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PHYS 111LV Friction Lab 03
发布时间：2024-06-21
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PHYS 111LV Friction Lab 03
Introduction/Background
Frictional are contact forces that resist motion.There are two types of friction:kinetic and static friction.
Static friction,like its name suggests,occurs in the case when the object is at rest.It points in the
direction exactly opposite to the sum of the other forces on the object.The magnitude of the static
friction force,fs,is whatever it has to be to prevent acceleration of the objcct up to a maximum value of
fs,max =μsN,
where N is the magnitude of the normal force and μs is called the coefficient of static friction.The
coefficient of statie friction has no units (is dimensionless)and depends on the material that the object is made of and the material of the surface it is on.
When an object is sliding on a surface,the friction force is in the opposite dircction to the velocity,and is called kinetic friction.The magnitude of the kinetic friction force,fr,is
fk =μxN.
Again,N is the magnitude of the normal force and μg is called the coefficient of kinetic friction,which depends on what the materials the object and the surface it is on are made of.
Procedure
In your labpaq lab kit,there should be a bag with two wooden blocks in it.One of the wooden blocks should have a mirror glued to one face and sand paper glued to the opposing face.
1)First,find and record the mass of each of the blocks using the digital scale.
2)Put a sheet of paper on a table,and tape or weight down the paper so it will not move.
3)Take the wooden block with no mirror or sandpaper,and place it on the paper.Take the digital
scale(the spring scale will probably not be sensitive enough),and rotate it so the scale plate is
vertically oriented.Tare (zero)the scale by pressing the on button once,and make sure you are in gram mode (use the M button to svitch modes).Slowly begin pushing the scale plate against the block (see figure)until the block moves.Just before the block imoved,the scale reading should be at its maximum value.Record this maximum value.Repeat until you have 10 measurements for this maximum value.
4)Convert your scale measurements to Newtons.Do this by noting that if the scale reads 1 g,this would be the weight of a l-gram mass.So the force the scale feels is mg=(0.001 kg)*(9.8 m/s²) =0.0098 N.So if the scale reads,say 11.2 g,then the force exerted on or by the scale plate is 0.0098*11.2N=0.10976 N.
5)Find the average of your maximum scale forces,and the standard error (see the uncertainty handout).
6)Using a free body diagram and the information you collected,find the coefficient of static friction of the wooden block on paper
7)Repeat parts 3)-6)of the procedure for the block with the mirror resting on the paper,and the block with the sandpaper resting on the paper.
8)Now go back to part 3(you may want to change the paper at this point,because the paper gets smoother as you sand it).Instead of looking at the maximum force of the wooden,push the
wooden block at a constant velocity over the paper and record the reading of the scale.This may be difficult because the scale reading will be jumping around,but do your best with finding the' central value of the scale reading.Record this.Repeat for a total of 10 measurements.Repeat parts 4)and 5)for these values.
9)Using the information you collected in I)and 8),find the coefficient of kinetic friction for the woodblock on paper
A note on uncertainties;
Use your standard error in your applied force as your uncertainty in the applied force.Your free body diagram will show that the force applied by the scale is the same size as the friction force.
The normal force that appears in the formulae for friction is the normal force between the surface and the object.In our case,your free body diagram will end up telling you that this is the same size as the force of gravity,mg.This is uncertain by 0.05 g,according to our rule for uncertainties in single measurements.Use this and your standard error in the friction force to find the uncertainty in the coefficients of friction.
Results that must be reported
In your Raw Data section,make sure you have a table of your measurement data.
The following results must be reported in your results section.
Your measured coefficient of static friction for the three surfaces,with uncertainties.
You measured coefficient of kinetic friction for the woodblock on paper,with uncertainty.
Questions
1.Compare your coefficient of kinetic friction for the wood on paper with the coefficient of static friction for the wood on paper.Can you say which one is larger (use the z'comparison to see if they are definitely different),and if so,which one is larger?
