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Lab 3: Measuring Static and Kinetic Friction in Physics 31210, Study notes of Physics

Physics

A lab experiment for Physics 31210 students to measure the coefficients of static and kinetic friction between a block and a wooden board. instructions for setting up the experiment, taking measurements, and analyzing data using graphical analysis and statistical methods.

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Physics 31210 Lab 3

Static and Kinetic Friction

Introduction:

Friction can thought of as a force that opposes motion. It never pushes! When friction is strong enough

to prevent an object from moving, we call it static friction. In this lab, you will measure the coefficients of

friction between the block and a wooden board. To study how an incline effects motion, you will draw force

diagrams that relate gravity, friction, and the normal force.

Static friction has a maximum value – it can only hold back so much force. Frictional force is related to

the normal force on an object by

s

(the coefficient of static friction):

Ff≤sN

. If an object is moving

with frictional effects, we call this kinetic friction and use

k

(the coefficient of kinetic friction). The

relationship between kinetic friction and normal force is

Ff=kN

.

Study I: Using the worksheet (page 5) draw and label the force diagram for the block. Determine the

relationship between

s

and θ. Don't forget to find

s

. Have a TA check your work.

Study II: Measuring Static Friction (

s

).

Imagine pushing on a very heavy box. You can push quite hard without any movement at all. Then suddenly

you reach the “break away” point and the box starts sliding.

1. Set up your inclined plane using the materials provided. Adjust your ramp so it is at a fairly small angle (the

block shouldn't slide when set down).

2. Choose one side of your block for all measurements . Place the block on it and carefully lift until it begins to

slide. Once you've got a rough idea of the angle, use a lab jack to do a fine adjustment and get the “just

slipping” point. Use the blue tape to reduce random uncertainty.

3. Measure the sliding angle with the

large wooden protractor. There are

a few different types of protractors

in lab and it is easy to misuse them

and introduce a systematic

uncertainty in your data. Get a

lesson in protractor reading if

necessary.

4. Repeat this measurement 5 times

(remember to always use the same side of the block). If there any obvious outliers in your data set, feel free

to retake a measurement. This is a measurement with a lot of natural variation, so repeating this

measurement 50 times would be appropriate (but not necessary).

Trial 1 2 3 4 5

θ

1

Read protractor here, on bottom edge of ramp

Line u p the center of the protractor with

the base corner of the ramp.

Partial preview of the text

Download Lab 3: Measuring Static and Kinetic Friction in Physics 31210 and more Study notes Physics in PDF only on Docsity!

Static and Kinetic Friction

Introduction: Friction can thought of as a force that opposes motion. It never pushes! When friction is strong enough to prevent an object from moving, we call it static friction. In this lab, you will measure the coefficients of friction between the block and a wooden board. To study how an incline effects motion, you will draw force diagrams that relate gravity, friction, and the normal force. Static friction has a maximum value – it can only hold back so much force. Frictional force is related to

the normal force on an object by  s (the coefficient of static friction): Ff ≤ s N^. If an object is moving

with frictional effects, we call this kinetic friction and use  k (the coefficient of kinetic friction). The

relationship between kinetic friction and normal force is Ff = k N^.

Study I: Using the worksheet (page 5) draw and label the force diagram for the block. Determine the

relationship between  s and θ. Don't forget to find ^  s. Have a TA check your work.

Study II: Measuring Static Friction (  s ).

Imagine pushing on a very heavy box. You can push quite hard without any movement at all. Then suddenly you reach the “break away” point and the box starts sliding.

1. Set up your inclined plane using the materials provided. Adjust your ramp so it is at a fairly small angle (the

block shouldn't slide when set down).

Choose one side of your block for all measurements. Place the block on it and carefully lift until it begins to slide. Once you've got a rough idea of the angle, use a lab jack to do a fine adjustment and get the “just slipping” point. Use the blue tape to reduce random uncertainty.
Measure the sliding angle with the large wooden protractor. There are a few different types of protractors in lab and it is easy to misuse them and introduce a systematic uncertainty in your data. Get a lesson in protractor reading if necessary.

4. Repeat this measurement 5 times

(remember to always use the same side of the block). If there any obvious outliers in your data set, feel free to retake a measurement. This is a measurement with a lot of natural variation, so repeating this measurement 50 times would be appropriate (but not necessary). Trial 1 2 3 4 5 θ Read protractor here, on bottom edge of ramp Line up the center of the protractor with the base corner of the ramp.

5. Use Graphical Analysis to do a quick statistical analysis:

a. Make a Manual Column numbered 1-5, name it Index. b. Make a second Manual Column, θ, and enter your five values

c. Plot Index vs. θ and hit the “STAT” button on the top bar. This will give you  and 

d. Calculate the standard deviation of the mean. Remember that the uncertainty:  =.

e. Don't bother to print the GA graph. Average

Standard Deviation

Standard Deviation of the Mean =/ N

6. Using your formulas from Study I, calculate  s and ^  s from  and ^ ^. Use radians in your

calculation of the uncertainty. Study III: Measuring Kinetic Friction (qualitative)

1. If you were to repeat calculation for  , this time to measure the kinetic friction on an object, what

would change in the equations? What would remain the same?

2. If a block is sliding down a ramp, is  F = 0? Always? Sometimes? Never?

3. To measure  k , you want to eliminate any complications to make it as much like the static case as

possible, i.e. when  F = 0. What is the acceleration of the block for this?

