FRICTION In thi experiment we will meaure the effect of friction on the motion of a body in contact with a particular urface. When a body lide or roll over another, it motion i oppoed by the force of friction between the urface of the two bodie. If a body i at ret on another body, there exit tatic friction between the two bodie. A certain amount of force i required to overcome the tatic friction and et the body in motion. Friction depend on the propertie of the urface of the bodie in contact, pecifically the urface roughne of the bodie. It alo depend on how tightly the bodie are preed againt one another. Ultimately, friction depend on electromagnetic interaction at the atomic or molecular level. THE THEORY Conider a bloc of ma m, reting on a flat urface (ee Fig. 1). To et the bloc in motion we apply a force F. Until F i large enough to overcome friction, the frictional force f i equal and oppoite to F and no motion tae place. In thi experiment you will tet the following empirical rule that applie to uch a ytem: f = ìn, where the proportionality contant i the coefficient of friction ì. We uually ditinguih between tatic friction, when an object i at ret, and inetic friction, when an object i already moving. Here, you will motly (but not excluively) tudy tatic friction and meaure the coefficient of tatic friction ì (uing apparatu imilar to figure 1. and 2.). In fact, for an inclined plane, uch a in Fig. 2, it can be hown that the coefficient of tatic friction, ì, i related to the mallet angle è for which the bloc begin to lide down the plane by: ì = tanè Figure 1 Figure 2. 1
DATA COLLECTION I. Static Friction with Horizontal Plane In the firt part of thi experi ment, you will deter mine the coefficient of tatic friction between aluminum and Teflon urface by tudying the relationhip between the normal force, N, and the minimal Figure 3 amount of force required to overcome the frictional force and et the bloc in motion. The etup i illutrated in Fig. 3 and 4. 1. Meaure the ma of the aluminum bloc, m b, on the balance pan cale. 2. Set the wooden board (with the Teflon heet attached) to the horizontal poition, and faten the pulley near the end of the board. Attach the aluminum bloc and the force gauge at the end of the tring. Set the bloc down on it larget ide in the middle of the Teflon heet and connect the force gauge to the motor. 3. Place the tring over the pulley wheel and mae ure that the pulley wheel i adjuted o that the tring between the aluminum bloc and the pulley wheel i level (otherwie, you will be exerting an extra normal force on the aluminum bloc when the force gauge pull it). 4. Mae ure that the tring connecting the force gauge to the motor and to the aluminum bloc are aligned (put ome tenion on the tring to accomplih thi). After the alignment, reduce the tenion of the tring by rotating the motor haft a bit. Reet the force gauge to zero by puhing the tare button on the ide of the gauge. 5. Mae ure that the force gauge i attached to the computer and that you are ready to 2
tae data: i) The power trip controlling the computer hould be on ii) Log on a uer (ee uername and paword indicated on the monitor face). iii) Double clic on Data Studio (Note that if the interface box ha been turned off, you will have to turn it on and then reboot the computer). iv) Open Activity Go up to C: Student temp p183 friction_meaure v) To get an empty run: Experiment Delete All Data Run vi) File Save Activity A 183E5your initial 6. When you are ready to tae data (mae ure the ample rate i 500Hz) clic on Start 7. Turn on the motor. You hould ee a plot of the force exerted on the bloc by the force gauge a a function of time. Poitive force may be in the negative y direction on the plot. See Fig. 5. 8. The force which jut overcome the tatic friction force, f, i F. The bloc then begin to move and when it i moving with a contant velocity, the force neceary to overcome the inetic friction force, f, i F. 9. The force will not necearily be contant after the bloc ha begun to move with a contant Figure 5 velocity (ee Fig. 5). Can you thin of an explanation for thi? 10. Clic on } Stop to top the data taing. 11. Record the value of F and F in Table 1 of the data heet. Repeat the meaurement 2 more time and record the value of F and F in Table 1 of the data heet. 12. Save the file by clicing on File Save Activity. 13. Mae ure the data window i the active window (a your intructor if you are not 3
