1
00:00:00,000 --> 00:00:00,680


2
00:00:00,680 --> 00:00:04,610
Let's say that I have a
huge, maybe frozen over lake,

3
00:00:04,610 --> 00:00:06,080
or maybe it's a big pond.

4
00:00:06,080 --> 00:00:09,750
So I have a huge surface of
ice over here-- my best attempt

5
00:00:09,750 --> 00:00:11,830
to draw a flat surface
of ice-- and I'm

6
00:00:11,830 --> 00:00:14,710
going to put two
blocks of ice here.

7
00:00:14,710 --> 00:00:17,650
So I'm going to put
one block of ice

8
00:00:17,650 --> 00:00:21,814
just like this, one block
of ice right over here.

9
00:00:21,814 --> 00:00:23,980
And then I'm going to put
another block of ice right

10
00:00:23,980 --> 00:00:26,220
over here.

11
00:00:26,220 --> 00:00:30,490
And then another block
of ice right over here.

12
00:00:30,490 --> 00:00:33,510
And these blocks of
ice are identical.

13
00:00:33,510 --> 00:00:35,230
They're both 5 kilograms.

14
00:00:35,230 --> 00:00:38,280
They are both 5 kilograms--
let me write this down.

15
00:00:38,280 --> 00:00:42,490
So they are both 5 kilograms.

16
00:00:42,490 --> 00:00:46,310
Or both of their masses, I
should say, are 5 kilograms.

17
00:00:46,310 --> 00:00:48,700
And the only difference
between the two

18
00:00:48,700 --> 00:00:51,620
is that relative to
the pond, this one

19
00:00:51,620 --> 00:00:56,120
is stationary-- this
one is stationary--

20
00:00:56,120 --> 00:00:59,310
and this one is moving
with a constant velocity--

21
00:00:59,310 --> 00:01:02,745
constant velocity.

22
00:01:02,745 --> 00:01:06,460
Constant velocity in the
right-wards direction.

23
00:01:06,460 --> 00:01:08,950
And let's say that
its constant velocity

24
00:01:08,950 --> 00:01:13,890
is at 5 meters per second--
5 meters per second.

25
00:01:13,890 --> 00:01:17,164
And the whole reason why I made
blocks of ice on top of ice

26
00:01:17,164 --> 00:01:19,330
is that we're going to
assume, at least for the sake

27
00:01:19,330 --> 00:01:23,540
of this video, that
friction is negligible.

28
00:01:23,540 --> 00:01:26,360
Now what does Newton's
First Law of Motion

29
00:01:26,360 --> 00:01:29,590
tell us about something that
is either not in motion--

30
00:01:29,590 --> 00:01:32,440
or you could view this as
a constant velocity of 0--

31
00:01:32,440 --> 00:01:35,314
or something that has
a constant velocity?

32
00:01:35,314 --> 00:01:36,730
Well Newton's First
Law says, well

33
00:01:36,730 --> 00:01:41,180
look, they're going to keep
their constant velocity

34
00:01:41,180 --> 00:01:43,930
or stay stationary, which is
the constant velocity of 0,

35
00:01:43,930 --> 00:01:47,110
unless there is some
unbalance, unless there

36
00:01:47,110 --> 00:01:49,774
is some net force
acting on an object.

37
00:01:49,774 --> 00:01:51,190
So let's just think
about it here.

38
00:01:51,190 --> 00:01:52,850
In either of these
situations, there

39
00:01:52,850 --> 00:01:56,140
must not be any unbalanced
force acting on them.

40
00:01:56,140 --> 00:01:58,532
Or their must not
be any net force.

41
00:01:58,532 --> 00:01:59,990
But if you think
about it, if we're

42
00:01:59,990 --> 00:02:02,280
assuming that these
things are on Earth,

43
00:02:02,280 --> 00:02:05,830
there is a net force
acting on both of them.

44
00:02:05,830 --> 00:02:08,780
Both of them are at the
surface of the Earth,

45
00:02:08,780 --> 00:02:10,949
and they both have
mass, so there

46
00:02:10,949 --> 00:02:14,460
will be the force of
gravity acting downwards

47
00:02:14,460 --> 00:02:15,900
on both of them.

48
00:02:15,900 --> 00:02:20,140
There is going to be the
downward force of gravity

49
00:02:20,140 --> 00:02:22,630
on both of these blocks of ice.

