SPH3UW/SPH4UI Unit 2.4 Friction Force Page o 8 ote Phyic Tool Box Kinetic Friction Force, : The ind o riction that act when a body lide over a urace. Static Friction Force, : Friction orce when there i no relative motion uch a trying to lide a box acro the loor. Coeicient o Static Friction,, the ratio o the magnitude o the maximum tatic riction to the magnitude o the normal orce. Magnitude o Static Friction Force: un Coeicient o Kinetic Friction,, the ratio o the magnitude o inetic riction to the magnitude o the normal orce. Magnitude o Kinetic Friction Force: un Fluid: ubtance that low and tae the hape o it container. Vicoity: internal riction between molecule caued by coheive orce. Laminar Flow: table low o a vicou luid where adjacent layer o a luid lide moothly over one another. Turbulence: irregular luid motion. Bernoulli Principle: Where the peed o a luid i low, the preure i high. Where the ame peed o the ame luid i high, the preure i low. When a body ret o lide acro a urace, we can alway repreent the contact orce exerted by the urace on the body in term o component o orce perpendicular and parallel to the urace. The perpendicular component i called the normal orce, n. The component parallel to the urace i the riction orce,. I the urace i rictionle, then the contact orce ha only a normal component. ( 0 ). Friction Diagram
SPH3UW/SPH4UI Unit 2.4 Friction Force Page 2 o 8 Horizontal Motion Minimizing Friction
SPH3UW/SPH4UI Unit 2.4 Friction Force Page 3 o 8 Example woman pull a loaded led o ma m 75g along a horizontal urace at contant velocity. The coeicient o inetic riction,, between the runner o the led and the now i 0.0, and the angle i 42. What i the magnitude o the orce T on the led rom the rope? y Solution: x ote: Becaue the led velocity i contant, it acceleration i zero in pite o the women pull. cceleration i prevented by a inetic rictional orce Since the acceleration i zero, then T F F 0 x component: T co u F 0 Tco 42 0.0F 0 y component: T in F F 0 g g m T in42 F 75g 9.8 0 2 We have two equation and two unnown. on the led rom the now. T co 42 F 0.0 T co42 m T in 42 75g 9.8 0 2 0.0 m T 0.0in 42 co42 0.075g 9.8 2 m 0.075g 9.8 2 T 0.0in 42 co 42 9
SPH3UW/SPH4UI Unit 2.4 Friction Force Page 4 o 8 Example bloc o ma m = 20.0 g on a rough, horizontal urace i connected to a ball o ma m 2 =0.0 g by a lightweight cord over a lightweight, rictionle pulley. orce o magnitude F=220 at an angle 30 with the horizontal i applied to the bloc a hown. The coeicient o inetic riction between the bloc and urace i 0.20. Determine the magnitude o the acceleration o the two object, given that the m 2 rie. Solution By creating the ree body diagram (ee above) o the two mae, we can put to gether a plan o attac. Given that the acceleration path i up and then to the right, we will mae that direction poitive. Becaue we have riction, it i a good idea to determine the ormal Force irt. F 0 F in F m g 0 F m g F in For ma 2 For ma F m a m g T m a 2 F co T F m a F co T F m a F co T m g F in m a F m a Since the tenion, T and acceleration, a, are the unnown, we need only add thee equation to eliminate the tenion variable and then olve or the acceleration variable.
SPH3UW/SPH4UI Unit 2.4 Friction Force Page 5 o 8 F co T m g F in m a F co m g F in m g m a m a m g T m a 2 2 2 Fco mg Fin m2g a m m m m 220co 30 0.220g 9.8 220 in 2 30 0g 9.8 2 20g 0g 2.5 Thereore ma rie at 2.5 m 2. Example Bloc in the diagram below weigh.50 and bloc B weigh 4.30. The coeicient o inetic riction between all urace i 0.20. Find the magnitude o the horizontal orce F neceary to drag bloc B to the let at a contant peed i and B are connected by a light, lexible cord paing around a ixed, rictionle pulley. Solution: The ideal pulley change the direction o the tenion but not it magnitude.
SPH3UW/SPH4UI Unit 2.4 Friction Force Page 6 o 8 T Kw 0 thereore T ukw Since we are moving at a contant peed, we have no acceleration F wb w ukw Tcord 0 F w w u w u w 0 B K K F u w u w u w u w K B K K K u w 3w K B 0.20 4.30 3.50.76 For ma B F 0 x F F T F F T 0 F m g m g m g T 0 B For Ma F F x 0 T 0 m g T 0 F T m g Thereore F m mb m T 0 F m g m g m g m g 0 B F 3 m g m g 0 B F 3 m g m g B 3 0.20.50 0.20 4.30.76
SPH3UW/SPH4UI Unit 2.4 Friction Force Page 7 o 8 Example bloc i given an initial velocity o 5.00 m/ up a 30.0 incline. How ar up the incline doe the bloc lide beore coming to ret i the coeicient o riction i 0.200? a F Solution: F Let irt determine the ormal orce (we will require that or riction. F 0 F co 30 F 0 g F g F co 30 mg co 30 F G ow let determine the acceleration n in 30 co30 in 30 co30 in 30 F ma F F ma gx F mg ma mg mg ma g g a ow or the topping ditance v v 2ad i v vi d 2a m m 0 25 m 26.597 2.89m Thereore the bloc will lide about.89 m m m a 2 2 m 6.597 2 0.2 9.8 co30 9.8 in 30
SPH3UW/SPH4UI Unit 2.4 Friction Force Page 8 o 8 Extra ote and Comment