Kinetic Friction – Wide Face of Block 1. Weigh the block and record its mass. 2. Wipe the surfaces of the plank and block with a moist paper towel. Make sure both are free of dirt and grit. 3. Clamp the pulley at the end of the plank and place the plank at the edge of a lab table. Place the block on the far end of the plank and attach a length of string to it. Drape the string over the pulley and hang the mass hanger from its end. The string should be short enough so that the block can slide the length of the plank before the mass hanger hits the floor. 4. Determine what weight must be added to the hanger so that the system (block, string and hanger with mass) moves at constant speed: Add a little mass to the hanger. Give the block a slight push to start it moving. If the block accelerates (speeds up), take a little mass off and try again. If the block decelerates (slows down), add a little more mass and try again. (Note: If the block accelerates from the weight of the mass hanger alone, put 20 or so grams on top of the block and treat that mass as part of the block.) If the block moves the length of the plank at roughly the same speed, you have found the necessary mass. Record the total hanging mass and its weight. The weight of the hanging mass is equivalent to the force of friction on the block (see equation 4 above). Record the total mass of and on the block and its weight. The weight on the block is equivalent to the normal force, F N . 6. Repeat the above process, adding 100 grams on top of the block for each new trial, for a total of six trials. Kinetic Friction - Narrow Face of Block 7. To test the hypothesis that the force of friction is independent of the surface area in contact, repeat the above experiment using the narrow face of the block: Set the block at the end of the plank on its narrow face and reconnect the string to the lower hook so that the string is horizontal. Determine the amount of hanging mass necessary to keep the block moving at constant speed after an initial push. Do this for 0 – 500 grams placed on the block, in 100-gram increments, for a total of six trials. May 05 57
PHYS 2211L LAB 5 Experiments Analysis 1. Calculate the normal force, F n , and the frictional force, f k , for each trial. Note that the normal forces are the same for the wide & narrow faces. 2. Calculate the coefficient of friction μ k for each trial. 3. Find the average values, the deviations, and the standard deviation for both data sets. 4. Calculate the percent difference between the average values for the wide & narrow faces. 5. Questions: a) In determining the kinetic friction force, f , why was it necessary that the block move at constant speed? b) Using Newton’s Laws, show how you would measure μ k for an accelerating block.
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