[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.33,0:00:01.37,Default,,0000,0000,0000,,- [Instructor] There are\Na lot of different types
Dialogue: 0,0:00:01.37,0:00:02.55,Default,,0000,0000,0000,,of forces in physics,
Dialogue: 0,0:00:02.55,0:00:03.54,Default,,0000,0000,0000,,but for the most part,
Dialogue: 0,0:00:03.54,0:00:07.24,Default,,0000,0000,0000,,all forces can be\Ncategorized as either being
Dialogue: 0,0:00:07.24,0:00:10.95,Default,,0000,0000,0000,,a contact force or a long range force.
Dialogue: 0,0:00:10.95,0:00:13.64,Default,,0000,0000,0000,,So contact forces as the name suggests
Dialogue: 0,0:00:13.64,0:00:14.79,Default,,0000,0000,0000,,requires the two objects
Dialogue: 0,0:00:14.79,0:00:16.31,Default,,0000,0000,0000,,that are exerting a force on each other
Dialogue: 0,0:00:16.31,0:00:19.13,Default,,0000,0000,0000,,to be touching or in contact.
Dialogue: 0,0:00:19.13,0:00:22.01,Default,,0000,0000,0000,,So tension, the normal\Nforce, frictional forces,
Dialogue: 0,0:00:22.01,0:00:26.00,Default,,0000,0000,0000,,these are all common everyday\Nexamples of contact forces.
Dialogue: 0,0:00:26.00,0:00:29.26,Default,,0000,0000,0000,,So, you know, this wire from this crane
Dialogue: 0,0:00:29.26,0:00:31.86,Default,,0000,0000,0000,,can exert a contact force\Ni.e. attention force
Dialogue: 0,0:00:31.86,0:00:34.40,Default,,0000,0000,0000,,on the wrecking ball but that wire
Dialogue: 0,0:00:34.40,0:00:36.81,Default,,0000,0000,0000,,can only exert that tension\Nforce on the wrecking ball
Dialogue: 0,0:00:36.81,0:00:39.19,Default,,0000,0000,0000,,if the wire is actually connected to
Dialogue: 0,0:00:39.19,0:00:41.35,Default,,0000,0000,0000,,i.e. touching the wrecking ball,
Dialogue: 0,0:00:41.35,0:00:43.87,Default,,0000,0000,0000,,if you forgot to tie the\Nwire to the wrecking ball,
Dialogue: 0,0:00:43.87,0:00:46.00,Default,,0000,0000,0000,,that wire is not gonna exert any tension
Dialogue: 0,0:00:46.00,0:00:47.44,Default,,0000,0000,0000,,on the wrecking ball.
Dialogue: 0,0:00:47.44,0:00:51.31,Default,,0000,0000,0000,,So these contact forces are\Nto be distinguished from
Dialogue: 0,0:00:51.31,0:00:52.66,Default,,0000,0000,0000,,long range forces.
Dialogue: 0,0:00:52.66,0:00:55.95,Default,,0000,0000,0000,,Sometimes these are called\Naction at a distance forces
Dialogue: 0,0:00:55.95,0:00:57.52,Default,,0000,0000,0000,,because they can be exerted
Dialogue: 0,0:00:57.52,0:00:59.69,Default,,0000,0000,0000,,on objects that are far\Naway from each other,
Dialogue: 0,0:00:59.69,0:01:01.45,Default,,0000,0000,0000,,so gravity is a common example,
Dialogue: 0,0:01:02.48,0:01:05.39,Default,,0000,0000,0000,,the earth can exert a\Ngravitational force on the moon
Dialogue: 0,0:01:05.39,0:01:07.56,Default,,0000,0000,0000,,even though the earth and\Nthe moon aren't touching.
Dialogue: 0,0:01:07.56,0:01:09.56,Default,,0000,0000,0000,,So that's a group long range force.
