Transcript Newton`s Laws and Forces

Physics
Newton’s Laws
And Forces
Aristotle on motion
Aristotle
believed
objects had either
“natural” or
“violent” motion
He also said
everything would
naturally STOP
moving
GALILEO:
Since he
experimented to
get EVIDENCE
for his
conclusions,
he is considered
to be the
father of
experimental
physics.
He also
figured
out that…
 Things
in motion tend to
STAY IN MOTION; they
do NOT tend to SLOW
DOWN OR STOP ON
THEIR OWN.
 In
fact, Galileo figured
out that it TAKES a force
to make something slow
down or stop. The usual
force is (he discovered):
FRICTION.
 So
in fact, GALILEO figured out
the principle behind what we call
Newton’s 1st Law
NEWTON’S 1ST LAW
 An
object at rest
tends to stay at
rest, and an object
in motion tends to
stay in motion in a
straight line at
constant velocity
unless acted on by
an outside net
force.
INERTIA refers to Newton’s 1st Law




Inertia is a measure of an object’s
RESISTANCE to CHANGE in state of motion.
The word inertia comes from the Italian word
for “laziness.”
Inertia is DIRECTLY PROPORTIONAL TO MASS
ONLY.
INERTIA IS NOT A FORCE!!! IT IS A
TENDENCEY OR CHARACTERISTIC!
What is the difference between
mass and weight?
MASS is the
amount of
MATTER in an
object.
Mass does NOT
change,
regardless of
location.
Mass is
measured in kg.
WEIGHT is the
pull due to
gravity.
–Weight
CHANGES due
to location!
–Weight is a
FORCE!
inertia…
 If
you are floating
around in space
outside the space
shuttle, and go up
to it and give it a
kick, what will be
the results?
Newton gives credit to Galileo
 Since
Galileo took all the heat for
speaking up about new ideas and
empirical evidence, Newton gives him
credit, saying:
 “If
I have seen far, it is because I have
stood on the shoulders of giants.”
What is force?
A
push or a pull.
A
NET force is the leftover
(or unbalanced) amount of force after
you add up all the forces acting in a
certain direction. (examples in a sec…)
 Only
NET forces cause
accelerations!
 No
net force means NO CHANGE.
Newton’s 2nd Law:
 The
acceleration of a body is
directly proportional to the net
force on it, and inversely
proportional to the mass.
 This
means Fnet = ma, but is
usually just written F = ma
FORCE UNITS
 Think
of Newton’s 2nd law:
F=m a
 N = kg m/s2
 Since
mass is in kg, and a is in
m/s/s, or m/s2, F or Forces are
measured in kg m/s2.
 That
is the SAME as a Newton, or N.
F = ma
100N = 100kg X 1.00m/s2
a = F/m
1.00m/s2 = 100N / 100kg
…Newton’s 2nd Law…
 This
makes sense…the
more massive something
is, the harder it is to
accelerate. And the
more force there is, the
easier something will
accelerate.
 Zero
net force
means zero
acceleration.
F = ma
W = mg
 To
calculate weight in metrics:
 Example:
 A 15 kg border collie
weighs how much?
F
= ma or W = mg = 15 kg x 10m/s2
 = 150 kg m/s2
 or 150 N [Newtons]
F = ma
W = mg
A
2 kg mass accelerating
at 3 m/s2 has how much
net force on it?
 If
you weigh 150
pounds, your mass is
about 68 kg. (1 kg =
2.2 lbs.) What is your
WEIGHT in Newtons
here on Earth?
SOLUTIONS
A
2 kg mass accelerating at 3 m/s2 has
how much net force on it?
F
= ma = 2kg X 3 m/s2 = 6 N
 If
you weigh 150 pounds, your mass is
about 68 kg. (1 kg = 2.2 lbs.) What is
your WEIGHT in Newtons here on
Earth?
W
= mg = 68kg X 10m/s2 = 680 N
F = ma
a = F/m
A
force of 16 N is pulling on a 2 kg
wagon. If there is no friction, how fast
is the wagon accelerating?
 If
the wagon above is going at a
CONSTANT VELOCITY with the 16N
pull, how much friction is there?
A
force of 16 N is pulling on a 2 kg
wagon. If there is no friction, how fast is
the wagon accelerating?
a
= F/m = 16N / 2kg = 8 m/s2
 If
the wagon above is going at a
CONSTANT VELOCITY with the 16N
pull, how much friction is there?
 16
N ! If going constant velocity,
acceleration is 0, & net force is 0 so
all forces are BALANCED (equal)
in any 1 direction.
Newton’s 3rd Law:
 Every
action [FORCE]
has an equal and
opposite reaction
[FORCE].
 There are NO single
forces. They are
always an
action/reaction pair.”
 For
example, you touch
the table with 1N of
Force, it touches you
back with 1N of Force
SIMULTANEOUSLY (not a
little later!).
 Newton’s
3rd Law says that an action
reaction pair would be hands pushing
on barbell, barbell pushing on hands.
 Then HOW can the barbell be
ACCELERATED upward?
 What OTHER action reaction pairs are
involved?
More on Newton’s 3rd…
 If
you smash the wall with your
fist with 80 N of force, the wall
will hit you right back with 80 N.
 But
what if you hold a piece of
paper in the air and try to hit it
with 80 N?
If the paper …
 cannot
support or hit you back with
80 N, can YOU really hit IT with 80
N?
 If
the paper can only hit you back
with 5 N, what happens to the other
75 N from your punch??!
HINT:
How do they accelerate?
Many forces act at ANGLES
Force of friction
Opposes
motion
Occurs
between
surfaces in
contact
Ff
=
μ FN
Considered “two dimensional” motion.
Remember that horizontal and vertical are
COMPLETELY INDEPENDENT of each other!!!
LINK TO APPLET
FOUR TYPES
OF FORCES
 Right
now we think there are 4
different types of forces. Einstein
was working on a “unified force
theory” before he died, and others
continue that work today,
believing that there is really only
one type of force.
The four types of forces
(weakest to strongest) are:
1.
2.
3.
4.
Gravitational
Weak [Nuclear]
Electromagnetic
Strong [Nuclear]
Gravitational force…
 Is
the
WEAKEST of all
the forces, but
acts over the
GREATEST
distances!
You are pulling and pulled by EVERY
ATOM IN THE UNIVERSE!!!
 The
FORCE of gravity=G (the
gravitational constant 6.67 X 10-11
N m2 / kg2) X mass1 X mass 2
divided by the square of the
distance between the two objects
(center to center)
Electromagnetic
forces…
 Are
the forces that act due to
electron configuration in atoms
 These
forces are responsible
for the PROPERTIES or
characteristics of elements,
compounds, and materials
 This
force
makes paint
stick to a wall,
makes
diamonds
extremely hard,
makes gold
malleable,
rubber stretchy,
water a bipolar
molecule, etc.
Strong nuclear force…
 This
is the STRONGEST
OF ALL forces, but acts
over the SMALLEST
distances—only within
the nucleus of an atom!
 What
do like charges
normally “try” to do?
 Strong
nuclear force
binds protons (and
neutrons if present)
VERY tightly
together in the
nucleus of an atom.
It takes a nuclear
fission reaction to
break these protons
apart.
Weak nuclear force…
 Is
the force involved with
radioactive decay, where
unstable elements give off
radiation (alpha, beta, gamma)
Great
site:
overview of
forces and
Newton’s laws