Friction: Crash Course Physics #6 CrashCourse English English (auto-generated)

Friction: Crash Course Physics #6YouTube↗ IA↗(long shot)
Education 1.2M views 13.4K likes 535 dislikes 05 May 2016 Show Contributors 6

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French Anthony Kyalumba

Portuguese (Brazil) Luca Miguel

Slovak Veda s titulkami

Spanish (Mexico) David F. Silva

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This episode of Crash Course Physics is sponsored by Audible.

this episode of crash course physics is sponsored by audible if you watched our

If you watched our episode on Newton’s laws of motion,

episode on Newton's law of motion then

then you’re practically an expert on FORCES, and the way they make things accelerate.

you're practically an expert on forces

But there’s something we left out -- something important when you’re trying to

and the way they make things accelerate but there's something we left out something important when you're trying

describe how things move in real life: FRICTION.

to describe how things move in real life

It stops a baseball player sliding into home plate.

friction it stops the baseball players sliding into home plate and it's the

And it’s the reason I can pick up this cup without it slipping through my fingers.

reason I can pick up this cup without it

Without friction, it would be tough to do … almost anything.

slipping through my fingers without friction it would be tough to do almost

But it also makes doing OTHER things…pretty hard.

anything but it also makes doing other things pretty hard

Anyone who has moved furniture knows that.

anyone who's moved furniture knows that

Luckily, friction is also a force, so, as physicists, we know what to do with it. [Theme Music]

luckily friction is also a force so as physicists we know what to do with

You’re moving into a new house, and you ask a friend to come help out, which he does, because he’s just nice like that.

you're moving into a new house and you asked a friend to come help out which he does because he's just nice like that

And you really do need his help, because you’ll have to move giant bookcases and a desk,

and you really do need his help because you have to move giant bookcases and a

and you can’t do all of that on your own.

desk and you can't do all of that on

And about halfway through the move-in, you’re rearranging the furniture in your new bedroom

your own and about halfway through the move-in you're rearranging the furniture

when you run into a problem:

in your new bedroom when you run into a

You’re pushing and pushing on your bookcase, but it just not sliding across the floor.

problem you're pushing and pushing on your bookcase but it's just not sliding

So you call your friend over from the other room, and with both of you pushing, the bookcase finally starts to slide.

across the floor so you call your friend over from the other room and with both of you pushing the bookcase finally

But it was really hard to get it moving -- and stay moving -- because there’s a force working

starts to slide but it was really hard to get it moving and stay moving because

against you - stopping the bookcase from sliding: friction.

there's a force working against you stopping the bookcase from sliding

And there are actually two kinds of friction.

friction and there are actually two

First, there’s KINETIC friction -- that’s the force that slows the bookcase down as it slides.

kinds of friction versus kinetic friction that's the force that slows the bookcase down as it slide and then

And there’s also STATIC friction — the force you had to overcome to get it moving in the first place.

there's also static friction the force that you have to overcome to get it

So let’s talk about kinetic friction first.

moving in the first place so let's talk about kinetic friction first it's a

It’s a resistive force, which just means that no matter what, it’ll act to resist you.

resistive force which just means that no matter what it will apt to resist you in

In other words, if you push the bookcase so it slides to the left, then the force of kinetic

other words if you push the bookcase so it slides to the left then the force of

friction is acting to the right.

kinetic friction is acting to the right

But if you move the bookcase to the right, then the friction will resist to the left.

but if you move the bookcase to the right then the friction will resist to

Kinetic friction will also often generate heat or sound or both.

the left kinetic friction will also often generate heat or sound or both so

So, what causes it?

what causes it to bookcase and the wood

The bookcase and the wood floor might LOOK smooth, but if you looked at their surfaces

floor might look smooth but if you looked at their surfaces under a

under a microscope, you’d see that they’re covered in tiny bumps and grooves.

microscope you'd see that they're covered in tiny bumps and grooves how's

As the bookcase slides across the floor, the surfaces catch on each other,

the bookcase slides across the floor the surfaces catch on each other and might

and might even start to form weak intermolecular bonds. That pulls on the bookcase, slowing it down.

even start to form weak intermolecular bonds that pulls on the bookcase slowing

But as the bookcase continues to slide, it breaks those bonds and moves past whatever bumps it was stuck on.

it down but as the book head continues to slide it breaks those bonds and Meuse pass whatever bumps it was stuck on the

The newly-freed molecules start to spring back, and the movement produces heat and sound.

newly freed molecules start to spring back and the movement produces heat and

Then the cycle continues with the next patch of floor.

