Inclined planes and friction
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Inclined Plane Force Components
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Ice Accelerating Down an Incline
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Force of Friction Keeping the Block Stationary
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Correction to Force of Friction Keeping the Block Stationary
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Force of Friction Keeping Velocity Constant
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Intuition on Static and Kinetic Friction Comparisons
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Static and Kinetic Friction Example
Force of Friction Keeping Velocity Constant Calculating the coefficient of kinetic friction (correction made in next video)
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- I want to make a quick clarification to the last video and then think about
- what friction up to when the block is actually moving
- In the last video, we started off with a block being stationary
- We knew that the parallel component
- of the force of gravity on that block was 49 N downwards, down the slope
- I went on the block was stationary so there must be an offsetting force
- then we said, that's the force of friction and it must be 49 N upwards
- and so they completely net out in that direction
- Now what we said is keep applying a little bit more force until we can budge
- this block to start accelerating downwards
- I said I kept applying a little bit more force until I get to 1 N
- and then the block started to budge
- So at that point, when it started to budge, I am applying this 1 N right over here
- There is already 49 N of force or the component of gravity in this direction
- Combined, we're providing 50 N to just start budging it
- to just overcome the force of friction
- The one thing I want to clarify here is that
- at the whole time the force of friction was not constant at 49 N
- When I wasn't messing with this block and the parallel component of force was 49 N
- then the force of friction was 49 N
- When I started to press on it, apply a little bit force
- maybe I applied 1/10 N on top of that
- then the force of friction was 49.1 N
- because it was still providing enough force so that this block was not moving
- Then I applied maybe 1/2 N
- and so the total force in the downward direction would have been 49.5 N
- But if it still was not moving
- then the force of friction was still completely overcoming it
- So the force of fiction must be 49.5 N
- All the way up to the combined force in the downward direction being 49.999999 N
- and then the force of friction was still 49.999999 N
- All the way until I hit 50 N and the block started to budge
- which tells us that the force of friction, or at least the force of static friction
- all of a sudden now couldn't keep up and it started to accelerate downwards
- So in that static scenario, the force of friction
- changed as I applied more or less force in this downward direction
- With that out of the way, let's take a different scenario
- Let me just redraw that same block
- since all this is getting messy
- So we have the same block
- As we said in the last video, we're now assuming wood on wood
- So this is the wedge
- This is the block right over here
- We know that the component of gravity that is parallel to the plane right there is 49 N
- We know the component of gravity that's perpendicular to the plane--
- we figured it out about 2 videos ago--
- is 49 square root of 3 N
- We know that this block is not accelerating in this normal direction
- So there must be some force counteracting gravity in that direction
- And that's the normal force of the wedge on the block
- So that is going in that direction
- at 49 square root of 3 N
- And now instead of assuming that this block is stationary
- let's assume that it's moving with a constant velocity
- So now are dealing with a scenario where the block has a constant velocity
- And for the sake of this video, we assume that constant velocity is downwards
- The constant velocity v is equal to--let's say--5 m/s down the wedge or down the ramp
- or in the direction that is parallel to the surface of the ramp, in this direction
- right over here. So that's the constant velocity
- So what are all the forces at play?
- And be very careful here
- There might be temptation to say, OK, you know there's a net force here. We're moving
- So maybe that's the net force causing the movement, but member, this is super important
- This is Newton's 1st law
- If you have a net force, if you have an unbalanced force, it'll cause it to accelerate
- And we are not accelerating here; we have a constant velocity
- We are not accelerating here
- So if you're not accelerating in that direction
- then that means that the force in that direction must be balanced
- So there must be some force acting in the exactly opposite direction
- that keeps this thing from accelerating downwards
- And so it must be exactly 49 N in the opposite direction
- And as you can imagine, this is the force of friction
- This right over here is the force of friction
- And the difference between this video and the last video is last time friction was static
- even at 49 N, the box was stationary
- You keep nudging it until you get to 50 N, then it start moving
- Here we're just jumping into this picture where
- we just see a box that's moving down the slope at 5 m/s
- So we don't know how much force it took to overcome static friction
- But we do know
- that there is some force of friction that is keeping this box from accelerating
- keeping it at a constant velocity
- that is completely negating the parallel component of the force of gravity
- parallel to the surface of this plane
- So given this, let's calculate another coefficient of friction
- but this is going to be the coefficient of kinetic friction
- because now we're moving down the block. I'll do a video on why
- sometimes a coefficient of static friction can be different
- than the coefficient of kinetic friction
- So the coefficient of kinetic friction, we'll write it as the Greek letter mu
- with a little k here for kinetic where it's a kind of moving friction
- It's going to be equal to the magnitude of the force of friction
- over the magnitude of the normal force
- And you can derive this experimentally
- You just observe this whole thing going on and you do the math with this block
- You knew the a component of gravity that's going in this direction
- you could figure out this coefficient of kinetic friction
- What's cool about this is, this is in general going to be true
- For any two materials that are like this. So you know, maybe this is
- a certain type of wood on a certain type of wood
- or a certain type of sandpaper on a certain type of sandpaper
- whatever you're talking about, and then you can use that to make predictions
- if the incline was different or the mass was different
- or even if you are on a different planet
- or someone was pressing down on this block would change the normal force
- So given this right here, let's figure out for the sake of doing it
- the coefficient of kinetic friction here
- The force of friction here completely offsetting the force of gravity parallel to the surface
- is 49 N
- And the normal force here, the force of contact between these two things
- this block and this wedge, is 49 sqrt of 3 N
- So we get one over the square root of 3
- Let's get the calculator out to get the accurate number here
- So we have one divided by the square root of three
- which gives us .58
- 0.58 and there's no units here, because the units cancel out. It's a unitless measurement
- And the interesting thing here is that the coefficient the way I've set up this problem
- the coefficient of kinetic friction is lower, if we assume the same materials
- than the coefficient of static friction
- And for some materials, they might not be that different
- but for other materials, the kinetic friction can be lower than static friction
- You never see a situation where the coefficient of static friction is lower than
- the coefficient of kinetic friction
- but you do see situations where the coefficient of kinetic friction is lower
- than the coefficient of static friction that
- once something is moving for some reason--
- we'll theorize why that might be--for some reason
- friction has a little less potent than when something is stationary
- So we can say this generally
- coefficient of kinetic friction is less than or equal to the coefficient of static friction
- It's a little bit easier, or friction provides a little less, or less than
- or equal to the force when something's moving than when something is stationary
- I'll think about that a little bit deeper in the next video
Be specific, and indicate a time in the video:
At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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