Coefficient of static friction with cable

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Discussion Overview

The discussion revolves around the measurement of the coefficient of static friction in a physics lab experiment. Participants explore the relationship between the speed of pulling an object and the observed coefficient of static friction, questioning the traditional understanding that velocity should not affect this coefficient.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant notes that the coefficient of static friction appeared to increase with faster pulling, which contradicts the expectation that velocity should not influence it.
  • Another participant suggests that pulling more suddenly may cause the block to "dig in" more, potentially affecting the measurement.
  • A different viewpoint argues that the observed increase in force during faster pulls is due to a higher peak force reading resulting from the rate of initial acceleration, rather than a change in the static coefficient of friction.
  • One participant questions the concept of "peak force" and whether it indicates a defect in the measuring instrument.
  • A participant compares the scenario to dragging a box with an elastic rope, suggesting that different pulling methods will yield different results in terms of force and stretch.
  • Another participant asserts that the only valid method for measuring static friction is to start with a stationary object and gradually increase the force until movement occurs, arguing that starting with a larger force is not a proper measurement technique.
  • It is mentioned that pulling faster results in measuring both friction and the force required to accelerate the block, complicating the measurement of static friction.

Areas of Agreement / Disagreement

Participants express differing views on the influence of pulling speed on the coefficient of static friction, with no consensus reached on the validity of the experimental approach or the interpretation of the results.

Contextual Notes

Participants highlight limitations in the experimental setup, including the potential impact of acceleration on force measurements and the method of applying force to the object. There is also uncertainty regarding the definition and implications of "peak force."

bendel
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We did a lab in physics class to determine the coefficient of static and sliding friction. basically we used a spring scale to determine how many Newtons of force was required to just get a block moving and then to keep it moving. We needed to do several trial runs with a slow, fast, and faster pull. After the block was moving, the coefficient of sliding friction were all about the same, however the coefficient of static friction became larger when we pulled faster. My teacher was unable to give me an explanation and told me that velocity should have nothing to do with coefficient of friction because it is not in the formula(Ff=uN). It makes sense to me that if you are trying to accelerate the object faster, it is going to take a lot more force to get it to move than if you are increasing the force little by little until it just barely moves. Can anyone help me understand?
 
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Welcome to PF!

bendel said:
… however the coefficient of static friction became larger when we pulled faster …

Hi bendel ! Welcome to PF! :smile:

I take it you mean, when you pulled more suddenly?

I suspect it was because you were pulling jerkily as you pulled harder, and perhaps the block "dug in" more.

Have you tried pulling something like a fridge (without wheels) across a floor? The more weight you let the leading edge take, the more difficult it is to move!

Yeah … try it with a fridge! :smile:
 
In the case of the faster pull, you created a larger jerk, which resulted in a higher peak force reading because of the rate of initial acceleration of the block. This had nothing to do with the static coefficient of friction.
 
:confused:

wot's a peak force?

are you saying it's a defect in the measuring instrument?
 
The way I'm thinking of it in my head is similar to the difference of throwing a box with an elastic rope attached off of a truck that is moving 60mph, and dragging a a box from behind the truck starting slow and accelerating. There is obviously going to be more of a stretch in the rope in the first scenario but what is the logical reason?
 
The only correct way to measure static friction is to start with a stationary object and slowly increase the force until the object starts moving. Only this way you can be sure that the force was just big enough to overcome static friction.

An experiment where you start to pull with a force big enough to cause movement has nothing to do with measuring static friction.
 
If you pull faster, and accelerate the block, you are measuring both friction and the force it takes to accelerate the block.
 

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