sound then the cycle continues with the

You end up with a force that resists any kind of sliding movement, and its strength depends

next patch of floor you end up with a force that resists any kind of sliding movement and it's strength depends

partly on what materials are sliding against each other.

partly on what materials are sliding

Rougher materials have more surfaces to catch on each other, which is why the bookcase will

against each other rougher materials have more surfaces to catch on each other which is why the

be easier to slide on the wood floor than if you’d tried it on carpet.

bookcase will be easier to slide on the wood floor than if you tried it on

And the way in which this roughness affects kinetic friction is called the coefficient of kinetic friction,

carpets and the way in which this roughness affects kinetic friction is called the coefficient of kinetic

and it’s different for pretty much every combination of materials.

friction and it's different for pretty much every combination of materials so

But there’s another factor at work in friction:

there's another factor at work in

How hard the materials are pressed together puts more of their surfaces in contact with each other.

friction how hard the materials are pressed together puts more of their surfaces in contact with each other and

And that’s where the normal force, or F_N, comes in.

that's where normal force or FN comes in you might

You might remember from our last episode, that when a force pushes a bookcase against

remember from our last episode that when a force pushes on a bookcase against the

the floor, the floor pushes right back.

floor the floor pushes right back that's

That’s Newton’s third law, and we call the force from the floor on the bookcase the NORMAL FORCE.

Newton's third law and we call the force from the floor on the bookcase the

In this case, the force from the bookcase on the floor is just its weight, mg.

normal force in this case the force from the bookcase on the floor is just its weight mg so the normal force would be

So the normal force would be equal to mg, but pointing up -- equal and opposite.

equal to mg but pointing up equal and

So - Now we know about the two main factors that affect the strength of kinetic friction:

opposite so now we know about the two main factors that affect the strength of

the coefficient of kinetic friction -- written as mu_k -- and the normal force, F_N.

kinetic friction the coefficient of kinetic friction written as mu K and the normal force F n it shouldn't be too

It shouldn’t be too surprising, then, that the equation for kinetic friction is just…

surprising then that the equation for

the coefficient times the normal force.

kinetic friction is just the coefficient

But that’s only half the picture. What about static friction?

times the normal force but that's only half the picture what about static

When you weren’t able to move the bookcase to begin with, that was because of the static

friction when you weren't able to move the bookcase to begin with that was

friction between the bookcase and the floor.

because of the static friction between the bookcase and the floor like kinetic

Like kinetic friction, it’s also a resistive force.

friction is also a resistive force but

But not only can its direction change -- its strength can, too.

not only can its direction change its strength can change to say you push on

Say you push on the bookcase lightly, with just one hand, and it doesn’t move.

the bookcase lightly with just one hand

Since it doesn’t have any acceleration, the bookcase must be in equilibrium --

and it doesn't move since it doesn't have any acceleration the bookcase must

meaning that the forces on it balance out.

be in equilibrium meaning that the forces on it balance out so the force

So the force from static friction must be exactly equal to the force from your hand,

from static friction must be exactly equal to the force from your hand but in

but in the opposite direction -- because of that equilibrium.

the opposite direction because of that

It’s not the same as the normal force, and it has nothing to do with Newton’s third law.

equilibrium it's not the same as the normal force and it has nothing to do

But say you really put your back into it, pushing as hard as you can.

with Newton's third law but say you really put your back into it pushing as

The bookcase still doesn’t move, so static friction must still be pushing back with the same amount of force you are.

hard as you can the bookcase still doesn't move so static friction must still be pushing back with the same

However, the static friction between two objects can change:

amount of force you are however the static friction between two objects can

Once your friend starts pushing too, the bookcase starts to slide, because the force from your

change once your friend starts pushing to the bookcase starts to slide because

teamwork is greater than the maximum amount of static friction -- which we write like this.

the force from your team work is greater than the maximum amount of static friction which we write like this and

And the relationship between the maximum static friction and the normal force is similar to

the relationship between the maximum static friction in the normal force is

the relationship between kinetic friction and the normal force.

similar to the relationship between kinetic friction and the normal force so

There’s even a coefficient of static friction, which measures the roughness between any two

as even a coefficient of static friction which measures the roughness between any

objects -- and we just write it as mu_s.

two objects and we just write it as mu s

And the maximum force of static friction is just equal to the coefficient of static friction times the normal force.

and the maximum force of static friction is just equal to the coefficient of static friction times the normal force

So now we know the equations for both kinds of friction and where they come from.

so now we know the equations for both kinds of friction and where they come

Which means we can use that information to figure out how to solve problems involving friction.

