SPH3U 3.1 Types of forces

0:03hello and welcome to 3.1 types of forces

0:07this is the start of unit 2 all about

0:09forces in unit one we learned about

0:12kinematics which was how things move in

0:14this unit we're going to be learning

0:16about why things

0:18move and that's called Dynamics Dynamics

0:21is the

0:24study of the causes of motion

0:31the causes of

0:36motion and of course our unit is called

0:39forces the most important thing here is

0:41a force a force is any

0:45push or

0:48pull anything that pushes something or

0:50pulls something that's called a

0:52force our units are called

0:55Newtons this is the unit for force

1:03and the conversion here is that 1 Newton

1:05is equal to 1

1:08kilg Time m/s squ 1 kilog m/s

1:14squar and we're going to see where that

1:15unit comes from

1:18soon to measure forces we can use

1:22devices called spring

1:27scales or Force

1:32sensors and basically a spring scale is

1:35um the more old-fashioned way of

1:38measuring something it's the analog way

1:40the idea is that we actually have a

1:42device that has a spring in it and when

1:43you pull it it measures how far it's

1:46been pulled and that tells you how much

1:47force has been applied a force sensor

1:50would be an electronic device that does

1:52the same thing it tells you how much

1:53force is being

1:57applied okay now we have two really

1:59important ways that we need to draw our

2:01problems here it's really important

2:03really really important when we're

2:04dealing with forces that we draw our

2:06problems so that we can understand

2:08exactly what's going on there's two ways

2:10of drawing our problems that's system

2:13diagrams and free body

2:15diagrams a system diagram is a simple

2:24sketch of all objects

2:32in a

2:37situation and we're going to do some

2:39examples down below so a system diagram

2:42is just a sketch a sketch of the

2:44situation whereas a free body diagram

2:47this is the one we're going to be using

2:48a lot in this unit this is again a

2:51simple dra uh simple drawing

3:00but this one is just of one

3:07object

3:10showing all

3:14forces acting on the object all forces

3:17on

3:19it so we've got our examples down below

3:22this first one here this is a system

3:28diagram and you can see that's a a

3:31sketch of the situation there's a hand

3:34holding a book now that's a pretty good

3:36sketch if you were to make a system

3:37diagram it probably wouldn't be this

3:39detailed and this one here this is a

3:42free body diagram and look at that

3:45difference here the free body diagram

3:47it's just a

3:48box with some lines coming out of it

3:51that's what every free body diagram is

3:52going to look like it's a box with some

3:55lines coming out of it each one of those

3:57lines is a force so each one of those

3:59lines is either a push or a pull on that

4:03object this is actually the free body

4:05diagram of that book there the book has

4:09gravity pulling it downwards and it has

4:11an applied force pushing it upwards

4:14that's from our

4:15hand okay let's get some terms here

4:17because we I just talked about these

4:19different

4:20forces we've got a whole list of

4:22different forces down here so we're

4:23going to we're going to go through

4:24through this list first

4:26off applied force we call this

4:31fa and each one of these forces is going

4:34to have a different name with a letter

4:36so fa is for the applied force this is a

4:41force caused by an

4:50object pushing or

4:57pulling if we have some object that's

5:00pushing or pulling something else that's

5:02giving it an applied

5:05force that's a pretty broad category and

5:07you're going to see the rest of these

5:08have more

5:09specific scenarios so tension ft notice

5:14there's an arrow over it these are all

5:17vectors tension

5:20is when

5:22a

5:24rope or

5:27string is pulling an object

5:35and notice it's only pulling tension can

5:37only pull it can't pull it can't push

5:41sorry and that hopefully that makes

5:43sense if you have something tied to a

5:45rope you can't push the Rope to push the

5:47object the Rope just sort of curls

5:51up okay our next one here is the normal

5:53force we're going to be be seeing a lot

5:55of this Force here the normal force FN

5:59um is when a

6:02surface when a

6:07surface pushes back on an

6:17object so for instance right now you're

6:20probably sitting in a chair or standing

6:23somewhere gravity is pulling you

6:25downwards and that's soon on our list

6:27gravity is pulling you downwards and the

6:29floor or your chair is pushing you back

6:32upward so that you don't fall down

6:34that's called the normal force that

6:36surface your chair or the floor is

6:39keeping you from falling

6:41downwards that's pushing you back

6:43upwards so that's a normal

6:45force and the other thing we can say

6:48here is that FN the normal

6:50force is always perpendicular

7:00to the

7:02surface whatever surface is giving us

