Force of Friction

(fL)2.2 N4.4 N6.8 N8.8 NKinetic Friction (fk)1.6 N2.6 N3.4 N4.2 NNormal Force (R)3.502 N6.943 N10.393 N14.008 NTable 3Graph 4Graph 5From the graphs,  and .Coefficients of Friction for Steel BlocksNumber of blocks1234Applied Force (F)Limiting Static Friction (fL)0.2 N0.6 N1.0 N1.6 NKinetic Friction (fk)0.1 N0.4 N0.8 N1.4 NNormal Force (R)0.996 N2.001 N3.002 N4.004 NTable 4Graph 6Graph 7From the graphs,  and .Contact Surface AreaNumber of Blocks234Pile UpSide by SidePile UpSide by SidePile UpSide by SideApplied Force F(Kinetic Friction fk)1.8 N2.0 N3.3 N2.8 N4.5 N4.0 NNormal Force R(Weight of the Blocks)3.727 N3.727 N5.561 N5.561 N7.186 N7.186 NTable 5Conclusion:The normal force of block is directly proportional to the force of friction.Different materials have different magnitudes of force of friction on the same plane.The contact surface area is independent of the kinetic friction and limiting static friction when normal force is kept constant.Discussion:Describe the shapes of the graphs plotted in Step 4 (Graph 1).When applied forces are in between 0 and 1.2 N, the graph is an inclining straight line with positive slope, which implies the static friction increases as the applied force increases. When applied force is larger than 1.2 N, friction decreases from 1.2 N to 0.8 N in a small time interval. Then a horizontal line is observed on the graph.Comment the case that when the applied force is smaller than the limiting static friction.When the applied force is smaller than the limiting static friction, the magnitude of static friction is equal to that of applied force but in different direction, according to the theory. Therefore, net force acting on the block is zero and no motion is resulted in this case.Explain why the coefficient of kinetic friction is smaller than the coefficient of static friction.Consider  and , where fL, fk and R are limiting static friction, kinetic friction and normal force respectively. Since fL > fk, μs > μk and R is kept constant, coefficient of kinetic friction must be smaller than coefficient of static friction.Compare the values of coefficients of friction for wooden blocks, steel blocks and bricks.Wooden BlocksBricksSteel Blocksμstatic0.70.640.4μkinetic0.60.320.3Table 6 – Coefficients of Friction for Different MaterialsBoth coefficients of steel blocks are smaller than those of bricks, which are smaller than those of wooden blocks.Referring to the table in Step 12 (Table 4), compare the results of the blocks in pile and placed side by side. State your findings on the effect of contact surface area.The applied forces used for the blocks in pile and placed side by side are approximately the same, and the normal forces are the same. Therefore, the contact surface area is independent of the kinetic friction when normal force is kept constant.State the sources of error and suggest improvements for the experiment.Drags of objects on the wooden plane will flatten the surface of the plane and change its roughness of the plane. The values obtained vary in different measurements.The roughness of the surface is not uniform throughout the wooden plane, i.e. some area of the plane is relatively smooth and some is otherwise. When dragging across different areas, inconsistent values of kinetic friction will be measured. Besides, when dragging the blocks along the plane in order to measure the kinetic friction, it is impossible to maintain constant speed by human hand.Sometimes the readings on the spring balance vary too fast and are difficult to observe. Systematic errors will rise. Data-logger should be used for more accurate results. - End of Report -