Full transcript
0:00finally Newton's third law well it's the
0:05idea for every action force there's an
0:08equal and opposite reaction force you've
0:10probably heard this in movies but when I
0:13push on that fridge it pushes back and
0:18this might sound silly but I want you to
0:20take a second and push on something with
0:23your finger and look at your finger when
0:26you push on and say a table or click the
0:29mouse your finger dimples from that push
0:32and that's because what you're pushing
0:34on is pushing back on your finger so
0:37take a look at that even if you just
0:39push your two fingers together you know
0:40they're both dimpled so when you push
0:42exert a force on something it exerts a
0:45force back and we're gonna see this
0:48comes up especially when we start
0:49talking about gravity this can be an
0:51important idea when I push on an object
0:53it pushes back figured I can do a quick
0:58demonstration for Newton's law good old
1:03well third law basically now most
1:06teachers were probably yell at me and
1:08say this is not the best way to
1:10demonstrate it it's more for momentum
1:12and energy but who cares I do think as
1:17long as you explain it carefully can be
1:18a good explanation also I've been told
1:21in the past when I did last year people
1:22thought it was a very good visual for
1:23this so hey plus I like playing with it
1:26that's so what do we got well do balls
1:30on strings that if you've never seen it
1:32before you move on and you're gonna
1:33release it and the whole idea we have
1:35here is I'm gonna release this ball and
1:37when it hits this it's going to exert a
1:39force on this ball but remember from
1:41Newton's law when one exerts a force on
1:43the other that one in turn exerts a
1:46force back for every action force we
1:49have a reaction one so let's just go
1:52ahead and do it before we talk more
1:54about it now you can see it goes back
1:57and forth so what really happened here
1:59well again I pulled this and this ball
2:02right here ends up exerting a force on
2:04this second one right here alright so
2:09what happened this ball exerted a force
2:11on this one but we're not having
2:13Vall exerting their force exactly back
2:15on it which actually is why it's stopped
2:17and then this one kept moving exerted
2:19force on this one which made this one
2:21exert a force back on this and it
2:23stopped and that carried all the way
2:24over to this one which had nothing to
2:26stop it until the force of gravity
2:28eventually brought it back down
2:31basically the easiest way of thing about
2:33this is what we have going in on one
2:35side should come out the other and let's
2:39start doing some more demonstrations we
2:41started off with just looking at one
2:42where I pull out - well then you have
2:46two coming out the other side but of
2:49course do pulling out three Oh drop it
2:53that is can you see that one alright
2:59let's try and do all four if I pull out
3:02four again the whole idea is we should
3:05have four going out the other side and
3:08that's why it just keeps alternating
3:10back and forth of course if we really
3:12wanted to we could do all five okay I
3:16mean at least some of you were laughing
3:17guessing what LR is gonna happen
3:18don't be too excited yeah exactly what
3:22other corpse yeah
3:24that being said we can start being more
3:27creative - you can have things like this
3:30where you pull one from each side and
3:32they keep adding the other side go you
3:35really want you could do two from each
3:37side oh honey go tangled up
3:41that's not far in the demonstration all
3:43right you can do two on one side one on
3:45the other and you can see an alternate
3:48switch status - what's going in should
3:51be coming out the other side you could
3:54do pull one out more so this one's going
3:56to end up having a bigger force when it
3:58hits compared to this one I'm only
3:59pulling a little so how can you even see
4:03it there we go
4:06yeah the sound effects you can only hope
4:08so ones being pulled out more than the
4:10other and it should again oh I'll have
4:14suspense for nothing so you can see it
4:18keeps alternating which side again is
4:20going up higher and eventually it's
4:23bleeding off because of noise and sound
4:25loss
4:26you need new things like you know pull
4:28up to and try and catch one midway
4:30through Oh easily
4:33Paul - yeah okay at this point a pickup
4:39just playing anyway okay there is a few
4:44more terms we have to define as well
4:46important terms that will be used in
4:48this course we can talk about the idea
4:50of mass we've talked about that here
4:52already mass is an inherent tree of
4:55something I have a certain amount of
4:59mass maybe too much at the moment but I
5:03have a certain amount of mass to me this
5:05is very different from weight by the way
5:07weight depends on the gravitational pull
5:10of Earth so my weight is not just my
5:14mass my mass would be the same no matter
5:17where I was on the universe my weight
5:20though would change if I went on the
5:22moon my weight would be less because the
5:24pull the gravitational pull there is
5:26less my mass though is my an actual
5:30inherent trait of me how much mass I
5:33have volume is how big something is
5:37really how big something is it's not its
5:42area except in three dimensions so how
5:44much volume something has how big it is
5:46you can think finally density is how
5:50much mass per unit volume how spread out
5:54your mass is so if something has a very
5:56low density its mass is spread out over
5:59a huge volume whereas if it's a high
6:02density it's a very compact object
6:04there's a lot of mass in a small package
6:08and you can see some examples of density
6:10here water is the most common case this
6:14is one case we're not using SI units
6:16because that should be kilograms per
6:18meter cubed but this is just a more
6:20illustrative way because water is the
6:22quantity of one something like wood has
6:25low density and would float whereas gold
6:27quite high density would sink and there
6:30is a simulation your book provides if
6:33you want to take a look at some of these
6:34volumes and how they be used so
6:36important quantities we're going to be
6:39talking about
6:39and these densities are just show a
6:41small range here on earth we're gonna
6:43see some incredible densities in this
6:46course we also have to talk about the
6:50idea of angular momentum we talked about
6:53momentum and this was that and basically
6:57the object's motion related to its mass
7:01and its velocity its mass and its
7:05velocity angular momentum is the very
7:08similar idea but it's also how spread
7:11out that mass is so it's when we're
7:14looking at a rotating body angular
7:16momentum is a measure of that body's
7:19rotation and it relates to how much mass
7:21it is how fast it's rotating and how
7:26spread out that mass is and the reason
7:29this is important is because what we
7:30have is this law this conservation of
7:33angular momentum angular momentum will
7:35be conserved meaning it stays constant
7:38but that doesn't mean that you can't
7:40change one of those quantities you can't
7:43you could change the how fast is
7:45spinning or you could change how spread
7:48out that mass is and one of the most
7:50common examples of this is the idea of a
7:53figure skater if you ever seen figure
7:56skating if you watch the Olympics you've
7:58seen the figure skaters start to spin
7:59and what they do is cross their arms and
8:01they start spinning faster this is
8:04conservation of angular momentum because
8:07what happens is in this one case oops in
8:10this one case the mass is more spread
8:12out their arms are spread out the mass
8:14is more distributed across a wider space
8:16when they cross their arms it's more
8:18compact and thus for angular momentum to
8:21stay the same velocity increases the
8:24angular velocity the speed at which
8:26they're spinning increases because the
8:29amount of mass how spread out it was
8:31changed the mass remains same but how
8:33spread out it was decreased