7:04this supplied Force FN is always

7:06perpendicular to that

7:09surface good two more forces on this

7:12list here we have

7:13FF this is for

7:16friction friction slows things down

7:19friction

7:22always

7:24tries to stop motion

7:34and it

7:37acts in the opposite

7:47direction of the

7:50motion so if I'm moving forwards there's

7:53going to be some friction that's slowing

7:55me down some friction that's forcing me

7:57backwards to Slow Me Down

8:00and we're going to be learning all about

8:03all of these different forces right now

8:04we're just getting a quick overview our

8:06last one here Gravity you're probably

8:09familiar with gravity this is the force

8:12of

8:18attraction well I'm going to stop for a

8:20second here Gravity is the thing that

8:22pulls you towards the Earth right it's

8:24what makes us heavy it's what makes us

8:27not float away from the earth but it's

8:29not just the Earth there is gravity

8:32between everything any two objects have

8:35gravity between them and the only reason

8:36why for us we only care about the

8:39Earth's gravity is because the Earth is

8:41so much bigger than anything else we

8:43deal with and it's also closer the sun

8:46is even bigger than than the earth but

8:48it's further away so we just feel the

8:50gravity of Earth but if you've got a pen

8:53in your hand you are experiencing

8:55Gravity from that pen you're

8:57experiencing Gravity from the table

9:00everything pulls itself to other other

9:03things so this is the force of

9:04attraction

9:08between any two

9:15objects now in this course we focus on

9:18Earth's

9:21gravity so we actually have an equation

9:24here for Gravity the force due to

9:26gravity is equal to mg

9:31and we've dealt with G already where G

9:35is 9.8 m per second

9:39squared and there we go I'm actually

9:42just going to turn that into a vector

9:44I'm going to say that this is 9.8 m/s

9:46squar

9:47down always pulling us towards the

9:50Earth so in this course we're just

9:52dealing with Gravity from the

9:54earth okay we've got a picture down here

9:58so I want to um

10:00I want to do a diagram where we're

10:01looking at all the forces on this

10:02picture here first off this is a sketch

10:04of the situation this is a system

10:08diagram good we're going to draw a free

10:11body

10:15diagram we always start with a

10:18box no artistic talent needed for this

10:21we just start with a box and we draw all

10:24the forces on this usually we can start

10:26with gravity gravity is usually easy

10:27because it's always going downwards

10:30towards the center of the earth okay

10:32let's look back at this picture here

10:34we've got gravity pulling us down the

10:37ground is pushing us back up so we're

10:39going to have a normal force upwards I

10:41can put that over

10:43here here's our normal

10:45force okay and then we have this girl

10:48over here is pushing the

10:51object if that girl is pushing it she's

10:53directly pushing it she's giving it an

10:55applied

10:57force notice that even she's on the left

11:00she's pushing it this way she's pushing

11:03it to the right so we draw the force

11:04coming out the right side of our

11:08box okay and then we have another Force

11:11here the boy is pulling on this rope

11:15remember rope is special so if we're

11:18dealing with rope that means we have a

11:19tension force again it's coming off in

11:21this

11:22direction

11:24ft and we're almost done but remember if

11:28something moving then there's probably

11:30friction slowing it down so we've got

11:32our tension going this way we've got our

11:34applied

11:35force friction is going to be slowing it

11:37down back in this

11:40direction FF and there's our free body

11:43diagram that's how we make free body

11:46diagrams okay let's pop on to the next

11:48page

11:49here we've got a bunch of situations and

11:51we're going to draw the system diagram

11:53and the free body diagram for each

11:56object remember fbd that stands for free

11:59body diagram every single problem that

12:02you do in this unit should have a free

12:03body

12:06diagram here's our

12:09system we have a cup sitting at rest on

12:12a table okay so I'm going to draw a

12:13little table here again I am not an

12:19artist so something like this here's the

12:23legs of the

12:26table cool that looks like a table and

12:29now we'll draw a free body diagram

12:32remember the system diagram we just draw

12:34the whole thing just a little sketch of

12:36what's going on we have a cup on the

12:38table but we want to draw the free body

12:41diagram just for the one object the cup

12:43and fbd is always just one object here's

12:46our cup it has gravity pulling it

12:50down and it has normal force pushing it

12:53back up so that it doesn't fall

12:55downwards that's

12:57it next one a large trunk in the

13:00basement is pulled by a rope tied to the

13:02right side of the trunk by a person the

13:05trunk does not move a trunk it's like a