50
00:02:22,630 --> 00:02:25,810
And that downward force of
gravity, the force of gravity,

51
00:02:25,810 --> 00:02:29,540
is going to be equal to
the gravitational field

52
00:02:29,540 --> 00:02:32,910
near the surface of the
Earth, times-- which

53
00:02:32,910 --> 00:02:37,220
is a vector-- times
the mass of the object.

54
00:02:37,220 --> 00:02:41,050
So times 5 kilograms.

55
00:02:41,050 --> 00:02:47,040
This right over here is 9.8
meters per second squared.

56
00:02:47,040 --> 00:02:49,290
So you multiply that times 5.

57
00:02:49,290 --> 00:02:53,000
You get 49 kilogram meter
per second squared, which

58
00:02:53,000 --> 00:02:56,080
is the same thing as 49 newtons.

59
00:02:56,080 --> 00:02:58,670
So this is a little bit
of a conundrum here.

60
00:02:58,670 --> 00:03:02,490
Newton's First Law
says, an object at rest

61
00:03:02,490 --> 00:03:04,250
will stay at rest, or
an object in motion

62
00:03:04,250 --> 00:03:06,610
will stay in motion, unless
there is some unbalanced,

63
00:03:06,610 --> 00:03:08,580
or unless there
is some net force.

64
00:03:08,580 --> 00:03:10,340
But based on what
we've drawn right here,

65
00:03:10,340 --> 00:03:13,320
it looks like there's
some type of a net force.

66
00:03:13,320 --> 00:03:16,780
It looks like I have 49 newtons
of force pulling this thing

67
00:03:16,780 --> 00:03:17,740
downwards.

68
00:03:17,740 --> 00:03:20,900
But you say, no, no no, Sal.

69
00:03:20,900 --> 00:03:23,860
Obviously this thing won't
start accelerating downwards

70
00:03:23,860 --> 00:03:25,200
because there's ice here.

71
00:03:25,200 --> 00:03:32,640
Its resting on a big
pool of frozen water.

72
00:03:32,640 --> 00:03:35,930
And so my answer to you is,
well, if that's your answer,

73
00:03:35,930 --> 00:03:42,350
then what is the resulting
force that cancels out

74
00:03:42,350 --> 00:03:44,990
with gravity to keep
these blocks of ice,

75
00:03:44,990 --> 00:03:47,600
either one of them,
from plummeting down

76
00:03:47,600 --> 00:03:51,320
to the core of the Earth?

77
00:03:51,320 --> 00:03:53,610
From essentially
going into free fall,

78
00:03:53,610 --> 00:03:56,240
or accelerating towards
the center of the Earth?

79
00:03:56,240 --> 00:04:01,210
And you say, well, I guess if
these things would be falling,

80
00:04:01,210 --> 00:04:04,400
if not for the ice,
the ice must be

81
00:04:04,400 --> 00:04:07,420
providing the
counteracting force.

82
00:04:07,420 --> 00:04:09,630
And you are absolutely correct.

83
00:04:09,630 --> 00:04:13,390
The ice is providing
the counteracting force

84
00:04:13,390 --> 00:04:15,750
in the opposite direction.

85
00:04:15,750 --> 00:04:17,839
So the exact magnitude
of force, and it

86
00:04:17,839 --> 00:04:20,220
is in the opposite direction.

87
00:04:20,220 --> 00:04:23,730
And so if the force of gravity
on each of these blocks of ice

88
00:04:23,730 --> 00:04:27,690
are 49 newtons downwards
it is completely

89
00:04:27,690 --> 00:04:32,150
netted off by the force of
the ice on the block upwards.

90
00:04:32,150 --> 00:04:37,560
And that will be a force 49
newtons upwards in either case.

91
00:04:37,560 --> 00:04:39,320
And now, hopefully,
it makes sense

92
00:04:39,320 --> 00:04:41,650
that Newton's First
Law still holds.

93
00:04:41,650 --> 00:04:45,100
We have no net force on this
in the vertical direction,

94
00:04:45,100 --> 00:04:47,810
actually no net force on
this in either direction.

95
00:04:47,810 --> 00:04:51,740
That's why this guy
has a 0 velocity

96
00:04:51,740 --> 00:04:53,140
in the horizontal direction.

97
00:04:53,140 --> 00:04:56,010
This guy has a constant velocity
in the horizontal direction.