Lower your ramp a couple of degrees and set down the block. Does it slide? Play around with this a little. Your block will slide at angles less than the one you measured for the limit of static friction. Why?
Try to find the smallest angle the block slides down on its own (you may have to nudge it to overcome static friction). You will be surprised at how slowly the block moves when it isn't accelerating.

12. Once your position versus time graph is truly a straight line, you've got constant velocity! Record the

equation of constant velocity from the DataStudio curve fit. You do not need to print the graph.

x  t  = ___________________________________________________________________

13. Measure the angle corresponding to constant velocity  = _ +/-

Here   can be taken to be the uncertainty in reading the protractor.

14. Calculate  k = ___ +/- _. The formula is the same as  s , you'll

need to explain why as you write up this lab. ^  k can be calculated given  and   (from #14)

Bonus Experiments: You must convincingly design an experiment and provide compelling (graphical) evidence to receive any bonus points for the following experiments. Furthermore, you must clearly explain the physics on the graph using appropriate labels (such as those in the figure below). I: (5 points) Use two force sensors to prove Newton's 3rd^ Law. II: (5 points) Use one force sensor and a heavy mass to study the transition from static to kinetic friction. You should see a figure that resembles the following from section 6-2 of Halliday, Fundamentals of Physics , 9e. Before you leave the lab:

1. Calculation of  s and  k and their errors.

Completed Study I – this is important for your theory section.
Statistical Analysis for Study II.
Printed graphs from Bonus experiments. Lab Report: For your report, you must explain the phenomena of static and kinetic friction. Clearly show all

your calculations, especially your derivation of  s , and your error propagation calculation. Your work on the

included sheets should be sufficient for this, but you should rewrite it if your calculations are hard to follow. Discussion questions:

1. Explain why ^ had a large spread of values in Study II.

2. Why did you only perform Study III once instead of 5 times? If you were to repeat the experiment, how

would the spread in values of ^ compare to those from Study II?

Compare the coefficients of kinetic and static friction – are they the same? Should they be? Why?
If you flipped the board (or block) over, would you expect to measure the same values for the coefficients of static and kinetic friction?

Study I: As demonstrated in class, develop a formula for  s ^ using free body diagrams. Draw a good force

diagram showing the components of W in the x- and y-directions:

Write down the weight in the x- and y-directions: Wx = Wy =

2. What are ax and ay? If  Fx=max and  Fy=may ,

what are Fx and Fx?

ax = Fx =

a y = Fy =

Using the force diagram, sum the forces in terms of N, Ff, and W:

Fx =

Fy =

4. Plug Ff = s N^ in for the frictional force in the

appropriate equation from box #3.

If you haven't yet, set Fx and Fy in box #3 to zero (from box #2).

6. You should have two equations with two unknowns. Eliminate the normal force and solve for  s in

terms of the angle θ symbolically. Get a TA to check this before moving on to step #7! x y

Lab 3: Measuring Static and Kinetic Friction in Physics 31210, Study notes of Physics

Related documents

Partial preview of the text

Download Lab 3: Measuring Static and Kinetic Friction in Physics 31210 and more Study notes Physics in PDF only on Docsity!

Static and Kinetic Friction

the normal force on an object by  s (the coefficient of static friction): Ff ≤ s N^. If an object is moving

with frictional effects, we call this kinetic friction and use  k (the coefficient of kinetic friction). The

relationship between kinetic friction and normal force is Ff = k N^.

relationship between  s and θ. Don't forget to find ^  s. Have a TA check your work.

Study II: Measuring Static Friction (  s ).

1. Set up your inclined plane using the materials provided. Adjust your ramp so it is at a fairly small angle (the

4. Repeat this measurement 5 times

5. Use Graphical Analysis to do a quick statistical analysis:

c. Plot Index vs. θ and hit the “STAT” button on the top bar. This will give you  and 

d. Calculate the standard deviation of the mean. Remember that the uncertainty:  =.

6. Using your formulas from Study I, calculate  s and ^  s from  and ^ ^. Use radians in your

1. If you were to repeat calculation for  , this time to measure the kinetic friction on an object, what

2. If a block is sliding down a ramp, is  F = 0? Always? Sometimes? Never?

3. To measure  k , you want to eliminate any complications to make it as much like the static case as

possible, i.e. when  F = 0. What is the acceleration of the block for this?

12. Once your position versus time graph is truly a straight line, you've got constant velocity! Record the

x  t  = ___________________________________________________________________

13. Measure the angle corresponding to constant velocity  = _________________ +/- ________________

Here   can be taken to be the uncertainty in reading the protractor.

14. Calculate  k = _________________ +/- _______________. The formula is the same as  s , you'll

need to explain why as you write up this lab. ^  k can be calculated given  and   (from #14)

1. Calculation of  s and  k and their errors.

your calculations, especially your derivation of  s , and your error propagation calculation. Your work on the

1. Explain why ^ had a large spread of values in Study II.

2. Why did you only perform Study III once instead of 5 times? If you were to repeat the experiment, how

would the spread in values of ^ compare to those from Study II?

Study I: As demonstrated in class, develop a formula for  s ^ using free body diagrams. Draw a good force

2. What are ax and ay? If  Fx=max and  Fy=may ,

what are Fx and Fx?

ax = Fx =

a y = Fy =

Fx =

Fy =

4. Plug Ff = s N^ in for the frictional force in the

6. You should have two equations with two unknowns. Eliminate the normal force and solve for  s in

N = Normal

Force

Ff = Frictional

Force

W = Weight

13. Measure the angle corresponding to constant velocity  = _ +/-

14. Calculate  k = ___ +/- _. The formula is the same as  s , you'll