13:07big um big wooden chest or a box so

13:11we're going to draw this big wooden

13:15box something like

13:18this okay and we've got a rope tied to

13:21it it's being pulled in this direction

13:25by some person here here's our person

13:30we're pulling it but the trunk doesn't

13:32move so we're trying to pull the trunk

13:35isn't actually moving that's our

13:39system and here's our

13:44fbd let's see gravity always a good one

13:49to start with normal

13:53force

13:56friction friction is keeping it from

13:58moving

13:59and we've got our applied force or in

14:02this case actually it's a tension force

14:03because we're pulling a rope ft now

14:06notice I've drawn that tension force up

14:09on an angle because probably if you look

14:12at our picture probably we're pulling it

14:14up on a bit of an angle from wherever it

14:16was tied Maybe not maybe it's a

14:19different situation but that's just my

14:20feeling about

14:23this all

14:25right now we're going to be learning by

14:27the way how to use these diagrams use

14:30these free body diagrams to solve the

14:32problem to figure out what all the

14:34different forces are but that's beyond

14:36us right now we're just learning how to

14:37draw them next one we have a baseball

14:40player sliding to the left across the

14:41ground okay here's our

14:46system all right this is where we need

14:48to get a bit artistic here we've got

14:50a

14:51uh plate maybe that the baseball player

14:54is sliding towards and we're going to

14:56have this this baseball player sort of

14:57sliding off like this this trying to

15:01catch that base before they're tagged

15:03out something like this and they're

15:05sliding in this direction

15:07perfect here's our fbd of that situation

15:11we want to draw the person here's our

15:14person

15:16gravity normal

15:18force and our last one

15:21here well maybe there's more than one

15:23but I guess I've spoiled it already we

15:25only have one more Force this person is

15:27sliding this way as they go They're

15:29slowing down

15:31right they're going to stop at some

15:34point all they have acting on them here

15:36is friction friction is slowing them

15:38down and there's no Force pushing them

15:41forwards remember if there was going to

15:42be a force over here something would

15:44have to be giving it that

15:46Force we don't have anything to give it

15:49a force

15:50forwards so that's our free body diagram

15:52of that

15:53picture last one a desk is pushed to the

15:58left across uh the

16:02floor we're going to draw our desk

16:07here something like this and we got some

16:10person here pushing it

16:15to the left goes this way it's usually a

16:19good idea on your system diagram to draw

16:21some sort of direction of

16:24motion and an

16:26fbd of our desk again we have

16:30gravity normal

16:34force we're pushing it to the left so it

16:36has an applied force we sometimes call

16:39this an action force as well

16:42and again we usually have friction so

16:46friction is going to be slowing it down

16:47a

16:49bit okay there we go there's there's

16:52some diagrams now this is as you've

16:54probably noticed a long lesson so we've

16:57still got a bit to go

17:00number three we want to calculate the

17:02net force net

17:04force net force we we call this F net

17:09like this so we can add this to our list

17:12of different forces but net force is a

17:15bit special net force is the total

17:23force and the way that works is um it's

17:26the sum

17:30of all

17:35forces acting on an

17:45object so we need to add them all

17:47together now they're going to be in

17:48different

17:49directions so we're we're probably going

17:51to have to do some

17:532D um

17:55vector addition

18:00sometimes when we're doing these sorts

18:01of

18:02problems but that's what our net force

18:04is and it's a very important concept

18:07when we take all the different forces

18:09acting on something we add them all up

18:12well if we have a force to the left and

18:13a force to the right if they're the same

18:16that adds to zero if we have a force up

18:18and a force down and they're the same

18:19that adds to zero but it could be

18:22unbalanced and we could have maybe the

18:25left force is larger than the right that

18:27means that our net force would be

18:29something like this or if the left one

18:32and the up one were were larger then we

18:34could have a net force on some angle

18:36here it's really there's a lot of

18:37different scenarios here and we're going

18:40to be learning in the next lessons why

18:43the net force is

18:45important for now we're just going to

18:46learn how to calculate it here it says

18:50that the floor exerts a normal force of

18:5136 Newtons up on a stationary chair the

18:55force of gravity on the chair is 36

18:57Newtons down draw the pre- bodybody

18:59diagram of the chair and use that to

19:01determine the net force on the chair