Dialogue: 0,0:01:09.56,0:01:12.84,Default,,0000,0000,0000,,Similarly, the electric force\Ncan exert a repulsive force
Dialogue: 0,0:01:12.84,0:01:15.13,Default,,0000,0000,0000,,on two charges if they're not touching,
Dialogue: 0,0:01:15.13,0:01:17.38,Default,,0000,0000,0000,,so not a contact force,
Dialogue: 0,0:01:17.38,0:01:19.42,Default,,0000,0000,0000,,and magnets can attract each other
Dialogue: 0,0:01:19.42,0:01:20.69,Default,,0000,0000,0000,,even if they're not touching,
Dialogue: 0,0:01:20.69,0:01:24.42,Default,,0000,0000,0000,,so those are all long range or\Naction at a distance forces.
Dialogue: 0,0:01:24.42,0:01:26.29,Default,,0000,0000,0000,,But I'll be honest with you here,
Dialogue: 0,0:01:26.29,0:01:29.41,Default,,0000,0000,0000,,this distinction is not\Nnearly as fundamental
Dialogue: 0,0:01:29.41,0:01:31.01,Default,,0000,0000,0000,,as it might seem at first.
Dialogue: 0,0:01:31.01,0:01:34.62,Default,,0000,0000,0000,,All of these forces that\Nwe call contact forces
Dialogue: 0,0:01:34.62,0:01:38.88,Default,,0000,0000,0000,,are really just an enormous\Nnumber of long range forces
Dialogue: 0,0:01:38.88,0:01:40.19,Default,,0000,0000,0000,,in disguise.
Dialogue: 0,0:01:40.19,0:01:42.93,Default,,0000,0000,0000,,In other words, these contact\Nforces, tension, normal force
Dialogue: 0,0:01:42.93,0:01:46.74,Default,,0000,0000,0000,,and friction are all arising\Nmicroscopically due to
Dialogue: 0,0:01:46.74,0:01:48.62,Default,,0000,0000,0000,,a bunch of long range forces
Dialogue: 0,0:01:48.62,0:01:50.91,Default,,0000,0000,0000,,acting over really short distances.
Dialogue: 0,0:01:50.91,0:01:53.25,Default,,0000,0000,0000,,So just cause they're\Ncalled long range forces
Dialogue: 0,0:01:53.25,0:01:56.64,Default,,0000,0000,0000,,doesn't mean they can't exert\Nforce over small distances,
Dialogue: 0,0:01:56.64,0:01:59.51,Default,,0000,0000,0000,,and in fact, all those forces arise,
Dialogue: 0,0:01:59.51,0:02:01.33,Default,,0000,0000,0000,,you know, cause all these forces to arise.
Dialogue: 0,0:02:01.33,0:02:03.72,Default,,0000,0000,0000,,So let me go through and explain\Nhow all these come about.
Dialogue: 0,0:02:03.72,0:02:06.00,Default,,0000,0000,0000,,So we'll start with tension here.
Dialogue: 0,0:02:06.00,0:02:07.45,Default,,0000,0000,0000,,Where does tension come from?
Dialogue: 0,0:02:07.45,0:02:09.96,Default,,0000,0000,0000,,Well, tension is the\Nforce exerted by a wire
Dialogue: 0,0:02:09.96,0:02:12.84,Default,,0000,0000,0000,,or a cable or a string,\Nsomething like that.
Dialogue: 0,0:02:12.84,0:02:14.13,Default,,0000,0000,0000,,And so these strings,
Dialogue: 0,0:02:14.13,0:02:15.78,Default,,0000,0000,0000,,they're made out of atoms and molecules,
Dialogue: 0,0:02:15.78,0:02:17.40,Default,,0000,0000,0000,,I'm trying to represent that over here.
Dialogue: 0,0:02:17.40,0:02:20.69,Default,,0000,0000,0000,,Your string is probably\Nmore than three atoms wide,
Dialogue: 0,0:02:20.69,0:02:22.79,Default,,0000,0000,0000,,but I didn't wanna have to\Ndraw an enormous number here,
Dialogue: 0,0:02:22.79,0:02:24.55,Default,,0000,0000,0000,,so I imagine you've got\Na certain number of atoms
Dialogue: 0,0:02:24.55,0:02:26.81,Default,,0000,0000,0000,,and molecules in your string,
Dialogue: 0,0:02:26.81,0:02:29.50,Default,,0000,0000,0000,,well, these atoms and molecules\Nare all bonded together,
Dialogue: 0,0:02:29.50,0:02:30.77,Default,,0000,0000,0000,,chemically bonded together,
Dialogue: 0,0:02:30.77,0:02:33.54,Default,,0000,0000,0000,,those are all electromagnetic bonds here,
Dialogue: 0,0:02:35.10,0:02:38.20,Default,,0000,0000,0000,,they don't wanna move away from\Ntheir equilibrium position,
Dialogue: 0,0:02:38.20,0:02:40.49,Default,,0000,0000,0000,,they have a position and if\Nthey get displaced from there,
Dialogue: 0,0:02:40.49,0:02:42.45,Default,,0000,0000,0000,,they wanna go back to that spot.
