## Physics Laboratory 5

### Measuring the coefficients of static and kinetic friction

In this laboratory you will measure the coefficient of static and the coefficient of kinetic friction for a wood block and a felt-covered block in contact with a metal track.  A force sensor is mounted on a cart which is attached to the block.  The force sensor measures the applied force.  The mass of the block, cart, and sensor is given in the video clip.

Open a Microsoft Word document to keep a log of your procedures and your results.  This log will form the basis of lab report.  Address the points highlighted in blue.  Answer all questions.

Experiment 1

Find the coefficient of static friction.

You will examine two video clips.  By stepping through these clips frame by frame you will determine the applied force needed to overcome the force of static friction.  As the magnitude of the applied force becomes greater than the magnitude of the maximum force of static friction the blocks will accelerate.  Using  fs(max) = μsN and N = mg, you will determine μs for the wood block and the felt-covered block.

Procedure:

To step through video clips frame-by-frame click the buttons below.  The "Video Analysis" web page will open.

Construct a table as shown below.  Insert this table into your log.

surface fs(max) (N) mass (kg) normal force (N) c. of static friction μs
wood
felt

Experiment 2

Find the coefficient of kinetic friction.

You will examine two video clips.  You will determine the  position of the block in each clip as a function of time by stepping through the video clip frame-by-frame and by reading the time and the position coordinates of the block off each frame.  You will construct a spreadsheet with columns for time and position and use this spreadsheet to find the velocity as a function of time.  The slope of a velocity versus time graph yields the acceleration of the block.  The force sensor measures the applied force and the mass of the accelerating object is given in the video clip.  Using Ftotal = ma and Ftotal = Fapplied - fk you will determine the force of kinetic friction fk.  The coefficient of kinetic friction μk can then be found from fk = μkN.

Procedure:

To play the video clip or to step through it frame-by-frame click the "Begin" button.  The "Video Analysis" web page will open.  Choose the friction_3.mp4 and the friction_4.mp4 video clips.

## Begin

For each video clip:

• Play the video clip and return it to frame 0.
• In the setup window choose to track the x-coordinate of one object.
• Calibrate your data as described in lab 1.
• Highlight and copy your data table.  Open Microsoft Excel, and paste the table into an Excel spreadsheet.
Your spreadsheet will have two columns, time (s) and  x (m).  Produce a graph of position (vertical axis) versus time (horizontal axis).  Label the axes.
• Right-click the data and choose "Add Trendline".  Choose Polynomial, Order 2. and under options click "Display equation on chart".  An equation of the form y = b1x2 + b2 x + b3 will be displayed where b1, b2, and b3 are numbers.
• For motion with constant acceleration the position varies with time as x = x0 + v0t + (1/2)at2.  Since we are plotting x versus t, the number b1 is the best estimate for a/2 from the fit.  Therefore the value of the acceleration determined from the fit is a = 2b1.
• Find the acceleration of the block in each clip.
• Construct a table as shown below.  Insert this table into your log.
surface acceleration(m/s2) Ftotal (N) Fapplied (N) fk (N) normal force (N) c. of kinetic friction μk
wood
felt
• Compare the coefficients of static and kinetic friction for the two surfaces.  Comment on your results.  Do they make sense?
• How good do you think are your results?  What factors do you think may be responsible for the largest uncertainties?

Experiment 3

Rub your hands together and describe what you feel.
Write down what you think is happening.

Open the Friction simulation.

What do you observe as you rub the two books together?  Hit RESET to do this several times.
Does what you observe in the simulation agree with what your thought was happening when you rubbed your hands together?