98
00:04:56,010 --> 00:04:57,570
And neither of them
are accelerating

99
00:04:57,570 --> 00:04:58,970
in the vertical direction.

100
00:04:58,970 --> 00:05:01,520
Because you have the force
of the ice on the block,

101
00:05:01,520 --> 00:05:03,470
the ice is supporting
the block, that's

102
00:05:03,470 --> 00:05:05,920
completely
counteracting gravity.

103
00:05:05,920 --> 00:05:09,740
And this force, in this example,
is called the normal force.

104
00:05:09,740 --> 00:05:14,570
This is the normal force--
it's 49 newtons upwards.

105
00:05:14,570 --> 00:05:17,040
This right here is
the normal force.

106
00:05:17,040 --> 00:05:20,150
And we'll talk more about the
normal force in future videos.

107
00:05:20,150 --> 00:05:22,610
The normal force
is the force, when

108
00:05:22,610 --> 00:05:24,380
anything is resting
on any surface that's

109
00:05:24,380 --> 00:05:25,630
perpendicular to that surface.

110
00:05:25,630 --> 00:05:27,463
And it's going to start
to matter a lot when

111
00:05:27,463 --> 00:05:29,692
we start thinking about
friction and all the rest.

112
00:05:29,692 --> 00:05:32,150
So what we'll see in future
videos, when you have something

113
00:05:32,150 --> 00:05:35,220
on an incline, and let's say
I have a block on an incline

114
00:05:35,220 --> 00:05:36,120
like this.

115
00:05:36,120 --> 00:05:38,440
The normal force
from the, I guess

116
00:05:38,440 --> 00:05:40,080
you could say, this
wedge on the block,

117
00:05:40,080 --> 00:05:44,364
is going to be perpendicular
to the surface.

118
00:05:44,364 --> 00:05:46,530
And if you really think
about what's happening here,

119
00:05:46,530 --> 00:05:48,760
it's fundamentally an
electromagnetic force.

120
00:05:48,760 --> 00:05:56,080
Because if you really zoomed
in on the molecules of the ice

121
00:05:56,080 --> 00:05:58,590
right over here, even better
the atoms of the ice here.

122
00:05:58,590 --> 00:06:01,840
And you really zoomed in on
the atoms or the molecules

123
00:06:01,840 --> 00:06:06,320
of the ice up here, what's
keeping this top block of ice

124
00:06:06,320 --> 00:06:09,030
from falling down
is that in order

125
00:06:09,030 --> 00:06:13,480
for it to go through its
molecules would have to kind

126
00:06:13,480 --> 00:06:16,070
of compress against, or I guess
it would have to get closer

127
00:06:16,070 --> 00:06:19,420
to, the water molecules
or the individual atoms

128
00:06:19,420 --> 00:06:21,530
in this ice down here.

129
00:06:21,530 --> 00:06:23,720
And the atoms, let me
draw it on an atomic level

130
00:06:23,720 --> 00:06:24,890
right over here.

131
00:06:24,890 --> 00:06:33,300
So maybe, let me draw one
of this guy's molecules.

132
00:06:33,300 --> 00:06:40,044
So you have an oxygen
with 2 hydrogens

133
00:06:40,044 --> 00:06:41,710
and it forms this big
lattice structure.

134
00:06:41,710 --> 00:06:45,980
And we can talk about more of
that in the chemistry playlist.

135
00:06:45,980 --> 00:06:48,880
And let's talk about this ice
as one of these molecules.

136
00:06:48,880 --> 00:06:53,500
So maybe it looks
something like this.

137
00:06:53,500 --> 00:06:55,810
And it has its 2 hydrogens

138
00:06:55,810 --> 00:06:58,710
And so what's keeping these guys
from getting compressed, what's

139
00:06:58,710 --> 00:07:02,200
keeping this block of ice
from going down further,

140
00:07:02,200 --> 00:07:05,330
is the repulsion between the
electrons in this molecule

141
00:07:05,330 --> 00:07:06,950
and the electrons
in that molecule.

142
00:07:06,950 --> 00:07:10,570
So on a macro level we view
this is kind of a contact force.

143
00:07:10,570 --> 00:07:13,110
But on a microscopic
level, on an atomic level,

144
00:07:13,110 --> 00:00:00,000
it's really just electromagnetic
repulsion at work.