ure). Then print out a copy of the data window (File Print) 14. Repeat tep #6 through #11 for m w=200, 400, and 800 gram added to the aluminum bloc. Mae ure to add thee weight in equal increment to the two crew, i.e. for m w=200 gram, add 100 gram to each of the bra crew poition on top of the aluminum bloc. Try to et the aluminum bloc down at approximately the ame poition on the Teflon heet each time (don t lide the aluminum bloc on the Teflon urface, et it down carefully in place). 15. If the graph on the computer creen become too crowded, remove the previou run from the diplay 16 Prepare one graph of the tatic frictional force, f, a a function of the normal force, N, uing the averaged data from Table 1 and another graph of the inetic frictional force, f, a a function of the normal force, N. 17. Fit traight line through the data point on each of the graph and extract the tatic and inetic coefficient of friction uing the lope of the line. II. Friction Dependence on Material 1. Remove all mae from the aluminum bloc. 2. Remove the Teflon heet and inert the rubber urface onto the wood. 3. Repeat tep #6 through #11 of Part I above (3 trial total), and record F and F in Table 2. 4. From the value of F and F and the value of the Normal force, N, calculate the value of ì and ì, the tatic and inetic coefficient of friction, and record them in Table 2. 4
III. Static Friction with Inclined Plane In thi part of the experiment, you will determine the coefficient of tatic friction, ì, between aluminum and Teflon urface by uing an inclined plane. The etup i hown in Fig. 6. 1. Put the Teflon urface bac on the wooden board. 2. Remove the pulley and move the wooden board near the cran ytem. Attach the rope and hoo near the end of the board (where the pulley wa attached). 3. Remove the tring from the bloc. Figure 6 4. Set the aluminum bloc down in the middle of the Teflon heet a before (don t lide the aluminum bloc on the Teflon urface, et it down carefully in place). 5. Turn the cran to raie the board. Raie the board a moothly a poible (jarring the board may et the bloc in motion prematurely) and record, in Table 3., the mallet angle è for which the bloc tart liding. The apparatu include a protractor o o mared in degree from 0 to 45. Tae a total of 4 meaurement of thi quantity and record them in Table 3. BEWARE: Hold the cran handle while lowering the board. The handle may hurt you if you let it go free!! 6. Repeat tep #4 and #5 for m w = 200, 400, and 800 gram added to the bloc (½ at each crew poition a in the firt part of the experiment). 7. Compute the average angle and value of ì for each value of m w. Then compute the average value of ì value from your five different m w meaurement of the coefficient of friction, ì. 5
IV. Friction Dependence on Surface Area In the lat part of thi experiment, you will ue the inclined plane, a in Part III. to invetigate the friction dependence on urface area. 1. Remove all mae from the aluminum bloc. 2. Set the bloc down on it ide in the middle of the Teflon heet. 3. Record the mallet angle è for which the bloc tart liding in Table 4. Repeat thi until you have a total of 4 meaurement, which you record in Table 4. 4. Compute the average coefficient of friction. 6
PHYSICS 183 - LAB Expt 5 Worheet Fall 2004 FRICTION STUDENT NAME DATE PARTNER S NAME LAB SECT Ma of the aluminum bloc g Table 1. Teflon Surface Trial m w: 0 200 gram 400 gram 800 gram N = (m +m )g b w 1 f = F 2 3 Avg. f = F Graph: 1) f a a function of normal force N. Slope of bet line going through (0,0) yield =... 2) f a a function of normal force N. Slope of bet line going through (0,0) yield =... 1
PHYSICS 183 - LAB Expt 5 Worheet Fall 2004 Table 2. Rubber Surface F F Trial 1 Trial 2 Bloc on rubber Trial 3 Average Calculated value = = Table 3. Non Horizontal Teflon Surface Beware that in the table your angle are given in degree, not in radian! Mae ure your calculator i et properly. Angle, (degree) mw 0 200 gram 400 gram 800 gram average = tan Average = Table 4: Angle (degree) = tan Bloc on it ide (Dependence on urface area) trial 1 trial 2 trial 3 trial 4 Average 2
PHYSICS 183 - LAB Expt 5 Worheet Fall 2004 QUESTIONS 1. From you graph of frictional force (F) v. N, what i the relationhip between frictional force and load (normal force). 2. Compare the value of obtained for teflon with the horizontal and inclined plane. Expre the difference in term of a percent difference. Do thee value agree within 20%? If not, what could caue the difference? 3. What do your meaurement from part III ay about the effect of load on the coefficient of friction? 3
PHYSICS 183 - LAB Expt 5 Worheet Fall 2004 4. What do your meaurement from part IV ay about the effect of urface area? 5. From your data, what can you ay about the difference between the coefficient of tatic and inetic friction? 6. Are there difference in the value of the coefficient of friction for different material? If o, what could the caue() of thee difference be? 7. If thi experiment i being ubmitted a a lab report, derive the relation = tan. At the end of the lab, turn in your worheet with the data and anwered quetion a well a your graph. 4