Full transcript
0:00of course there's a big question
0:03great capita has these three laws why
0:07are these laws true why are these things
0:10true and that's where the work of Newton
0:14would come in very famous scientist but
0:17first I wanted to find a couple more
0:19terms couple more terms that will be
0:21useful for our description here for one
0:24thing we have to define between speed
0:27and velocity and I think I've actually
0:29kind of used both interchangeably for
0:31now but they're not the same thing
0:33speed is how fast something travels
0:38velocity is speed with a direction so
0:42not only do we have to say how fast
0:45something's going but we have to
0:46describe in what direction and I mean
0:49when you think about it that's the full
0:51picture if I told you that this cheetah
0:53was going I don't know I don't have to
0:56remember how fast cheetahs travel but if
0:57I was to say 50 kilometres an hour
0:59great that might be an interesting
1:02number the number gets a lot more
1:05interesting if I say 50 kilometers
1:07straight at you all of a sudden that's a
1:10much more relevant number to you you're
1:12probably a lot more concerned it I mean
1:15generally you should be giving a proper
1:17direction say I don't know 50 kilometres
1:19north but that's the difference between
1:21speed and velocity velocity is speed
1:24with a direction so we're gonna be using
1:27those the idea of velocity quite a bit
1:29in this course now momentum is also
1:32something that comes up in science quite
1:33a bit it's as it says it's a measure of
1:36some body's motion well basically it is
1:39your velocity and mass combined so it's
1:44important the reason we talk about
1:46momentum a lot of times is sure
1:47something might have a certain velocity
1:48but momentum kind of tells more of the
1:52picture saying something's coming at you
1:54with I don't know again 50 kilometers an
1:56hour north
1:58well how much mass does the object have
2:01that might be an important measure I
2:02mean a mouse is running at me 50 comet
2:05an hour
2:05well not as worried but a big cheetah
2:09that's a lot more momentum coming at me
2:12a lot more scary
2:15okay now we can start looking at
2:18Newton's laws and trying to describe why
2:20Kepler's laws work because Newton really
2:23started studying physical motion and you
2:27know in some ways he built on some of
2:28the ideas from Galileo Galilei that we
2:30talked about before because Newton's
2:32first law is actually pretty similar to
2:33one of the things we were talking about
2:35before and it's the idea of the object
2:38movement an object moving or staying at
2:42rest is Newton's first law as it says
2:44here it's basically that if something is
2:47at rest its gonna stay at rest or if
2:49something is moving it's gonna keep
2:51moving unless acted upon by an outside
2:55force so I mean if I'm looking at my
2:58phone on the table right now it's not
3:00gonna just often start moving unless
3:02something pushes it a force causes it to
3:04move well the second parts a little
3:07harder to picture but it's the idea that
3:09something will keep moving in a straight
3:13line unless stopped and this seems to go
3:15against our experiences but that's
3:18because we all have tons of forces
3:20around us at all times that actually act
3:22to stop an object that is moving and
3:25it's worth noting by the way that
3:27Newton's first law is often called the
3:29law of inertia because when inertia is
3:32this idea of an object's tendency to
3:35keep moving inertia how much as
3:39describes the tendency of an object to
3:41keep moving so hence again this law is
3:43often called law of inertia and really
3:46an estimate of inertia is mass the more
3:50mass something has the more it's the
3:54harder it is to stop it and this shows a
3:57nice example what happened is this truck
3:59was driving along and they didn't
4:01actually put anything to seal the cement
4:05down to keep it from moving within the
4:07back of this truck so when this person
4:09applied the brakes the cement block was
4:11still moving with the speed of the truck
4:14and thus friction between it and the
4:17base of the container wasn't enough to
4:20stop it
4:21eventually the truck itself had to stop
4:23it so inertia it's ten
4:26somebody keep moving and the more mass
4:28something has the harder it is to get it
4:32to stop which is something you probably
4:34have experienced in your everyday life I
4:35mean when I try and push a pencil across
4:39the table it's pretty easy it's got a
4:42little bit of mass but if you've ever
4:43had to help a friend move and have tried
4:45to move something like our fridge or a
4:48couch you know that's hard to get moving
4:53but sometimes if you can get that fridge
4:55moving once you do it's a little easier
4:57to keep sliding it it's harder to get it
4:59initially moving once it starts to move
5:01it's usually a little bit easier that's
5:02not our idea coming in - okay so
5:08Newton's second law and actually know
5:10what I'm gonna go back to Newton's first
5:11off SEC okay I want to clarify it's very
5:16difficult to picture this idea of
5:18something once it starts moving keeps
5:20moving forever but that's because again
5:22a ball rolling across the floor there's
5:24friction between the floor and the ball
5:26and that is the force that's causing it
5:29to stop if we got rid of that friction
5:31it would keep sliding forever and in
5:33fact if you've ever seen like a puck
5:36sliding on ice you see it keeps
5:38traveling a lot further because there's
5:40less friction between the puck and the
5:42ice there's still some friction so we'll
5:44come to a stop eventually but as you
5:46reduce the friction if there's no force
5:48an object keep moving you know keep
5:49moving in the direction it's moving so
5:52even if you want a ball to go a
5:53different way you have to cause it to by
5:56giving it a force on nudge okay now
6:01Newton's second law starts to qualify
6:03this a bit more because what it talks
6:07about I mean this description here talks
6:10about acceleration and that force in a
6:11mass using these Newton's second law is
6:14often just written as a formula what we
6:17see and it might often be written as f
6:19net equals MA okay well what are we
6:22saying F is the force which is nice F
6:26for force a for acceleration M for mass
6:28nice and logical well in this case the
6:31net force means we're considering all of
6:32the forces acting on an object and I'll
6:35come back to why that's important a
6:36second but let's take a look
6:39acceleration as we saw before was the
6:43change in velocity or change in
6:46direction we'll see that's kind of
6:47important way to consider it too but
6:49it's a change in velocity so f if I'm
6:53pushing on something well its velocity
6:56will change I mean if I push on my phone
6:59it'll start sliding across the table its
7:00accelerating if you're in your car and
7:03you slam on the brakes that's a force
7:05causing you to decelerate go slower but
7:09the amount of force is not the only
7:13thing that's important the harder I push
7:15then the faster this thing will
7:18accelerate the more the harder I push on
7:20my phone the faster it goes if I pushed
7:22too hard it goes flying off the table
7:23but it also relates to mass it's backed
7:26an idea of that fridge example it's a
7:28lot easier for me to push my phone than
7:31it is to push a say fridge so if this
7:36acceleration inversed me proportional to
7:39mass so the more mass the harder it is
7:41to cause an acceleration the more force
7:43at the exert to cause an acceleration
7:46and the reason we use the net force this
7:48is we have to sum all of the force being
7:50applied to this object I mean a silly
7:53example would be if I start pushing on
7:55the fridge and my friends standing on
7:57the other side and pushing back well the
7:59fridge is gonna be a lot harder to move
8:01because we're counteracting each other
8:03so we have to consider all the forces I
8:05mean there's more proper examples not
8:07ones where my friends is messing with me
8:09a better one might be considering the
8:11friction between the ground and the
8:13fridge but you summarize all the forces
8:16to figure out how much of a total
8:18acceleration if me and my friend stand
8:20on the same side of the fridge and push
8:21our forces combined and it's easier to
8:24move that fridge