This is power point 3

1. How
The
Universe
Works:
Mo2on
&
Forces
Lecture
3
AST
102,
Prof.
Rice
Monday
10:10
–
11:50
am
September
16,
2013

2. AMer
today’s
class,
you
should
be
able
to…
… Define
speed,
velocity
and
accelera2on
and
explain
their
rela2onship
… Interpret
a
plot
of
an
object’s
mo2on
… Explain
Newton’s
First
Law
… Use
Newton’s
Second
and
Third
Laws
to
describe
mo2on
in
response
to
forces
… Describe
the
four
fundamental
forces

3. Propor2onali2es
can
be
very
useful.
“The
area
of
a
square
is
propor2onal
to
the
length
of
a
side,
squared.”
A
∝
s2
side
=
z
side
=
2z
area
=
A
area
=
?

4. The
volume
of
a
sphere
is
V
=
4/3
π
R3.
Which
expression
is
a
propor2onality?
V
∝
π
R3
V
∝
4/3
π
R3
V
∝
4/3
R3
V
∝
R3

5. Two
2mes
larger.
Four
2mes
larger.
Six
2mes
larger.
Eight
2mes
larger.
The
volume
of
a
sphere
is
propor2onal
to
the
radius
cubed.
If
we
double
the
radius
of
the
sphere,
the
volume
is
now…

6. V∝r3
(2r)3
=
23
x
r3
=
8
x
r3
Eight
2mes
larger.
The
volume
of
a
sphere
is
propor2onal
to
the
radius
cubed.
If
we
double
the
radius
of
the
sphere,
the
volume
is
now…

7. Sphere
A
has
a
radius
twice
as
large
as
Sphere
B.
What
is
the
ra2o
of
volumes
of
the
spheres?
Va
/
Vb
=
1/8
Va
/
Vb
=
8
Va
/
Vb
=
4/3
Va
/
Vb
=
3/4

8. The
area
of
a
circle
is
A
=
π
R2.
Circle
X
is
three
2mes
the
radius
of
circle
Y
(RX
=
3RY).
What
is
the
ra2o
of
the
area
of
circle
X
to
the
area
of
circle
Y?
AX
/
AY
=
6
AX
/
AY
=
9
AX
/
AY
=
1/6
AX
/
AY
=
1/9

9. Speed:
distance
traveled
per
unit
2me
V
=
t
d
Units:
“miles
per
hour”
…expressed
frac2onally:
V
=
hr
miles
Slope
is
“rise
over
run”
d
t
Δd
Δt
V
=
Δt
(s)
Δd
(m)

10. A
speedometer
reads
the
instantaneous
speed:
the
speed
you
are
traveling
at
that
instant.
Imagine
driving
from
campus
to
Atlan2c
City.
d
t
Prejy
slow
leaving
CSI...
…but
much
faster
on
the
expressway.
The
tangent
to
the
curve
gives
the
instantaneous
speed.
CSI
278
NJ
Turnpike

11. Shallow
slope,
low
speed
Steep
slope,
high
speed
An
analogy:

12. Speed:
rate
of
change
of
distance
per
unit
2me
Accelera2on:
rate
of
change
of
speed*
per
unit
2me
a
=
Δt
ΔV
“Zero
to
60
in
4
seconds!”
a
=
4
s
60
mi/hr
=
s
6.7
m/s
a
=
6.7
m/s2
“6.7
meters
per
second,
per
second”
*
In
a
straight
line!

13. Can
accelera2on
be
nega2ve
while
speed
is
posi2ve?
Yes
No

14. d
t
Yes!
Nega2ve
accelera2on
means
slowing
down.

15. If
the
speed
of
a
car
is
non-­‐zero
(V≠0),
can
the
accelera2on
of
the
car
be
zero?
Yes
No
Depends
on
the
velocity

16. Yes
Example:
a
car
cruising
in
a
straight
line
at
65
mph
has
zero
accelera2on.
If
the
speed
of
a
car
is
non-­‐zero
(V≠0),
can
the
accelera2on
of
the
car
be
zero?

