gravitational Force
2. Electromagnetic Force
3. Strong Nuclear Force
4. Weak Force
The strongest of these forces is….
The Strong Nuclear Force
(it holds the nucleus of all atoms
together)
The weakest of these forces is….
Gravity
The unit for Force is the Newton, N
If someone was pushing on you with one Newton
of Force, it wouldn’t hurt much.
One Newton of Force is about the same as the
weight of a quarter pound hamburger patty.
Inertia
Inertia: an object’s resistance to a change in its
motion
Mass: how you measure inertia unit: kg
Newton’s First Law of Motion:
“The Law of Inertia”
An object at rest
remains at rest unless
acted upon by a net
external force
An object in motion
continues that motion
unless acted upon by
a net external force.
Newton’s First Law of Motion:
“The Law of Inertia”
An object at rest
remains at rest unless
a net external force
acts on it.
An object in motion
continues that motion
unless a net external
force acts on it.
Newton’s law of inertia confirmed
what Galileo concluded:
Once an object is moving, it
requires no additional force to
keep it moving. It will continue to
move in a straight line unless a
NET force acts upon it.
I knew it
first!!
Stupid humor of the day
Why did the chicken cross the road?
Issac Newton: Chickens at rest tend to stay
at rest, chickens in motion tend to cross
roads.
Second Law
F

a
m
or F  ma
( “the granddaddy of all physics equations”!)
Third Law
For every force, there is an equal but opposite
force.
Action / Reaction forces
For every force, there is an equal but opposite
force.
Action / Reaction forces
Third Law
For every force, there is an equal but opposite
force.
Action / Reaction forces
For every force, there is an equal but opposite
force.
Action / Reaction forces
The Action / Reaction forces do NOT act on
the same object.
I kick the wall (Action)
The wall kicks me back (Reaction)
• While driving down the road, an
unfortunate bug strikes the
windshield of a bus.
• The bug hit the bus and the bus hit
the bug.
• Which of the two forces is greater:
the force on the bug or the force on
the bus?
For every force, there is an EQUAL by
opposite force!
A child pulls on a 5.0 kg wagon with a net
force of 20.0 N. What is the wagon’s
acceleration?
m = 5.0 kg
Fnet = 20.0 N
a = Fnet / m
a = 20.0 / 5.0 =
a = 4 m/s2
A dog pulling a man on a sled over ice can
accelerate the sled at 2 m/s2. If the sled
and man have a combined mass of 150.0
kg, what was the net force on the sled?
m = 150.0 kg
a = 2 m/s2
Fnet = ma
Fnet = 150.0 kg x 2 m/s2 = 300 N
When released, the lift provides a
net force of 6 N on a 2 kg balloon.
If it started at rest, how fast will it
be moving in 4.0 s?
Fnet = 6 N
m = 2 kg
vo = 0
t=4s
vf = ?
vf = vo + at, so we need “a”.
a = Fnet / m
a = 6 / 2= 3 m/s2
vf = 0 + 3 x 4
vf = 12 m/s
A 1500 kg. car starts from rest and is moving
at 10 m/s after 5.0 s. What was the net
force on the car?
vf = vo + at
m = 1500 kg
a = ( v f – v o) / t
Vo = 0
a = (10 – 0) / 5 = 2 m/s2
Vf = 10 m/s
Fnet = ma
t = 5.0 s
Fnet = 1500 kg x 2 m/s2
Fnet = ?
Fnet = 3000 N
Fnet = ma,
so we need to find the acceleration.
The Calculus
connection
When Newton first wrote the relationship we
call the second law, it was not it the form
we use now. Instead, he wrote
dp
F=
dt
dp dmv
dv
F= 
m
 ma
dt
dt
dt