from which means we can use that information to figure out how to solve problems involving friction just like

Just like with the other force-related problems, we start with a free body diagram.

with other forceful two problems we start with a free body

That helps sort out what’s pushing where, so we can get an overview of the situation.

diagram that helps sort out was pushing where so we can get an overview of the

Next, we decide on some axes: where are x and y? And which way is positive?

situation next we decide of some axis where x and y in which way is positive

That’ll be useful when ramps come up, because if we want, we can just decide that x points

that'll be useful when ramps come up because if we want we can just decide

down the ramp and y points straight into it.

there X points down the ramp and Y

The next thing we’ll need to do is separate vectors into components along our new x and y axis

points straight into it the next thing we'll need to do is separate vectors into components along our new X and y

because strange things happen to the normal force when you have something (like that bookcase) sitting on a ramp.

axis because strange things happen to the normal force when you have something like that bookcase sitting on a ramp the

The normal force is pointing out of the ramp, because it’s perpendicular to the surface,

normal force is pointing out of the ramp because it's perpendicular to the

but it’s NOT equal and opposite to the force of gravity, because THAT'S pointing straight down.

surface but it's not equal and opposite to the force of gravity because that's

Instead, we’ll have to use trigonometry to separate the force of gravity into two

pointing straight down instead we'll have to use trigonometry to separate the

perpendicular components: one pointing down along the ramp, and one pointing into it.

force of gravity into two perpendicular components one pointing down along the ramp and one pointing into it and the

Then, the normal force will just be equal and opposite to the part of the gravitational

normal force would just be equal and opposite to the part of the

force that’s pointing into the ramp.

gravitational force that's pointing into

So, here’s how to separate and diagram everything that’s going on:

the ramp so here's how to separate in diagram everything that's going on the

The angle between the force of gravity and the component pointing into the ramp is called theta.

angle between the force of gravity and the component pointing into the ramp is

And it’s the same as the incline of the ramp.

called theta and it's the same as the

Those make two sides of a right triangle. And since the component pointing into the

incline of the ramp those make two sides of a right-angled triangle and since the

ramp is adjacent to the angle, we know that it’s equal to (mg) x (the cosine of that angle)

component pointing into the ramp is adjacent to the angle we know that it's equal to mg times the cosine of that

-- and so is the normal force.

angle and so is the normal force the

The other component, pointing ALONGSIDE the ramp, is OPPOSITE the angle.

other component pointing alongside the ramp is opposite the angle so again

So, again using trigonometry, we know that it’s equal to (mg) x (the sine of the angle).

using trigonometry we know that it's equal to mg times the sine of the angle

So! You’ve drawn your free-body diagram, you’ve decided on axes, and separated the

so you've drawn your Freebody diagram you've decided on axis and separated the

force of gravity into two useful components.

force of gravity into two useful

Now, we can finally set up our equations! That’s where Newton’s second law --

components now we can finally set up our equations that's where Newton's second law F net

F(net) = ma -- comes in handy.

equals MA comes in handy they'll

There’ll actually be two of these -- one for the x direction, down the ramp, and one

actually be two of these one for the x-direction down the ramp and one for

for the y direction, into the ramp.

the y-direction into the ramp so let's

So let’s say you’ve got the bookcase where it needs to go, and now you’re ready to

say you've got the bookcase where it needs to go and you're ready to get

get something else out of the moving van -- this time, a 40-kilogram box with a very fragile vase inside it.

something else out of the moving band this time a 40 kilogram box with a very fragile bars insider is heavy so let's

It’s heavy, so let’s say you put it down on the ramp leading from the back of the truck

say you put it down on the ramp leading

to the ground -- which happens to be angled at 30 degrees -- and then walk away to get a drink of water.

from the back of the truck to the ground which happens to be angled at 30 degrees and then you walk away to get a drink of

You also happen to know that the coefficient of static friction between the box and the ramp is 0.50.

water you also happen to know that the coefficient of static friction between the box and the ramp is 0.5 oh what is

What is the box’s acceleration?

the box's acceleration if it's zero

If it’s zero, it’ll stay put and your vase should be okay when you walk away from it.

it'll stay put and your vast should be okay

If not, that means the box will start sliding down the ramp, and you may have a problem.

when you walk away from it if not that means the box will start sliding down the ramp and you may have a problem the

The first thing we need to do is draw a free-body diagram of this box.

first thing we need to do is draw a Freebody diagram of this box so the

For our coordinate system, we’ll have x along the ramp, with the positive side at the bottom,

coordinate system will have X along the ramp with the positive side at the

and y perpendicular to the surface of the ramp, with the positive side also toward the bottom.