17. Velocity
is
speed
and
direc2on
Accelera2on
is
the
rate
of
change
of
velocity
per
unit
2me
Velocity
changes
in
circular
mo2on
(because
the
direc2on
changes),
even
at
a
constant
speed.
Therefore,
there
is
accelera2on
in
circular
mo2on!

19. Imagine
you
throw
a
ball
straight
up
in
the
air.
At
its
highest
point,
the
ball’s…
Velocity
and
accelera2on
are
both
zero
Velocity
is
non-­‐zero,
but
accelera2on
is
zero
Velocity
is
zero,
but
accelera2on
is
non-­‐zero
Velocity
and
accelera2on
are
both
non-­‐zero

20. Imagine
you
throw
a
ball
straight
up
in
the
air.
At
its
highest
point,
the
ball’s…
Velocity
is
zero,
but
accelera2on
is
non-­‐zero

21. The
ball
reaches
the
top…
…and
pauses
(speed
=
0)
But
no2ce:
accelera2on
(Δv/Δt)
is
always
the
same!

22. Aristotle
(c.
350
BC):
Natural
state
of
a
body
is
rest.
Plenty
of
evidence!
If
you
want
to
keep
something
moving,
you
must
compel
it
to
move:
you
must
force
it
to
move
to
overcome
its
natural
tendency
to
come
to
rest.

23. Galileo
(c.
1600):
Maybe
natural
state
of
a
body
is
whatever
it
is
already
doing!
What
is
unnatural
about
being
in
mo2on?
If
something
is
in
mo2on,
maybe
I
need
to
force
it
to
stop?
Counterintui2ve?
(Opposite
of
what
you
might
think?)

24. A
force
is
a
push
or
pull
one
object
exerts
upon
another
object.

25. Newton’s
Laws
of
Mo2on
1.
A
body
will
remain
at
constant
velocity
unless
acted
on
by
a
net
external
force.

26. A
marble
is
rolled
around
a
circular
piece
of
metal,
as
shown.
The
metal
ends
at
X
(the
circle
is
not
complete.
When
the
ball
gets
to
X,
which
path
does
it
take?
B
C
D
X
A

27. A
marble
is
rolled
around
a
circular
piece
of
metal,
as
shown.
The
metal
ends
at
X
(the
circle
is
not
complete.
When
the
ball
gets
to
X,
which
path
does
it
take?
C
X

28. Newton’s
Laws
of
Mo2on
1.
A
body
will
remain
at
constant
velocity
unless
acted
on
by
a
net
external
force.

29. Newton’s
Laws
of
Mo2on
1.
A
body
will
remain
at
constant
velocity
unless
acted
on
by
a
net
external
force.
2.
F
=
m
•
a

30. F
=
m
•
a
or…
a
=
F
m
The
accelera2on
due
to
a
par2cular
amount
of
force
depends
on
the
body’s
mass.
Newton’s
2nd
Law
says
mass
is
a
measure
of
just
how
badly
an
object
“wants”
to
remain
at
constant
velocity.
Mass:
a
measure
of
stuff,
a
quan2ty
of
ma:er.

31. A
soccer
player
first
kicks
a
soccer
ball,
then
kicks
a
bowling
ball
with
the
same
strength
of
kick.
Which
ball
leaves
her
foot
going
faster?
The
bowling
ball,
it’s
heavier
The
soccer
ball,
it’s
lighter
Neither,
both
travel
at
the
same
speed

32. A
soccer
player
first
kicks
a
soccer
ball,
then
kicks
a
bowling
ball
with
the
same
strength
of
kick.
Which
ball
leaves
her
foot
going
faster?
The
bowling
ball,
it’s
heavier
The
soccer
ball,
it’s
lighter
Neither,
both
travel
at
the
same
speed
a
=
F
m

33. Earth
completes
a
circular*
orbit
around
the
Sun
every
365.25
days.
Is
Earth
accelera2ng?
Yes
No
*
Well,
almost
exactly
circular.

34. Earth
completes
a
circular*
orbit
around
the
Sun
every
365.25
days.
Is
Earth
accelera2ng?
Yes!
*
Well,
almost
exactly
circular.
So…
is
there
a
force?

