Static friction — Experiment pulling different blocks across a surface

[Kolika28]

Homework Statement

So in lab at school we did an experiment where we pulled blocks with different surfaces(iron, wood, wool felt) across the desk (made out of wood) with a spring balance. We were given the task to measure the weight to the blocks and how many newtons we needed to make them move. And the we reapeted this 5 times for each block. We were then asked to find the maximum force of static friction and coeffcient of static friction. But how?

Relevant Equations

So I know that
$F_s=\mu*N$

I know that I am supposed to use the equation I wrote, but how do I find the maximum force of static friction and coeffcient of static friction if none of them are given beforehand? I can't have to unknowns in an equation. We then did the same thing, the only difference was then to measure the max force of kinetic friction. I really appreciate some help :)

 

[Lnewqban]

Did you measure and record the weight and needed pulling force for each block?

 

[Kolika28]

Did you measure and record the weight and needed pulling force for each block?

I assume by "pulling force", you mean what we measured with the spring balance? Then yes, we did!

 

[Lnewqban]

If you try to push a heavy piece of furniture, you will feel that more force is needed to start moving it than to keep sliding it over a flat floor.
The reason is that for the same combination of materials and roughness of surfaces in contact, the coefficient of static friction is always a little higher than the coefficient of dynamic friction.

 

[Kolika28]

If you try to push a heavy piece of furniture, you will feel that more force is needed to start moving it than to keep sliding it over a flat floor.
The reason is that for the same combination of materials and roughness of surfaces in contact, the coefficient of static friction is always a little higher than the coefficient of dynamic friction.

But when it comes to the to numbers I'm supposed to find, is the force of static friction the number I read from the spring balance? And I'm afterwards supposed to use that number and the weight I found, to determine the coefficent of static friction?

 

[Lnewqban]

When pulling each block until make it start sliding, you should have observed an increasing force (while the spring stretched), which reached a maximum value (just the instant before the block started moving) and then, decreased some while the blocks were moving.
Which of those values corresponded to the requested "maximum force of static friction" for each block?

 

[Kolika28]

When pulling each block until make it start sliding, you should have observed an increasing force (while the spring stretched), which reached a maximum value (just the instant before the block started moving) and then, decreased some while the blocks were moving.
Which of those values corresponded to the requested "maximum force of static friction" for each block?

I guess the biggest value you observe before the decrease, is the "maximum force of static friction"? Because as you said above: you feel more force is needed to make an object move than to keep it sliding? Correct me if I'm wrong?

 

[Lnewqban]

Correct!

Now, from that conclusion, you have one of the terms of each equation.

 

[Kolika28]

Correct!

Now, from that conclusion, you have one of the terms of each equation.

Thank you so much for your help! I really appreciate it! :)

 

[Lnewqban]

You are welcome.
Did you find the value of each N?

That coefficient is not more than the Fs/N rate.
The greater the pulling force to initiate movement (the value of Fs) respect to N, the higher the coefficient.
For dynamic friction, N remains the same, but Fk is lower than Fs.

Please, see:
http://hyperphysics.phy-astr.gsu.edu/hbase/Class/PhSciLab/friction.html

 

[haruspex]

I assume by "pulling force", you mean what we measured with the spring balance? Then yes, we did!

Is the spring balance reading in Newtons or kg? Just checking.

 

[Kolika28]

You are welcome.
Did you find the value of each N?

That coefficient is not more than the Fs/N rate.
The greater the pulling force to initiate movement (the value of Fs) respect to N, the higher the coefficient.
For dynamic friction, N remains the same, but Fk is lower than Fs.

Please, see:
http://hyperphysics.phy-astr.gsu.edu/hbase/Class/PhSciLab/friction.html

Yes, I did find the value of each N when doing the experiment.
But I have one question: how do we calculate the coefficent if both the mass and the pullforce we used, have uncertainties?

 

[Kolika28]

Is the spring balance reading in Newtons or kg? Just checking.

It reads Newtons!

 

[haruspex]

Yes, I did find the value of each N when doing the experiment.
But I have one question: how do we calculate the coefficent if both the mass and the pullforce we used, have uncertainties?

You can use the variance in the five readings to estimate the error in the mean. Provided that is small compared to the average reading, you can then combine the error in the two weights to get the error in the coefficient. (The percentage errors add.)

 

[Lnewqban]

Yes, I did find the value of each N when doing the experiment.
...

As the values of the pulling forces were in Newtons, the value of each N should be the calculated mass times the acceleration of gravity (in Newtons), or simply the average measured weight of each block (in Newtons).

 

[Kolika28]

Thank you so much! I have been sick for the last couple of days, so I'm sorry it took some time to reply! I only have one question left. We did also try to drag an iron block across the desk with three different velocities. We were supposed to see if the kinetic frictional force was independent of the speed. And our measurements told us that the force is independent. But how come? I tried to search a bit online, but I don't find many websites giving a clear explanation. Some websites says the kinetic frictional force is dependent, and some says that is not.

 

[haruspex]

We did also try to drag an iron block across the desk with three different velocities. We were supposed to see if the kinetic frictional force was independent of the speed. And our measurements told us that the force is independent. But how come? I tried to search a bit online, but I don't find many websites giving a clear explanation. Some websites says the kinetic frictional force is dependent, and some says that is not.

Friction is a complicated phenomenon. As a result, it is presented in a simplified form to students at first: kinetic frictional force and max static frictional force are proportional to the normal force, the constant factor depending on whether it is static or kinetic.
In reality, the kinetic force can have some speed dependence and both can have some area dependence. See e.g.
https://www.researchgate.net/publication/51152968_Dependence_of_kinetic_friction_on_velocity_Master_equation_approach

 

[Lnewqban]

Thank you so much! I have been sick for the last couple of days, ...
And our measurements told us that the force is independent. But how come?

You are welcome.
Hope you are feeling better.
Did you expect a different result?
If so, would you like to explain it?

 

[Kolika28]

You are welcome.
Hope you are feeling better.
Did you expect a different result?
If so, would you like to explain it?

Thank you so much, I feel much better
Hmmm, I don't know what I expected to be honest. I just didn't understand why it would be independent. But I see know that the formula for kinectic friction is just dependent on mass and the coefficent of friction. So it does make sense now!

Friction is a complicated phenomenon. As a result, it is presented in a simplified form to students at first: kinetic frictional force and max static frictional force are proportional to the normal force, the constant factor depending on whether it is static or kinetic.
In reality, the kinetic force can have some speed dependence and both can have some area dependence. See e.g.
https://www.researchgate.net/publication/51152968_Dependence_of_kinetic_friction_on_velocity_Master_equation_approach

Thank you so much, it was a great explanation!