Cause of Friction. Friction is caused by the microscopic roughness between surfaces like two gears locking together. S. Evans

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2 Cause of Friction Friction is caused by the microscopic roughness between surfaces like two gears locking together.

3 Factors Affecting Friction Factors affecting friction: 1) The condition of the surfaces in contact (rough versus smooth) called coefficient of friction, Note: The coefficient of friction is the ratio of the force of friction between the two bodies and the force pressing them together (normal force). F F friction n

4 Causes of Friction Factors affecting friction: 2) The apparent weight (normal force) of an object (heavy versus light). Friction does NOT depend on area of contact

5 Friction Friction forces are parallel to contact surfaces and oppose relative motion or impending (intended) motion. F s Static Friction, F s no relative motion Kinetic Friction, F k - relative motion

6 Friction Is it easier to get an object to move or to keep it moving once it is in motion? Once moving the push only has to overcome friction not the object s inertia. Static Friction > Kinetic Friction Retrieved from: cyfair3.schoolwires.net/ /lib/.../unit_6_friction.pp...

7 Static Friction, Prevents an object from starting to move. Acts against intended motion When an attempt is made to move an object on a surface, static friction slowly increases to a MAXIMUM value F smax s F F s n

8 Kinetic Friction, F k For horizontal motion, if the applied force is the same magnitude as the kinetic friction, then the object will maintain its current state of motion (constant velocity or at rest). F k k F n

9 Friction Problems

10 Friction Problems This problem is to demonstrate how: - Force of static friction is often not equal to FsMAX - Coefficient of friction is a constant between 2 surfaces. - How the force of friction increases if the normal force increases 1) A 2.0 kg block is sitting on a floor. The largest horizontal force that can be applied to the block before it will start to move is 10. N. Once the block starts moving it only takes 6.0 N to keep it moving at constant velocity. a) Find the coefficient of static friction between the block and floor. b) Find the force of friction acing on the block if a horizontal force of 4.0 N [E] acts on the block. c) Find the maximum magnitude of static friction acting on the block if a 1.5 N block is place on top of it. d) Find the coefficient of kinetic friction.

11 Friction Problems a) The block is not moving and the largest force that can be applied before it starts to move is FsMAX (10. N) F F 10 smax s ( mg) s 0.51 s b) The horizontal applied fore 4.0N[E] is less than the FsMAX so the block remains at rest. Since Fnet = 0, the applied force must be balanced by the static friction force so Fs = 4.0 N [W]. n

12 Friction Problems c) Total mass = = 3.5 kg Note that the same contact surfaces are involved so s will still be F smax F s ( mg) s n N

13 Friction Problems d) Once moving we deal with Kinetic friction. Constant velocity means Fnet =0 so F (Fk and Fapplied will be in opposite directions) k F applied Fk k F Fk k Fn Fk F g n 6.0 N 2.0kg( m ) 2 s

14 2) Friction Problems This problem is to demonstrate how: - - How friction can not make an object move.

15 2) Friction Problems This problem is to demonstrate how: - - How friction can not make an object move.

16 Friction Problems This problem is to demonstrate how: - - How friction affects pulleys 3) Given m 1 = 10. kg and m 2 = 5.0 kg (assume pulley is massless): s a) What value of would stop the block from sliding? = 0.50 k b) If the box is sliding and = 0.20, what is the acceleration? a=1.96 m/s 2 c) What is the tension of the rope? 39N

17 Friction Problems

18 Friction Problems

19 Friction Problems

20 4) Friction Problems This problem is to demonstrate: - friction when the normal force is horizontal

21 4) Solution: Friction Problems

22 Friction Problems This problem is to demonstrate: - the force that hits the puck is finished and the FBD of the puck coming to stop will ONLY have the force of friction on it in the horizontal. 5) A hockey puck is hit with a force and given an initial speed of 20m/s on a frozen pond. The puck remains on the ice and slides 120m before coming to rest. Determine the coefficient of friction between the puck and the ice.

23 Friction Problems This problem is to demonstrate: - the force that hits the puck is finished and the FBD of the puck coming to stop will ONLY have the force of friction on it in the horizontal. - how you do not need to be given the mass all the time since sometimes it cancels out.

24 Friction Problems 6) A crate having a falls horizontally off the back of a truck which is traveling at 30 m/s. Compute the coefficient of kinetic friction between the road and the crate if the crate slides 45 m on the ground with no tumbling along the road before coming to rest. Assume that the initial speed of the crate along the road is 30m/s. (hint: find acceleration first answer is: 1.02)

25 Solving More Difficult Friction Problems When applied force is at an Angle: Retrieved from cyfair3.schoolwires.net/ /lib/.../unit_6_friction.pp...

26 Solving More Difficult Friction Problems When applied force is at an Angle: 7) a) b) Retrieved fromwww.pa.msu.edu/~schmidt/phy231s08/lecture6.ppt

27 Solving More Difficult Friction Problems Inclined Planes: HMST ONLY

28 Solving More Difficult Friction Problems #8) HMST ONLY

29 #9) HMST ONLY Solving More Difficult Friction Problems 9) An object is initially at rest on top of a 4.0 m high ramp inclined at 30. degrees to the horizontal. The coefficient of kinetic friction between the object and the ramp is The object is released. Find a) The acceleration down the ramp. [answer:.2 2 ] 3 s m b) The speed of the object at the bottom of the ramp. [answer: 7.1 ] m s

30 Solving More Difficult Friction Problems Homework from Textbook : Pg 171 #1,3 Pg 172 #3,5,6, Pg 174 #1,2 Pg 177 #1,3,4 Pg 178 #3,5,6,8 Review Qs: Pg 198 #1-12, 29 Pg 210 # 40,47,50,66,69-70