Frictional Force Experiment

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floringr
Posts: 13
Joined: Tue Nov 29, 2011 2:06 pm
Occupation: Student: 11th grade
Project Question: Finding frictional force
Project Due Date: 12/5/15
Project Status: I am finished with my experiment and analyzing the data

Frictional Force Experiment

Post by floringr »

I recently did the following experiment for AP Physics 2:
Your goal is to determine the force of friction between the plunger and walls of a plastic syringe through the analysis of the work done during a thermodynamic process. You must also develop and deploy a different method that can be used to validate your results.

My group measured the pressure inside a syringe at room temperature. Then we boiled a beaker of water, and put the syringe inside the boiling water. We extended the syringe so that it added a certain amount of volume, then waited for the pressure to catch up with the pressure it was at room temperature. This way the pressure is kept constant. We measured the length we moved the syringe as well. This is our data from trial 1:

Pressure (inside the syringe) at room temperature: 100 kPa
Temperature at room temperature: 22.7 C
Temperature in hot water: 93.4 C
Volume added to the syringe by extension: +14 mL
Length syringe was extended: 4.45 cm

For the second part of the experiment, to validate the results, we used a spring scale to pull downwards on the syringe until it was moving at a constant speed, then the applied force was recorded: 1200 N. Is there any way to find the frictional force from that? Remember, this is a separate procedure from the one above, and will be used to calculate the error.

Is there any way to calculate the frictional force from this data? I feel like we are missing something, but if there is a way to calculate it just from what was recorded (and yes, this is all we have) then any help would be greatly appreciated.
prbeck
Former Expert
Posts: 12
Joined: Wed Oct 28, 2015 10:31 am
Occupation: Teacher

Re: Frictional Force Experiment

Post by prbeck »

I am assuming that you are trying to determine the static frictional force of the syringe. This is the constant force that will allow the plunger in the syringe to move.

In the thermodynamic case you heated a syringe up to the boiling point and observed the pressure of the syringe after it stopped moving as 100 kPa. So the 100kPa was the point where the pressure was no longer able to move the syringe plunger. The problem you have here is that Pascals are a unit force over an area. To change this to a force you would need to multiply it times the area that the pressure was acting on. 100kPa is 100,000 N divided by a square meter of area. To give you an example, assuming the area of the plunger would be about 0.1meters squared then you would have a force of 10,000 N (100, 000N /m squared multiplied by 0.1 m squared) where the plunger stopped working. You can assume that if the pressure was raised above that point that the plunger would start to move again (to actually start moving the syringe plunger again you would have to have a higher force because you would have to overcome the force of inertia).

In your case of spring loading the plunger by a spring scale you are directly measuring the static force that it takes to move the plunger. In your case it takes 10,000 N to move the plunger so that is the friction force needed to move the plunger at a constant speed.

Therefore the 10,000 N is the static friction force while the 100 kPa force should be slightly below that force since the plunger had stopped moving once the pressure dropped to that point.
prbeck
Former Expert
Posts: 12
Joined: Wed Oct 28, 2015 10:31 am
Occupation: Teacher

Re: Frictional Force Experiment

Post by prbeck »

I have to amend what I wrote earlier. To find the area that was resisting the movement of the plunger, you would need to know the diameter and length of the plunger end of your syringe. Your Area =pi*radius squared*length of plunger surface. You are missing the syringe plunger data. Would it be possible to get that information?

In paragraph three I wrongly used 10,000 N instead of the 1200N that is the correct value. I saw it all too late.
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