bottom and Y perpendicular to the surface of the ramp with the positive side also towards the bottom and we have

And we have a whole bunch of forces here:

a whole bunch of forces here the normal

The normal force is pointing out of the ramp.

force is pointing out of the ramp

There’s also the force of static friction, which, right now is pointing up the ramp -

there's also the force of static friction which right now is pointing up

since it’s resisting the part of the gravitational force pulling the box down.

the ramp since it's resisting the part of the gravitational force pulling the

Speaking of which: the force of gravity is pointing straight down --

box down speaking of which the force of gravity is pointing straight down not

NOT along either of our axes, so we separate it into components:

along either of our axis so we separate

(mg)(cos)(theta) pointing into the ramp, and (mg)(sin)(theta) pointing down along the ramp.

it into components mg coz-theta pointing into the ramp and mg sine-theta pointing

Now we set up our net force equations along each axis:

down along the route now we set up our net force equations along each axis the

The box isn’t going anywhere along the y axis -- nothing’s going to make it start

box isn't going anywhere along the y axis nothing's going to make it start

rising above the surface of the ramp, right?

rising above the surface of the ramp

So we know that all the forces acting along the y axis should add up to zero, since they’re all balanced out.

right so we know that all the forces acting along the y axis should add up to zero since they're all balanced out but

But the forces along the x axis, along the ramp, are a different story.

the forces along the x axis along the

If the box has no acceleration (it’s standing still, or moving at a constant velocity) they’ll also add up to zero.

ramp are a different story if the box has no acceleration it's standing still or moving at a constant velocity they'll

But if it DOES have acceleration along the ramp, then the forces will add up to its mass, times that acceleration.

also add up to zero but if it does have acceleration along the ramp then the forces will add up to its mass times

So, to figure out if the box slides, all we need to do is find out if the part of the gravitational force

that acceleration so to figure out the box slides all we need to do is find out if the part of the gravitational force

pushing it down the ramp, (mg)(sin)(theta), is greater than the maximum static friction resisting it.

pushing it down the ramp mg sine-theta is greater than the maximum static

In other words: if down the ramp is the positive direction, is the net force positive or zero?

friction resisting it in other words if down the ramp is the positive direction is the net force positive or zero the

The forces here are the gravitational force pushing the box down the ramp, (mg)(sin)(theta),

forces here are the gravitational force pushing the box down the ramp mg

and the maximum static friction. But we write the maximum static friction as negative, because

sine-theta and the maximum static friction but we write the maximum static friction as negative because it's

it’s pointing in the negative direction on our x-axis -- up the ramp.

pointing in the negative direction on our x axis up the round so adding them

So, adding them together, we get this equation.

together we get this equation

Multiplying together the different values that make up (mg)(sin)(theta),

multiplying together the different values that make up mg sine-theta we

we find that it’s equal to 196.20 Newtons.

find that is equal to 196 point two zero

And the maximum static friction is just equal to the coefficient of (static friction) x

Newtons and the maximum static friction is just equal to the coefficient of

(the normal force) -- so, plugging in the numbers, we find that it’s 169.91 Newtons.

static friction times the normal force so plugging in the numbers we find that it's 169 point nine one and Newtons

Which means that the net force is 26.29 Newtons.

which means that the net force is twenty

So, yes, there’s a net force pushing the box down the ramp, and it’s going to slide.

six point two nine Newtons so yes there's a net force pushing on the Box down the ramp

Which means I have some bad news for you about that vase...

and it's going to slide which means I have some bad news for you about that

But today, you learned about the two types of friction:

bars but today you learned about two

kinetic friction and static friction, and how they act to resist applied forces.

types of friction kinetic friction and static friction and how they act to resist applied forces we also talked

We also talked about how to use friction to describe an object’s motion,

about how to use friction to describe an object's motion including how to

including how to calculate the normal force when that object’s on a ramp.

calculate the normal force when that objects on a ramp finally we calculated

Finally, we calculated whether a box on a ramp would start sliding down.

whether a box in a ramp would start

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You can head over to their channel to check out amazing shows like Deep Look, The Good Stuff, and PBS Space Time.

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This episode of Crash Course was filmed in the Doctor Cheryl C. Kinney Crash Course Studio

episode of crash course was filmed in the doctor Cheryl C Kinney crash course

with the help of these amazing people and our Graphics Team is Thought Cafe.

studio with the help of these amazing people and our graphics team is thought cafe you

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