Dialogue: 0,0:02:42.45,0:02:44.56,Default,,0000,0000,0000,,So that's what it means\Nto be in a solid here.
Dialogue: 0,0:02:44.56,0:02:48.10,Default,,0000,0000,0000,,So this wire, if you connect\Nto a heavy load to it
Dialogue: 0,0:02:48.10,0:02:49.20,Default,,0000,0000,0000,,like a wrecking ball,
Dialogue: 0,0:02:49.20,0:02:51.27,Default,,0000,0000,0000,,that wrecking ball is gonna\Ntry to rip these atoms
Dialogue: 0,0:02:51.27,0:02:52.54,Default,,0000,0000,0000,,and molecules apart,
Dialogue: 0,0:02:52.54,0:02:55.23,Default,,0000,0000,0000,,it's gonna try to pull\Nthem away from each other,
Dialogue: 0,0:02:55.23,0:02:56.50,Default,,0000,0000,0000,,but they don't wanna move\Naway from each other,
Dialogue: 0,0:02:56.50,0:02:59.39,Default,,0000,0000,0000,,in other words, they try\Nto restore themselves
Dialogue: 0,0:02:59.39,0:03:01.72,Default,,0000,0000,0000,,as this distance between these atoms
Dialogue: 0,0:03:01.72,0:03:03.18,Default,,0000,0000,0000,,and molecules gets bigger,
Dialogue: 0,0:03:03.18,0:03:05.72,Default,,0000,0000,0000,,and it does, you'll stretch\Nyour string or your wire
Dialogue: 0,0:03:05.72,0:03:08.59,Default,,0000,0000,0000,,sometimes imperceptibly, but a little bit,
Dialogue: 0,0:03:08.59,0:03:10.34,Default,,0000,0000,0000,,these distances get bigger,
Dialogue: 0,0:03:10.34,0:03:13.09,Default,,0000,0000,0000,,that force holding them\Ntogether gets bigger,
Dialogue: 0,0:03:13.09,0:03:14.90,Default,,0000,0000,0000,,so more tension force occurs
Dialogue: 0,0:03:14.90,0:03:18.84,Default,,0000,0000,0000,,and this is the microscopic\Norigin of that tension force.
Dialogue: 0,0:03:18.84,0:03:21.03,Default,,0000,0000,0000,,These atoms and molecules\Nwanna restore themselves
Dialogue: 0,0:03:21.03,0:03:23.89,Default,,0000,0000,0000,,to their previous length and
Dialogue: 0,0:03:23.89,0:03:25.53,Default,,0000,0000,0000,,to do that they have to\Npull harder and harder.
Dialogue: 0,0:03:25.53,0:03:27.07,Default,,0000,0000,0000,,Now, this won't last forever,
Dialogue: 0,0:03:27.07,0:03:29.20,Default,,0000,0000,0000,,you hang a heavy enough load over here,
Dialogue: 0,0:03:29.20,0:03:31.81,Default,,0000,0000,0000,,you'll overwhelm these\Nelectromagnetic bonds
Dialogue: 0,0:03:31.81,0:03:34.60,Default,,0000,0000,0000,,and you'll rip these molecules apart
Dialogue: 0,0:03:34.60,0:03:36.26,Default,,0000,0000,0000,,and that's what happens\Nwhen your string breaks.
