Static and kinetic friction example | Forces and Newton's laws of motion | Physics | Khan Academy

Thinking about the coefficients of static and kinetic friction. Created by Sal Khan.

Watch the next lesson: https://www.khanacademy.org/science/physics/forces-newtons-laws/tension-tutorial/v/the-force-of-tension?utm_source=YT&utm_medium=Desc&utm_campaign=physics

Missed the previous lesson? https://www.khanacademy.org/science/physics/forces-newtons-laws/inclined-planes-friction/v/intuition-on-static-and-kinetic-friction-comparisons?utm_source=YT&utm_medium=Desc&utm_campaign=physics

Physics on Khan Academy: Physics is the study of the basic principles that govern the physical world around us. We'll start by looking at motion itself. Then, we'll learn about forces, momentum, energy, and other concepts in lots of different physical situations. To get the most out of physics, you'll need a solid understanding of algebra and a basic understanding of trigonometry.

About Khan Academy: Khan Academy offers practice exercises, instructional videos, and a personalized learning dashboard that empower learners to study at their own pace in and outside of the classroom. We tackle math, science, computer programming, history, art history, economics, and more. Our math missions guide learners from kindergarten to calculus using state-of-the-art, adaptive technology that identifies strengths and learning gaps. We've also partnered with institutions like NASA, The Museum of Modern Art, The California Academy of Sciences, and MIT to offer specialized content.

For free. For everyone. Forever. #YouCanLearnAnything

Subscribe to Khan Academy’s Physics channel: https://www.youtube.com/channel/UC0oGarQW2lE5PxhGoQAKV7Q?sub_confirmation=1
Subscribe to Khan Academy: https://www.youtube.com/subscription_center?add_user=khanacademy

So I have got this block of wood here that has a mass of 5 kilograms

and it is sitting on some dirt and we are near the surface of the earth

and the coefficient of static friction between this type of wood and this type of dirt is 0.60

and the coefficient of kinetic friction between this type of wood and this type of dirt is 0.55

This was measured by someone else long ago

or you found it in some type of a book someplace

And let's say we push on this side of the block with a force of a 100 N

What is going to happen?

So the first thing you might realize is if there is no friction

if this was a completely frictionless boundary and there is

no air resistance, we are assuming that there is no air resistance in this example

That in this dimension, in the horizontal dimension

there would only be one force here, this 100 N force

It would be completely unbalanced and that would be the net force

and so you would have a force going in that direction of a 100 N on a mass of 5 kilograms

Force = Mass times acceleration

acceleration and force are vector quantities

So you would have the force divided by the mass

would give you 20 meters per second of acceleration in the rightward direction

That is if there were no friction

but there is friction in this situation

So let's think about how we'll deal with it

So the coefficient of friction tells us

So this right here is the ratio between the magnitude of the force

that I have called the budging force

The amount of force you need to apply to get this thing to budge

to get this thing to start moving. So we can start using the coefficient of kinetic friction

It's the ratio between that and the magnitude of the force of contact

between this block and the floor or ground here

And the magnitude of that force of contact is the same thing

as the normal force that the ground is applying on the block

the magnitude of the normal force the ground is applying on the block

Then once its moving

then we can say that this is going to be--this will then be equal to

this over here will be equal to the force of friction

So this is the force that really overcome friction

and this over here will be equal to the force of friction

The magnitude of the force of friction over the force of contact

the contact force between those two, so over the normal force

and it makes sense

that the larger the contact force

the more that these are being pressed together

the little at the atomic level, they kind of really get into each others grooves

the more budging force you would need

or the more friction force would go against your motion

And in either situation

the force of friction is going against your motion

So even if you push it in that way

sounds like force of friction is all of a sudden going to help you

So let's think about what the necessary force will we need

to overcome the force of friction right here in the static situation

So the force of gravity on this block

is going to be the gravitational field which is 9.8 m/s^2 times 5 kilograms

9.8 m/s times 5 kilograms gives 49 kilogram meters per second or 49 newtons down

This is the force, the magnitude of the force due to gravity

the direction is straight down towards the center of the earth

The normal force, and that force is there because this block is not accelerating downwards

