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Physics Friction Force Dr. Lu Nov. 6, 2018Topic: Friction and Friction ForceLearning Targets(1).I can calculate the friction force on an object by multiplying the coefficient of friction of the contact surface and the normal force on the object from the surface.(2). I can find the net force acting on an object by subtracting the friction force from the applied force.(3) I can calculate the acceleration of an object by the net force from Newton’s 2nd Law of Motion UnpackingCoefficient of frictionNormal forceApplied forceNet forcea = net force / mFriction – power point about frictionFriction Force Ff = u Fn Friction force exists from the contact surface between two object, it is against to motion of each object. If the surface condition is rough ( not smooth), the friction force will be greater. If the object is on the surface of earth, friction force is proportional to the weight of an object. Or, Friction force = Ff = u W= u [mg] ; where u = coefficient of friction; Fn = normal force from the surface; w = weight; w= mg u (coefficient of friction) is depending on the surface condition. force = applied force = friction force Fnet = Fa = Ff ??????? F net = maExample 1A child pulls a wagon at a constant velocity along a level sidewalk. The child does this by applying a 22- newton force to the wagon handle, which is inclined at 35° to the sidewalk as shown below.What is the magnitude of the force of friction on the wagon?(1) 11 N (2) 13 N (3) 18 N (4) 22 N[Solution]The wagon is moving at a constant velocity because the net force = 0 nThe horizontal component of the pulling force of Force (22 n) is Fcosθ.Applied force = Fcosθ; Friction force = Ff Fnet = Fcosθ - Ff = net force = 0So, Ff = Fcosθ = 22[cos 350] = 18 n answer is (3)Example 2 The diagram below shows a 4.0-kilogram object accelerating at 10. meters per second2 on a rough horizontal surface.What is the magnitude of the frictional force Ff acting on the object?[Solution]Fnet = Fa - Ff = applied force – friction force = ma = (4kg)(10 m/s2 ) = 40 n = 50 n – Ff So, Ff = 50 n -40 n = 10 nGroup Work Group A – problem 1 Group B - problem 4 Group C – problem 3Group D - problem 1 Group E – problem 2 Group F – problem 41.Problem 1A force of 60. newtons is applied to a rope to pull a sled across a horizontal surface at aconstant velocity. The rope is at an angle of 30. degrees above the horizontal. (A)Calculate the magnitude of the component of the 60.-newton force that is parallel to the horizontal surface. [Show all work, including the equation and substitution with units.] [2](B)Determine the magnitude of the frictional force acting on the sled.2. Problem 2A 10.-kilogram box, sliding to the right across a rough horizontal floor, accelerates at –2.0 meters per second2 due to the force of friction.(A)Calculate the magnitude of the net force acting on the box. [Show all work, including the equation and substitution with units.] [2](B) Draw a vector representing the net force acting on the box. Begin the vector at point P and use a scale of 1.0 centimeter = 5.0 newtons. [2] C) Calculate the coefficient of kinetic friction between the box and the floor. [Show all work, including the equation and substitution with units.] 3. Problem 3 As represented in the diagram below, a constant 15-newton force, F, is applied to a 2.5-kilogram box, accelerating the box to the right at 2.0 meters per second squared across a rough horizontal surface. Calculate the magnitude of the net force acting on the box. [Show all work, including the Equation and substitution with units.] [2]Determine the magnitude of the force of friction on the box. [1]4.Problem 4 The diagram below represents a 4.0-newton force applied to a 0.200-kilogram copper block sliding to the right on a horizontal steel table.Exit Ticket1.The diagram below shows a block sliding down a plane inclined at angle θ with the horizontal.As angle θ is increased, the coefficient of kinetic friction between the bottom surface of the block and the surface of the incline will (1) decrease (2) increase (3) remain the same2. In the above problem, as angle θ is increased, the friction force will (1) decrease (2) increase (3) remain the same 3. If there is a friction force from the contact plane (inclined plane) and the box, how do you express the friction force in terms of normal force and kinetic coefficient of friction, u? (A) Determine the weight of the block. [1](B) Calculate the magnitude of the force of friction acting on the moving block. [Show all work, Including the equation and substitution with units.] [2](C) Determine the magnitude of the net force acting on the moving block. [1](D)70 Describe what happens to the magnitude of the velocity of the block as the block slides across the table. [1]Reflection and Summary (2 min)Did you achieve the learning target today?Make appointment with teacher for additional help when needed.Group InformationGroup A : Normad, Jonathan, and Brenda - Normad can help team membersGroup B: Wens, Jacqueline, Philip, and Zoha - Wens can help team membersGroup C: Noah, Fredie, Jepth, Gabriella- Noah can help members.Group D; Naomi and SandraGroup E: Elyzha, Giovanni,Cassandra, and Kamra- Elyzha can help membersGroup F: Tim, Judy, Glenny, TylerHomework: Review problem1.The graph below represents the velocity of an object traveling in a straight line as a function of time. Determine the magnitude of the total displacement of the object at the end of the first 6.0 seconds graphically and algebraically.2. The graph below shows the relationship between speed and elapsed time for a car moving in astraight line. (A) Determine the magnitude of the acceleration of the car. [1](B) Calculate the total distance the car traveled during the time interval 4.0 seconds to 8.0 seconds.[Show all work, including the equation and substitution with units.] [2]Friction Force3. A force of 10. newtons toward the right is exerted on a wooden crate initially moving to the right on a horizontal wooden floor. The crate weighs 25 newtons. (Jan 2003)(a) Calculate the magnitude of the force of friction between the crate and the floor. [Show all work,including the equation and substitution with units.] [2](b) Draw and label all vertical forces acting on the crate. [1]? On the diagram in your answer booklet, draw and label all horizontal forces acting on thecrate. [1](d) What is the magnitude of the net force acting on the crate? [1](e) Is the crate accelerating? Explain your answer. [1]4.The driver of a car made an emergency stop on a straight horizontal road. The wheels locked and the car skidded to a stop. The marks made by the rubber tires on the dry asphalt are 16 meters long, and the car’s mass is 1200 kilograms.(June 04)(A) Determine the weight of the car. [1](B) Calculate the magnitude of the frictional force the road applied to the car in stopping it. [Show all work, including the equation and substitution with units.] [2](C)Calculate the work done by the frictional force in stopping the car. [Show all work, including the equation and substitution with units.] [2](D) Assuming that energy is conserved, calculate the speed of the car before the brakes were applied. [Show all work, including the equation and substitution with units.] 5. The diagram below represents a block at rest on an incline.Which diagram best represents the forces acting on the block? (Ff = frictional force,FN = normal force, and Fw = weight.) ................
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