Dialogue: 0,0:03:36.26,0:03:38.68,Default,,0000,0000,0000,,So that's the microscopic\Norigin of tension
Dialogue: 0,0:03:38.68,0:03:42.86,Default,,0000,0000,0000,,but you don't have to draw an\NAvogadro's number of arrows,
Dialogue: 0,0:03:42.86,0:03:45.43,Default,,0000,0000,0000,,we just represent the\Ntension with one arrow up,
Dialogue: 0,0:03:45.43,0:03:47.29,Default,,0000,0000,0000,,it turns out you can pretty much summarize
Dialogue: 0,0:03:47.29,0:03:51.25,Default,,0000,0000,0000,,all of those microscopic\Nelectromagnetic chemical bonds
Dialogue: 0,0:03:51.25,0:03:54.42,Default,,0000,0000,0000,,with one arrow that we call tension.
Dialogue: 0,0:03:54.42,0:03:56.58,Default,,0000,0000,0000,,So how about the normal force,\Nwhere does that come from?
Dialogue: 0,0:03:56.58,0:03:58.06,Default,,0000,0000,0000,,Well, this is kind of the opposite.
Dialogue: 0,0:03:58.06,0:03:59.60,Default,,0000,0000,0000,,Tension is a pulling force,
Dialogue: 0,0:03:59.60,0:04:02.93,Default,,0000,0000,0000,,the normal force is the force\Nthat tries to prevent two
Dialogue: 0,0:04:02.93,0:04:05.39,Default,,0000,0000,0000,,objects from getting\Nsmashed into each other.
Dialogue: 0,0:04:05.39,0:04:07.10,Default,,0000,0000,0000,,So now instead of the atoms and molecules
Dialogue: 0,0:04:07.10,0:04:08.47,Default,,0000,0000,0000,,trying to get ripped apart,
Dialogue: 0,0:04:08.47,0:04:11.30,Default,,0000,0000,0000,,the atoms and molecules\Nin this green box here
Dialogue: 0,0:04:11.30,0:04:13.96,Default,,0000,0000,0000,,due to its weight are\Ntrying to get shoved into
Dialogue: 0,0:04:13.96,0:04:15.59,Default,,0000,0000,0000,,the atoms and molecules this table,
Dialogue: 0,0:04:15.59,0:04:17.33,Default,,0000,0000,0000,,so I've tried to represent that here,
Dialogue: 0,0:04:17.33,0:04:19.34,Default,,0000,0000,0000,,again, the box and the\Ntable are made out of more
Dialogue: 0,0:04:19.34,0:04:21.50,Default,,0000,0000,0000,,than these number of atoms and molecules,
Dialogue: 0,0:04:21.50,0:04:24.29,Default,,0000,0000,0000,,but you've got your atoms\Nmolecules of the box,
Dialogue: 0,0:04:24.29,0:04:26.82,Default,,0000,0000,0000,,atoms and molecules of the table,
Dialogue: 0,0:04:26.82,0:04:28.90,Default,,0000,0000,0000,,they won't get moved into each other,
Dialogue: 0,0:04:28.90,0:04:31.11,Default,,0000,0000,0000,,there's gonna be an electron cloud
Dialogue: 0,0:04:31.11,0:04:33.12,Default,,0000,0000,0000,,around these atoms and\Nmolecules of the box
Dialogue: 0,0:04:33.12,0:04:35.01,Default,,0000,0000,0000,,and similarly for the table,
Dialogue: 0,0:04:35.01,0:04:37.42,Default,,0000,0000,0000,,there's gonna be an\Nelectromagnetic repulsion
Dialogue: 0,0:04:37.42,0:04:38.89,Default,,0000,0000,0000,,when they try to overlap
Dialogue: 0,0:04:38.89,0:04:40.84,Default,,0000,0000,0000,,and other quantum mechanical effects,
Dialogue: 0,0:04:40.84,0:04:43.82,Default,,0000,0000,0000,,it turns out it's surprisingly\Ncomplicated to explain
Dialogue: 0,0:04:43.82,0:04:47.02,Default,,0000,0000,0000,,why matter is solid and it\Ncan't penetrate each other,
Dialogue: 0,0:04:47.02,0:04:50.24,Default,,0000,0000,0000,,but the enormous number of\Nelectromagnetic interactions
Dialogue: 0,0:04:50.24,0:04:52.97,Default,,0000,0000,0000,,and other quantum mechanical\Neffects between these
Dialogue: 0,0:04:52.97,0:04:56.19,Default,,0000,0000,0000,,atoms and molecules are\Nthe microscopic origin
Dialogue: 0,0:04:56.19,0:04:57.20,Default,,0000,0000,0000,,of the normal force.
