#### Measuring the coefficients of friction of a wooden block

First, we need to arrange the sensors and computer to read them.

1. Get a computer, LabPro and one force sensor. Use the adaptor at the end of the force probe cable to connect the force probe to CH1 of the LabPro. Be sure the slide switch on the force sensors is at 10N.
3. Run the Logger Pro program by double-clicking on the "drag_block_mwr.cmbl" icon. This will set up the screen and sensors for today's experiment.
4. If you see a couple of windows pop up -- just press "OK" to each.
5. Zero the sensor: place the sensor on the table and zero it by typing (CNTRL-0). Check the zero by collecting data with the sensor horizontal and not touching anything.
6. Calibrate the sensor by providing a known force. Choose Experiment -> Calibrate to bring up the calibration menu. Be sure the slide switch is in 10 N position. Hold sensor vertically and enter zero for the force. Now hang a total of 550 grams = 0.55 kg from the sensor's hook (be sure to include the mass of the hanger) and enter the weight in Newtons: W = (mass in kg)*(g). Finally, press "Done" to finish the calibration.
(Beware the Tare: There is a "tare" button on the side of the force sensor. Be very careful not to accidentally press it. If you do, you will need to re-zero the sensors, but will not need to recalibrate.)

Now you can do the experiment.

1. Get one of the wooden "friction blocks" from the instructor. These blocks are covered with a fabric on two faces, and have a hook attached to one end. Also take out two of the heavy mass bars from your equipment set.
2. You'll need a piece of string about 1 or 2 feet long.
3. Measure the mass of the "friction block" and each of the mass bars.

Now, place the block on the table, motionless. Put one of the mass bars on top of it. Tie one end of a short piece of string to the block's hook, and the other end of the string to the force sensor's hook. Be sure to do the following every time you want to make measurements:

• first, click on the "Zero" button while the string is loose; this re-sets the sensor to read "0 Newtons" when there is no force acting
• then, click on the "Collect" button to start reading the force sensor
After you have started collecting data, wait for a second or two (you should see some data start to appear on the graph), then very gently pull on the force sensor. The computer will display the force with which you are pulling the block.

If you apply a very small force, the block won't move. In this situation, you are measuring the force of static friction between block and table.

If you apply a larger force, the block will start to slide. If you continue to pull as the block moves with a constant speed, you are measuring the force of kinetic friction between block and table.

Your job is to use the measured forces to figure out as many of the following as you can:

1. coefficient of static friction between wood and table
2. coefficient of kinetic friction between wood and table
3. coefficient of static friction between fabric and table
4. coefficient of kinetic friction between fabric and table