Friction force and constant velocity

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Discussion Overview

The discussion centers around the relationship between force, friction, and velocity in the context of a block of wood being pulled across a surface. Participants explore concepts related to friction forces, acceleration, and the effects of varying forces on speed, touching on both theoretical and practical aspects of physics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that doubling the pulling force will double the acceleration of the block, but the speed will depend on the net force and may not simply double.
  • There is a question about whether the force of friction would double if the constant velocity of the block is doubled, with some arguing that it would not, as kinetic friction is independent of velocity in a simplified model.
  • One participant mentions that while it feels harder to maintain higher speeds due to increased power expenditure, the force needed to maintain constant speed does not change with velocity.
  • Another participant introduces the idea that fluid friction is dependent on velocity, contrasting it with normal sliding friction, which typically does not vary much with speed.
  • There is a discussion about the terminology surrounding viscosity and drag, with participants clarifying their understanding of these concepts and how they relate to forces.
  • A participant provides an analogy comparing the movement of a marble pulled by different forces, emphasizing that the acceleration to reach a specific speed can vary significantly based on the force applied.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between force, friction, and velocity, particularly regarding whether friction increases with speed. The discussion remains unresolved with multiple competing perspectives presented.

Contextual Notes

Some claims rely on simplified models of physics, and there are references to real-world complexities that may not be fully addressed. The discussion includes varying interpretations of terms like viscosity and drag, which may lead to confusion.

m_wu
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Does the speed of a block of wood being pulled across a surface go twice as fast if you pull twice as hard? If yes/no, why?

Thanks to anyone who will answer that!
 
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The net (horizontal) force on the block will determine the block's acceleration, the rate at which its speed changes. If you double the net force, the acceleration is doubled. You need more info to find the speed, since it changes.

If the net force is zero, the speed will remain constant.
 
Would the force of friction double if you double the constant velocity of the wooden block? If yes, why?

Thank you again.
 
Would the force of friction double if you double the constant velocity of the wooden block?

No. It requires more force to accelerate up to high speed, but once at high speed, the force needed to maintain that speed is the same as the force needed to maintain a lower speed. Subjectively (I mean if you are using your own muscles to do the pushing), it feels harder to go faster, but that is because the power expended is higher. Power = Force times speed, so you get tired faster.
 
Originally posted by m_wu
Would the force of friction double if you double the constant velocity of the wooden block?
In the simplified freshman physics model, no. The kinetic friction is independent of velocity. In the real world, if you're just talking surface contact, I would imagine that the friction would actually decrease with velocity.
 
Fluid friction depends strongly on velocity. Normal sliding friction usually doesn't depend on velocity much at all.
 
Originally posted by Stingray
Fluid friction
By this, do you mean "viscosity" or something else. I've heard an old man call "viscosity" "pipe friction" (and then an argument ensued. Arguing with old men can be quite frustrating if you care to convence them).
 
I meant viscosity as well as what would more precisely be called "pressure or inertial drag" (the main component of aerodynamic drag that's due to fluid being forced out of the way from an oncoming object).
 
Originally posted by Stingray
I meant viscosity as well as what would more precisely be called "pressure or inertial drag" (the main component of aerodynamic drag that's due to fluid being forced out of the way from an oncoming object).
I didn't think this was the same thing as viscosity, but is was imprecise for me to say "viscosity" for force, when it is a coefficient, not a force. I thought that there could be drag without viscosity, though.
 
  • #10
Yes, you're right. My "as well as" meant that I was mentioning two different things.

I think it is common practice to call a force viscosity, isn't it?
 
  • #11
Originally posted by Stingray
Yes, you're right. My "as well as" meant that I was mentioning two different things.
Oh. I misunderstood. Sorry.




Originally posted by Stingray
I think it is common practice to call a force viscosity, isn't it?
I don't know. I don't ever talk about fluid dynamics with anyone. My first year phys book speaks of viscosity as a coefficient, an makes a point of not calling it a force.
 
  • #12
A 1 ounce marble pulled by a connected string with your hand moving at 1 mph will go no faster or slower than the same marble pulled by an 80 ton hydraulic jack whoes piston moves at 1 mph.
 
  • #13
The acceleration TO the 1 mph speed is much greater with the 80 ton hydraulic jack, however.
 
  • #14
By this, do you mean "viscosity" or something else. I've heard an old man call "viscosity" "pipe friction" (and then an argument ensued. Arguing with old men can be quite frustrating if you care to convence them).

Especially if they are right! "Pipe friction" doesn't cover everything, but it's a good off-the-cuff representation.

I think it is common practice to call a force viscosity, isn't it?

No, it is not. "viscosity" is more like the "coefficient of friction" that determines the force than the force itself.
 
  • #15
Originally posted by HallsofIvy
Especially if they are right! "Pipe friction" doesn't cover everything, but it's a good off-the-cuff representation.
I was trying to explain to him that my "viscosity" was his "pipe friction." It was a disaster. He just wasn't havin' any of that newfangled technical jargon.
 

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