Dialogue: 0,0:04:57.20,0:04:59.88,Default,,0000,0000,0000,,So again, it's, you know,\Naction at a distance
Dialogue: 0,0:04:59.88,0:05:01.71,Default,,0000,0000,0000,,over a small scale,
Dialogue: 0,0:05:01.71,0:05:04.58,Default,,0000,0000,0000,,which really bugs people out,\Nthey're like, wait a minute,
Dialogue: 0,0:05:05.66,0:05:07.55,Default,,0000,0000,0000,,are two things ever actually touching?
Dialogue: 0,0:05:07.55,0:05:09.30,Default,,0000,0000,0000,,You know, as you sit in a chair,
Dialogue: 0,0:05:09.30,0:05:12.44,Default,,0000,0000,0000,,do the atoms and molecules of your pants
Dialogue: 0,0:05:12.44,0:05:14.69,Default,,0000,0000,0000,,actually physically touch?
Dialogue: 0,0:05:14.69,0:05:17.80,Default,,0000,0000,0000,,Hard to actually define\Nwhat it means touching here,
Dialogue: 0,0:05:17.80,0:05:20.44,Default,,0000,0000,0000,,you know, you've got these\Namorphous electron clouds,
Dialogue: 0,0:05:20.44,0:05:22.43,Default,,0000,0000,0000,,how do you define\Nwhether they're touching?
Dialogue: 0,0:05:22.43,0:05:24.93,Default,,0000,0000,0000,,Hard to do, but good news,\Nwe don't have to do it,
Dialogue: 0,0:05:24.93,0:05:27.40,Default,,0000,0000,0000,,we can actually just\Nsummarize microscopically
Dialogue: 0,0:05:27.40,0:05:29.62,Default,,0000,0000,0000,,all of these microscopic interactions
Dialogue: 0,0:05:29.62,0:05:33.96,Default,,0000,0000,0000,,as one big normal force and that helps us
Dialogue: 0,0:05:33.96,0:05:36.33,Default,,0000,0000,0000,,both calculationally and conceptually
Dialogue: 0,0:05:36.33,0:05:38.47,Default,,0000,0000,0000,,knock it to at last here.
Dialogue: 0,0:05:38.47,0:05:39.76,Default,,0000,0000,0000,,Now, you might be disturbed here,
Dialogue: 0,0:05:39.76,0:05:40.65,Default,,0000,0000,0000,,you might be like, wait a minute,
Dialogue: 0,0:05:40.65,0:05:43.16,Default,,0000,0000,0000,,this whole video is about contact forces,
Dialogue: 0,0:05:43.16,0:05:43.99,Default,,0000,0000,0000,,you're telling me,
Dialogue: 0,0:05:43.99,0:05:46.31,Default,,0000,0000,0000,,we don't even know if two\Nsurfaces are in contact,
Dialogue: 0,0:05:46.31,0:05:48.17,Default,,0000,0000,0000,,well, I'm saying it's hard to define
Dialogue: 0,0:05:48.17,0:05:50.37,Default,,0000,0000,0000,,but here's a good way to define it,
Dialogue: 0,0:05:50.37,0:05:52.37,Default,,0000,0000,0000,,your pants, atoms and molecules
Dialogue: 0,0:05:52.37,0:05:56.05,Default,,0000,0000,0000,,are contacting the seat's\Natom molecules as soon as
Dialogue: 0,0:05:56.05,0:05:57.65,Default,,0000,0000,0000,,you notice that force
Dialogue: 0,0:05:57.65,0:05:59.76,Default,,0000,0000,0000,,preventing them from\Nmoving into each other.
Dialogue: 0,0:05:59.76,0:06:01.83,Default,,0000,0000,0000,,So as soon as you could\Ndetect this normal force,
Dialogue: 0,0:06:01.83,0:06:03.04,Default,,0000,0000,0000,,that's as good a way as any
Dialogue: 0,0:06:03.04,0:06:06.46,Default,,0000,0000,0000,,to define two surfaces\Nas being in contact.