19:03okay every single Force problem should

19:06start with a free body

19:08diagram gravity normal force I can't

19:12tell you how important it is to draw one

19:14of these pictures it always helps you

19:16solve it in this case our net

19:20force here we can see we have FN upwards

19:24FG downwards

19:27so I want to add add them together

19:29they're in opposite directions so I can

19:30do this by saying it's going to be

19:33FN well let's let's add them as vectors

19:35here let's do some proper vector

19:37addition here FN plus

19:40FG this is what we're dealing with here

19:43so FN is 36 Newtons

19:48up and FG is 36 Newtons

19:57down now

20:00if you remember from unit one up and

20:03down those are opposite directions so to

20:05add these together I need to flip the

20:07second one I can say this is minus 36

20:10Newtons

20:11up 36 down becomes NE 36

20:16up and now we can do this here we have

20:1836 minus 36 that gives us a grand total

20:22of 0 Newtons and zero doesn't have a

20:25direction 0 is just zero

20:28we also don't need to do the right

20:30sigfigs for zero Z is zero that's all we

20:33have excellent therefore the net

20:41force is

20:46zero let's try another one here the

20:49figure to the right shows all the forces

20:51acting on an object use the free body

20:53diagram to calculate the net

20:55force we have our

20:57fbd now now notice we have some forces

21:00in the X some forces in the

21:04Y just like when we were doing our

21:06vector addition here we're going to

21:09separate those we're going to focus on X

21:11and Y as two separate pieces so I can

21:14get the

21:16fnet

21:18X the net X Force that's going to be we

21:22have 62 to the right minus 45 because

21:25that 45 is to the left here I'm

21:28simplifying things a bit um which I

21:31would recommend doing when you're

21:32solving doing it more in this faster way

21:36where we're implicitly doing our

21:38directions just with positive and

21:40negative as opposed to always writing

21:42out the the directions like we did above

21:44whatever is more comfortable for you go

21:46with it okay so 62 minus 45 we end up

21:50with 17

21:53Newtons and now we can say that this we

21:56said positive was to the east so 17

21:59Newtons

22:00East we can do the same thing for y here

22:04we have 52

22:05up 52

22:09down that means our y force is

22:13zero X is 17 Y is zero that means that

22:18our net force if we add those

22:22together as

22:24vectors our net

22:27force is going to be 17 Newtons

22:33East we can do a there four there that's

22:37it that's how we can find our net

22:40force now obviously we could have a more

22:42complicated scenario where we have some

22:45net X force and we have some net y force

22:48and we have to add them as

22:51vectors getting the

22:53hypotenuse that's something that can

22:55happen but mostly we'll be looking at um

22:58this sort of

23:00result all right last page here and now

23:03this is a a new topic so we've learned

23:07about all our different forces we've

23:09learned about how to do diagrams we've

23:11learned how to get the net force and now

23:13this is this is

23:16separate we're not going to be talking

23:18about this idea a whole lot but this is

23:20what all of our forces are built on

23:23there are four fundamental forces in the

23:27universe in everything as far as we know

23:29there's four types of forces and

23:32everything in the universe is made up of

23:33those four types of forces which is a

23:36big deal we're still as physicists we're

23:39trying to find a way to combine these

23:41into one

23:42force gravity is the main problem with

23:45that but anyway as far as we know

23:46there's four types of

23:48forces

23:50gravitational well you know a bit about

23:52gravitational force now so this acts

23:55between any two objects

24:06and the interesting thing about gravity

24:09is that this is only

24:16attractive there's no repulsion so that

24:19means it

24:20only pulls it doesn't push

24:28and that's different from all the other

24:29forces every other Force we have here

24:32can push things or it can pull things

24:33gravity as far as we know it can only

24:35pull things towards each

24:37other that's our first type of force our

24:40next force is

24:42electromagnetic well electromagnetic

24:44this is

24:47caused by electric

24:56charges so if we have two electric

24:59charges they interact some way they have

25:01a force between them it can

25:09attract or

25:12repel so it can push or

25:16pull and you've probably heard something

25:18like that before that like charges repel

25:21and opposite charges attract that sort

25:23of thing that's what we're talking about

25:25here and this is what holds

25:31atoms and molecules

25:41together so it's what causes the

25:43electrons to stay orbiting the nucleus

25:47and it's what causes multiple atoms to

25:49join together into a

25:52molecule

25:54now in this course not just in this

25:57course in in in physics almost all the

