Difference between static and kinetic friction

I always read that the static friction applies when the object is not moving. If it starts moving then you have kinetic friction.

But suppose the case of a block on conveyor belt. The block is moving, but I was told that that friction was static friction.

Or suppose the Death Wall. I've also been told that pilots don't fall because they suffer static friction, which cancels the weight.

So how can I know if I have static or kinetic friction if when a block is moving you can have static friction?
 
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Orodruin

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The relevant information is whether the objects are moving relative to each other or not. In the case of the conveyor belt, the block and conveyor belt are not moving relative to each other.
 
The relevant information is whether the objects are moving relative to each other or not. In the case of the conveyor belt, the block and conveyor belt are not moving relative to each other.
Why not?
If they were not moving relative to each other there wouldn't be relative motion between the surfaces and the block wouldn't move
 

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If they were not moving relative to each other there wouldn't be relative motion between the surfaces and the block wouldn't move
Put a block on a conveyor belt and the block and belt are both moving at the same speed in the same direction - zero motion relative to one another.

It’s the same thing if the block is sitting still on the surface of the earth. There’s no relative motion between the block and the earth, even though the surface of the earth is moving very rapidly because of the earth’s rotation and orbit around the sun.
 
Put a block on a conveyor belt and the block and belt are both moving at the same speed in the same direction - zero motion relative to one another.

It’s the same thing if the block is sitting still on the surface of the earth. There’s no relative motion between the block and the earth, even though the surface of the earth is moving very rapidly because of the earth’s rotation and orbit around the sun.
Yes, I understand that. But if there's no motion relative to each other why is there static friction? Because "friction opposes relative motion between surfaces". And in this case we know that the block is moving due to friction.
I don't know if my question is clear. I don't say that there's relative motion between the surfaces, but that there should be relative motion between the surfaces so that there's friction and the block moves
 

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Because "friction opposes relative motion between surfaces". And in this case we know that the block is moving due to friction.
The block is not moving because of friction, the block is moving because of inertia..
Static friction between the block and the belt only comes into play if some outside force tries to speed up or slow down the block down so that it is no longer at rest relative to the belt; imagine, for example, someone standing on the belt and trying to slide block around.

(In fact, such a force is always present. Air resistance is trying to slow the block down, but is opposed by static friction so that the net force on the block is zero. Thus, there’s always something for friction to do).
 
The block is not moving because of friction, the block is moving because of inertia..
Static friction between the block and the belt only comes into play if some outside force tries to speed up or slow down the block down so that it is no longer at rest relative to the belt; imagine, for example, someone standing on the belt and trying to slide block around.

(In fact, such a force is always present. Air resistance is trying to slow the block down, but is opposed by static friction so that the net force on the block is zero. Thus, there’s always something for friction to do).
So tell me if I'm right: when a box is dropped on a conveyor belt it feels friction which accelerates it on the direction of the belt and when it moves with the same velocity that the belt the box doesn't feel friction anymore
 

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: when a box is dropped on a conveyor belt it feels friction which accelerates it on the direction of the belt and when it moves with the same velocity that the belt the box doesn't feel friction anymore
It's better to think in terms of the forces between objects than in terms of inanimate objects like blocks and belts "feeling" things (it doesn't matter here but it will in more complex problems) but with that said, yes, pretty much right.

When the block is first dropped on the belt it will be moving relative to the belt and dynamic friction will tend to accelerate the block in the direction of the belt's travel. Very quickly, after some transient stuff that is difficult to model/analyze and doesn't change the outcome, the block ends up moving along with the belt and static friction between the block and belt will keep it that way.
 
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sophiecentaur

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when a box is dropped on a conveyor belt it feels friction
T box and belt start off with different velocities. The box will gain momentum over a finite time and that will be true for a rough surface, or if both surfaces having lateral vertical edged grooves (gear teeth) or, eventually for fairly slippery surfaces. The times will just be different but the final result will be the same. Even the gear teeth will distort a bit, in a very brief time. We call the change in momentum Impulse. All this is without slavishly using the term "friction". There will be forces between box and belt. What you choose to call them is of very little consequence.
Describing friction as a force which tends to resist relative motion (not just motion) is fine. Sometimes it slows things up (car brakes). Sometimes it speeds things up (a sprinter's shoes). No confusion if you forget what some elementary Science teacher told you.
 

Mister T

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But if there's no motion relative to each other why is there static friction?
It's by definition.

Static friction is the friction force that exists when there is no relative motion between the surfaces. Automobile tires, for example, may leave a perfect imprint on some fresh snow. This means static friction was at play.
 

sophiecentaur

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@Like Tony Stark you really don't need to worry so much about rules concerning 'names' in Science. Why a name is chosen for anything is a very secondary importance to understanding the way things actually work.
 

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