Dialogue: 0,0:06:06.46,0:06:08.68,Default,,0000,0000,0000,,So let's look at some other forces.
Dialogue: 0,0:06:08.68,0:06:10.64,Default,,0000,0000,0000,,So how about the frictional force?
Dialogue: 0,0:06:10.64,0:06:13.18,Default,,0000,0000,0000,,What are the microscopic\Norigins of the frictional force?
Dialogue: 0,0:06:13.18,0:06:15.80,Default,,0000,0000,0000,,Well, you know, the frictional\Nforce is the force that
Dialogue: 0,0:06:15.80,0:06:19.66,Default,,0000,0000,0000,,resist two surfaces from being\Ndragged across each other.
Dialogue: 0,0:06:19.66,0:06:21.22,Default,,0000,0000,0000,,Why is there a resistive force?
Dialogue: 0,0:06:21.22,0:06:23.92,Default,,0000,0000,0000,,Well, if you zoomed in on these surfaces,
Dialogue: 0,0:06:23.92,0:06:25.80,Default,,0000,0000,0000,,a table, no matter how smooth it looks,
Dialogue: 0,0:06:25.80,0:06:27.40,Default,,0000,0000,0000,,even if you just wiped it down,
Dialogue: 0,0:06:27.40,0:06:28.57,Default,,0000,0000,0000,,if you zoomed in close enough,
Dialogue: 0,0:06:28.57,0:06:30.57,Default,,0000,0000,0000,,you'd be shocked at all the little
Dialogue: 0,0:06:30.57,0:06:32.89,Default,,0000,0000,0000,,crevices and cracks and valleys involved,
Dialogue: 0,0:06:32.89,0:06:34.60,Default,,0000,0000,0000,,the whole world you don't know about
Dialogue: 0,0:06:34.60,0:06:36.28,Default,,0000,0000,0000,,unless you look at it microscopically,
Dialogue: 0,0:06:36.28,0:06:39.38,Default,,0000,0000,0000,,and similarly for this purple\Nbox, maybe it's cardboard,
Dialogue: 0,0:06:39.38,0:06:41.96,Default,,0000,0000,0000,,if you zoomed in microscopically,
Dialogue: 0,0:06:41.96,0:06:45.73,Default,,0000,0000,0000,,again, it's astonishing how\Nnot smooth those surfaces are.
Dialogue: 0,0:06:45.73,0:06:48.47,Default,,0000,0000,0000,,So obviously, if you tried\Nto drag one across the other
Dialogue: 0,0:06:48.47,0:06:49.97,Default,,0000,0000,0000,,and these are bumping into each other,
Dialogue: 0,0:06:49.97,0:06:52.57,Default,,0000,0000,0000,,these hills and valleys are\Nrunning into each other,
Dialogue: 0,0:06:52.57,0:06:53.94,Default,,0000,0000,0000,,that's gonna be a problem
Dialogue: 0,0:06:53.94,0:06:55.65,Default,,0000,0000,0000,,that's gonna cause a resistive force.
Dialogue: 0,0:06:55.65,0:06:59.15,Default,,0000,0000,0000,,You might break this, you\Nknow, yellow hill off,
Dialogue: 0,0:06:59.15,0:07:00.50,Default,,0000,0000,0000,,sometimes they just bust off,
Dialogue: 0,0:07:00.50,0:07:04.27,Default,,0000,0000,0000,,yep, that's gonna be a\Nresistive cause of friction.