26:00forces that we talk about when I talk

26:02about tension and I talk about um

26:05friction and I talk about uh applied

26:08force almost all of those boil down to

26:11electromagnetic forces so

26:16almost all

26:18forces that we deal

26:21with are electromagnetic

26:28which is kind of interesting to think

26:30about if I push

26:32something what's happening is and I'm

26:35just going to draw on this paper here

26:37what happens is all the molecules in my

26:39hand let's say this is my this is my

26:41hand here all the little molecules in my

26:45hand come up against all the little

26:47molecules in the thing I'm pushing let's

26:49say I'm pushing the wall and as they get

26:52closer the protons and electrons that

26:54are in my hand are repelling the protons

26:57and electrons that that are in the wall

26:59and they push each other and that's

27:03what's pushing that's what a pushing

27:05what an applied force is it's really

27:08just electromagnetic it's the same story

27:10with tension and with friction and all

27:12of these

27:14forces the only real one we'll be

27:16looking at that isn't that way is our

27:17gravitational force and again gravity is

27:19weird we don't fully understand gravity

27:22yet okay we have two more types of

27:24forces strong nuclear and weak nuclear

27:27strong nuclear here this

27:32keeps the

27:35protons uh actually let me just uh erase

27:38that I'm sorry this this it holds the

27:41nucleus

27:49together so we've got a whole bunch of

27:51protons together you might have thought

27:54it's weird that in the nucleus of an

27:56atom we can have a whole bunch of

27:58protons and for some reason they all

28:00want to stay together they're like

28:02charges so shouldn't they want to repel

28:04each other shouldn't they want to all

28:05fly away well this is they don't because

28:09there's this strong nuclear force that

28:12holds the nucleus

28:14together it holds the protons near each

28:16other so this it keeps the

28:23protons

28:26from repelling each other

28:38and it also keeps them

28:41from getting too close to each

28:53other they serve a double purpose

28:55because it would be just as bad if a

28:57proton and another proton got too close

28:59to each other and ended up fusing

29:02together that would be a a problem so

29:05the strong nuclear sort of keeps the

29:07nucleus all together and I'll point out

29:10that this is by far the

29:15strongest of the four

29:17forces we're going to see that just

29:19below here that's by far the strongest

29:21of the four forces and that's what holds

29:22the nucleus together but you don't see

29:24it outside of that nucleus and finally

29:27we have weak nuclear here and this is

29:29what

29:31holds a single

29:38Proton or

29:40Neutron

29:42together so this is even smaller if you

29:46um if you zoom in on a proton it's made

29:50up of these things called quarks and the

29:52weak nuclear sort of binds the quarks

29:56together

30:02so now we're really getting into the

30:04serious How the Universe Works part of

30:07physics and like I said we're not going

30:09to dwell too much on this in this course

30:11but it's important that you've got this

30:12Foundation to understand what the forces

30:14are everything we talk about in this

30:16course is going to boil down to these

30:19four

30:20forces okay one last little chart and

30:23then we're done so we've got our four

30:25types of forces

30:28we're just going to fill out the

30:29approximate strength of them and how far

30:33they operate so if we say that gravity

30:36has a strength of one we'll just start

30:39at that we'll say gravity has a strength

30:40of one then electromagnetic force is

30:45much stronger it has a strength of 10 20

30:48it's 10 the 20 times as strong as

30:50gravity is way stronger strong nuclear I

30:54said this is the strongest one 10 38 and

30:58weak nuclear not far behind at 10 25 so

31:02notice gravity is way weaker than

31:04everything

31:06else our range gravity operates on a

31:10range basically infinite you can be on

31:12the other side of the universe and we

31:14still have some amount of gravity

31:16gravitational force from an object

31:19electromagnetic force same thing

31:21basically infinite range strong nuclear

31:24and weak nuclear are on very small

31:26ranges and then they disappear

31:28so strong nuclear even though it's the

31:31strongest things need to be very close

31:34together less than 105 M and weak

31:38nuclear even

31:41smaller

31:44108 last column here the effect

31:49gravity can only

31:53attract whereas all these other ones can

31:56attract

31:59and

32:04repel and that's it those are our forces

32:08I'm sorry this has been such a long

32:09lesson but we really got through quite a

32:11lot at the bottom there's a bunch of

32:14problems don't be deceived they're

32:16actually pretty short questions each one

32:17of them so go ahead and give those a try

32:20and see you in the next lesson