35. In
the
old
Bri2sh
measuring
system,
the
unit
of
force
is
a
pound
(lb).
In
the
metric
system,
the
unit
of
force
is
a
Newton
(N).
Hang
on!
My
weight
is
a
force?
In
the
metric
system,
the
unit
of
mass
is
a
kilogram
(kg).
One
carbon
atom
has
mass
of
2
x
10-­‐26
kg!!

36. 0
sec:
distance
fallen
0m,
v
=
0
m/s
1
sec:
distance
fallen
~5m,
v
~
10
m/s
2
sec:
distance
fallen
~20m,
v
~
20
m/s
3
sec:
distance
fallen
~45m,
v
~
30
m/s
Accelera2on
of
gravity
on
Earth
~
10
m/s2

37.
Scale
Fgr
=
m
•
a
m
But
why
doesn’t
the
scale
fall
through
the
floor?
A
2
kg
object
weighs
~20
N.
The
force
of
gravity
pulls
the
box
onto
the
scale.
The
strength
of
the
pull
is
the
weight
of
the
box.

38. Newton’s
Laws
of
Mo2on
3.
If
an
object
(A)
exerts
a
force
on
another
object
(B),
then
object
B
exerts
a
force
of
equal
magnitude
in
the
opposite
direc2on
on
object
A.
1.
A
body
will
remain
at
constant
velocity
unless
acted
on
by
a
net
external
force.
2.
F
=
m
•
a

39. A
rocket
exerts
a
force
on
gases,
expelling
them.
The
expelled
gases
exert
a
force
of
equal
magnitude
on
the
rocket,
but
in
the
opposite
direc2on.

40. Imagine
you
are
standing
on
a
wheeled
cart,
and
you
throw
a
rock
as
shown.
Is
the
cart
put
into
mo2on?
Yes,
it
moves
to
the
leM.
Yes,
it
moves
to
the
right.
No,
it
remains
in
place.

41. When
a
moving
cue
ball
strikes
a
sta2onary
8
ball,
which
exerts
more
force?
The
cue
ball
exerts
more
force.
The
8
ball
exerts
more
force.
The
balls
exert
equal
force.

42. There
are
four
fundamental
forces:
• Gravita2onal
• Electromagne2c
• Weak
nuclear
• Strong
nuclear
Long
range,
very
weak,
ajrac2ve
Long
range,
ajrac2ve
or
repulsive
Short
range,
very
strong,
ajrac2ve
Very
short
range,
transmuta2onal
All
four
fundamental
forces
play
important
roles
in
astronomy.

43. The
electromagne2c
force
is
the
source
of
all
chemistry
and
biology.
+
-­‐
+
+
Likes
repel…
…and
opposites
ajract.
This
force
forms
atoms,
molecules,
and
cells,
and
is
the
cause
of
most
“contact
forces.”
While
it
acts
at
infinite
distances,
it
usually
cancels
itself
out
far
from
paired
charges.

44. Gravity
is
the
weakest
of
the
four
forces.
The
electromagne2c
repulsion
between
two
protons
is
1043
2mes
stronger
than
the
gravita2onal
ajrac2on
between
them!
+
+
Gravity
is
very
important
in
astronomy
because
it
acts
over
enormous
distances
and
is
always
ajrac2ve.

45. The
strong
nuclear
force
is
the
strongest
of
the
four
fundamental
forces.
It
must
be
strong
to
overcome
electromagne2c
force
inside
an
atomic
nucleus.
But
it
acts
only
over
very
short
range,
and
is
not
felt
between
even
neighboring
atoms.
This
force
plays
a
cri2cal
role
in
nucleosynthesis.

46. The
weak
nuclear
force
makes
the
Sun
shine.
This
force
is
“weak”
in
comparison
to
the
other
nuclear
force,
but
is
s2ll
1038
2mes
stronger
than
gravity!
Like
the
strong
force,
the
weak
force
is
very
short
range.
The
weak
force
is
important
in
nuclear
reac2ons,
where
it
can
change
sub-­‐atomic
par2cles
from
one
type
into
another.
It’s
a
cri2cal
part
of
how
the
Sun
makes
its
energy.

47. Next
Class
² HTUW:
Gravity
&
Orbits
Wednesday,
September
18
² Read
sec2ons
4.4
and
4.5
² Homework
2
due
Sunday
night
September
22
(Chapter
4)