So there must be some force that completely balances off the force of gravity

And in this example, it is the normal force

So it is acting 49 newtons upward

and so these net out. And that's why this block does not accelerate upwards or downwards

So what we have is the budge the

magnitude of the budging force, needs to be equal to, over the magnitude of the normal force

well this thing right over here is going to be 49 newtons

Is equal to 0.60

Or we could say that the magnitude of the budging force

is equal to 49 newtons times the coefficient of static fiction

Or that's 49 newtons times 0.60

And remember coefficient of friction are unitless

So the units here are still going to be in newtons

So this 49 times .6 gives us 29.4 newtons

This is equal to 29.4 newtons

So that's the force that's started to overcome static friction

which we are applying more than enough of

so with a 100 newtons, we would just start to budge it

and right when we are in just in that moment

where that thing is just starting to move

the net force--

so we have a 100 newtons going in that direction

and the force of static friction is going to go in this direction--

maybe I could draw it down here to show it's coming from right over here

The force of static friction is going to be 29.4 newtons that way

and so right when I am just starting to budge this

just when that little movement--

because once I do that, then all of a sudden it's moving

and then kinetic friction starts to matter, but just for that moment

just for that moment I'll have a net force of 100 - 29.4

to the right, so I have a net force of 70.6 N

for just a moment while I budge it

So just exactly while I'm budging it

While we're overcoming the static friction, we have a 70.6 N net force in the right direction

And so just for that moment, you divide it by 5 kg mass

So just for that moment, it will be accelerating at 14.12 m/s^2

So you'll have an acceleration of 14.1 m/s^2 to the right

but that will just be for that absolute moment, because once I budge it

all of a sudden the block will start to be moving

And once it's moving, the coefficient of kinetic friction starts to matter

We've got the things out of their little grooves

and so they're kind of gliding past each other on the top, although there still is resistant

So once we budge it, we'll have that acceleration for just a moment

Now all of a sudden, the coefficient of kinetic friction comes to play

And the force of friction, assuming we're moving

the magnitude of the force of friction will always go against our movement

is going to be--remember, our normal force is 49 N

So we can multiply both sides of this times 49

We get 49 N times 0.55 which is equal to 26.95 N

This is the force of friction; this is the magnitude

and it's going to go against our motions

So as soon as we start to move in that direction, the force of friction

is going to be going in that direction

So once we start moving, assuming that I'm continuing to apply this 100 newtons of force

what is the net force? So I have 100 N going that way

and I have 26.95 going that way

Remember, with vectors, I don't have to draw them here

I can draw all of their tails start at the center of mass of the

object. I can draw them whatever, but remember this is acting on the object

If we want to be precise, we can show it on the center of mass because

we can view all of these atoms as one collective object

But anyway, what is the net force now?

We have 100 N to the right; we have 26.95 to the left

100 minus 26.95

100 N that I'm applying to the right

- 26.95 N which is the force of friction to the left always acting against us

means that there's a net force to the right of 73.05

So once we're moving, we have a net force to the right of 73.05 N

This is the net force and it's acting to the right

Right after we budge it, how quickly will this accelerate?

Well, 73.05 divided by the mass, divided by 5 kg, gives us 14.61

So the acceleration once we're moving is going to be 14.61 m/s squared

to the right

So I really want to make sure you understand what's happening here

We always have enough force to start budging it

but right when we budged it

we overcome the static friction for just a moment

our acceleration was slower

because we're overcoming that static friction

but once we budged it and once it's moving

and assuming that we're continuing to apply a constant force over here

then all of a sudden, the force of friction since

we're kind of bump it along the top now and not stuck in their grooves

we're now using the coefficient of kinetic friction

And so once it's moving, the net force becomes greater in the rightward direction because

you can kind of view that force of friction will become less once it starts moving

And so now the force of friction went down a little bit to 26.95 N

And so now we're accelerating to right at a slightly faster rate 14.61 m/s^2

So right when you budge it, it accelerates at 14.1 m/s^2

but just for a moment, almost unnoticeable moment once it starts moving

Then you're going to be going to the right with this constant acceleration