Dialogue: 0,0:07:04.27,0:07:05.38,Default,,0000,0000,0000,,Sometimes they don't bust off,
Dialogue: 0,0:07:05.38,0:07:07.41,Default,,0000,0000,0000,,maybe they just like bend and bounce back
Dialogue: 0,0:07:07.41,0:07:08.98,Default,,0000,0000,0000,,but even if they do,
Dialogue: 0,0:07:08.98,0:07:10.50,Default,,0000,0000,0000,,still gonna cause a frictional force
Dialogue: 0,0:07:10.50,0:07:12.30,Default,,0000,0000,0000,,and add to this friction,
Dialogue: 0,0:07:12.30,0:07:15.33,Default,,0000,0000,0000,,and it's not just that\Nbut sometimes even like
Dialogue: 0,0:07:15.33,0:07:17.80,Default,,0000,0000,0000,,the atoms and molecules\Nin the surface over here,
Dialogue: 0,0:07:17.80,0:07:19.38,Default,,0000,0000,0000,,look, this pot doesn't look too bad,
Dialogue: 0,0:07:19.38,0:07:20.96,Default,,0000,0000,0000,,it looks like they could\Nslide across each other
Dialogue: 0,0:07:20.96,0:07:22.30,Default,,0000,0000,0000,,pretty well,
Dialogue: 0,0:07:22.30,0:07:25.76,Default,,0000,0000,0000,,but there can be adhesion\Nlike molecular bonds that form
Dialogue: 0,0:07:25.76,0:07:28.11,Default,,0000,0000,0000,,between those atoms and molecules\Nthat are near each other,
Dialogue: 0,0:07:28.11,0:07:30.41,Default,,0000,0000,0000,,that can also contribute to friction.
Dialogue: 0,0:07:30.41,0:07:32.35,Default,,0000,0000,0000,,So again, astonishingly complicated,
Dialogue: 0,0:07:32.35,0:07:35.08,Default,,0000,0000,0000,,there's actually lots of\Nquestions to still be answered
Dialogue: 0,0:07:35.08,0:07:36.22,Default,,0000,0000,0000,,in studying friction,
Dialogue: 0,0:07:36.22,0:07:39.15,Default,,0000,0000,0000,,and the study of friction\Nis called tribology.
Dialogue: 0,0:07:39.15,0:07:42.21,Default,,0000,0000,0000,,Shockingly, a lot of\Nquestions to this day but,
Dialogue: 0,0:07:42.21,0:07:44.11,Default,,0000,0000,0000,,the good news is you can summarize
Dialogue: 0,0:07:44.11,0:07:47.84,Default,,0000,0000,0000,,all of those microscopic\Ninteractions as one force
Dialogue: 0,0:07:47.84,0:07:50.09,Default,,0000,0000,0000,,we call friction that resists
Dialogue: 0,0:07:50.09,0:07:52.57,Default,,0000,0000,0000,,the two surfaces from\Nsliding over each other.
Dialogue: 0,0:07:52.57,0:07:54.97,Default,,0000,0000,0000,,So you don't have to do\Na lot of calculations
Dialogue: 0,0:07:54.97,0:07:57.75,Default,,0000,0000,0000,,and microscopically\Nzoom in on the surface,
Dialogue: 0,0:07:57.75,0:07:59.51,Default,,0000,0000,0000,,we can pretty much account for all of it
Dialogue: 0,0:07:59.51,0:08:01.93,Default,,0000,0000,0000,,by simply drawing it as\None big resistive force
Dialogue: 0,0:08:01.93,0:08:03.77,Default,,0000,0000,0000,,of friction backwards.
Dialogue: 0,0:08:03.77,0:08:06.67,Default,,0000,0000,0000,,So recapping, contact\Nforces are those forces
Dialogue: 0,0:08:06.67,0:08:09.51,Default,,0000,0000,0000,,that require the two objects\Ninteracting to be touching
Dialogue: 0,0:08:09.51,0:08:11.19,Default,,0000,0000,0000,,for that force to occur,
Dialogue: 0,0:08:11.19,0:08:13.86,Default,,0000,0000,0000,,but we've seen that these\Ncontact forces are actually due
Dialogue: 0,0:08:13.86,0:08:17.58,Default,,0000,0000,0000,,to a mind bogglingly large\Nnumber of long range forces
Dialogue: 0,0:08:17.58,0:08:19.79,Default,,0000,0000,0000,,all acting over a very short distance,
Dialogue: 0,0:08:19.79,0:08:22.21,Default,,0000,0000,0000,,but you can summarize all\Nthose long range forces
Dialogue: 0,0:08:22.21,0:08:24.56,Default,,0000,0000,0000,,as a single contact force
Dialogue: 0,0:08:24.56,0:08:28.08,Default,,0000,0000,0000,,when doing most introductory\Nphysics problems.